CISCO CCNP ROUTESIMPLIFIED

Your Complete Guide to Passing the CCNP ROUTE Exam

Paul Browning (LLB Hons) CCNP,MCSE

Farai Tafa dual CCIE

This study guide and/or material is not sponsored by,endorsed by or affiliated with Cisco Systems, Inc.Cisco, Cisco Systems, CCDA, CCNA,CCDP, CCNP, CCIE, CCSI, the CiscoSystems logo and the CCIE logo are trademarks orregistered trademarks of Cisco Systems, Inc in theUnited States and certain other countries. All othertrademarks are trademarks of their respective owners.

Copyright Notice

Copyright 2004, 2005, 2007, 2008, 2009, 2010,2011 Paul Browning all rights reserved. No portion ofthis book may be reproduced mechanically,electronically or by any other means, includingphotocopying without written permission of thepublisher.

ISBN: 978-0-9557815-7-5

Published by:Reality Press Ltd.

Midsummer Court314 Midsummer Blvd.Milton KeynesMK9 2UBhelp@reality-press.com

LEGAL NOTICE

The advice in this book is designed to help you achievethe standard of Cisco Certified Network Engineerwhich is Ciscos foundation internetworkingexamination. A CCNA is able to carry out basic routerand switch installations and troubleshooting. Before youcarry out more complex operations it is advisable toseek the advice of experts or Cisco Systems, Inc.

The practical scenarios in this book are meant toillustrate a technical point only and should be used onyour privately owned equipment only and never on alive network.

INTRODUCTIONThank you for purchasing Cisco CCNP ROUTESimplified.

The ROUTE exam is acknowledged as one of Ciscosmost difficult exams and many who start on the journeytowards the coveted CCNP certification fall down atthis first hurdle. Perhaps it is due to the overwhelmingamount of information they need to digest andunderstand, or perhaps it is because what they aretrying to learn never seems to sink in.

If you have ever struggled to retain what you have reador have struggled to apply your knowledge to a livenetwork, then we know how you feel; we felt the sameway, which is why we decided to develop our ownlearning materials. Cisco CCNP ROUTE Simplifiedhas been written with the busy student in mind. Weunderstand that many of you are studying around a full-time job, as well as family and other commitments. For

this reason, we decided to strip out all of the usual fluff you find in most Cisco manuals and give you theinformation you need to pass your ROUTE exam ANDapply what you have learned to a live Cisco network.

Passing your ROUTE exams will give you a massiveboost in confidence, as well as dramatically improveyour ability as a Cisco network engineer. Once youunderstand the major routing concepts, you will find theother CCNP materials much easier to understand andapply. Passing the CCNP certification will also give youa very strong foundation for becoming an IT contractor,consultant, designer, or even a voice or securityengineer.

In order to become a CCNP engineer, you will need topass the following exams (preferably in this order):

642-902 ROUTEImplementing Cisco IPRouting642-813 SWITCHImplementing Cisco IPSwitched Networks642-832 TSHOOTTroubleshooting and

Maintaining Cisco IP Networks

We have broken down each chapter of this book tomatch exam requirements and have added moreinformation when needed to give you a deeperunderstanding of the technologies introduced. Eachchapter ends with a review and the book finishes withseveral hands-on labs for you to follow on our liveracks (at http://racks.howtonetwork.net) or on yourown home lab.

Aim to take your ROUTE exam after around 60 to 90days of studying 2 to 3 hours per day. If you are amember of www.howtonetwork.net, then please ensurethat you make use of the other study materials, includingthe flash cards and exams, as well as the CCIE-moderated forum.

Best of luck with your studies.

ABOUT THE AUTHORSPaul Browning

Paul Browning is the author of CCNA Simplified, whichis one of the industrys leading CCNA study guides.Paul previously worked for Cisco TAC but left in 2002to start his own Cisco training company in the UK. Paulhas taught over 2,000 Cisco engineers with both hisclassroom-based courses and his online Cisco trainingsite, www.howtonetwork.net. Paul lives in the UK withhis wife and daughter.

Farai Tafa

Farai Tafa is a Dual CCIE in both Routing andSwitching and Service Provider. Farai currently worksfor one of the worlds largest telecoms companies as anetwork engineer. He has also written workbooks forthe CCNA, CCNP, and Cisco Security exams. Farailives in Washington, D.C. with his wife and daughter.

P A R T 1

Theory

CHAPTER 1 Internet Protocol Routing

Fundamentals

Welcome to the ROUTE course of the Cisco CertifiedNetwork Professional (CCNP) certification program.The focus of this guide is to pick up on IP routingconcepts where the Cisco Certified NetworkAdministrator (CCNA) certification program left off. Inaddition, this guide will introduce and explain, in detail,additional relevant concepts that are mandatoryrequirements of the current ROUTE certification exam.

The ROUTE certification exam focuses primarily on theimplementation of IP routing protocols within theenterprise network. In addition, you are also expectedto demonstrate a solid theoretical and practicalunderstanding of routing solutions to support remoteoffices as well as mobile workers. These concepts willbe covered in detail in this guide. This chapter lays thefoundation for the core concepts that will be describedthroughout this guide. The Internet Protocol routingfoundation topics that will be covered in this chapterinclude the following:

Internet Protocol Routing Fundamentals Flat and Hierarchical Routing Algorithms IP Addressing and Address Summarization Administrative Distance Routing Protocol Metrics Prefix Matching Building the IP Routing Table or RIB Routing Protocol Classes The Objectives of Routing Protocols On-Demand Routing (ODR)

INTERNET PROTOCOL ROUTINGFUNDAMENTALSA routing protocol allows a router to learn dynamicallyhow to reach other networks. A routing protocol alsoallows the router to exchange learned networkinformation with other routers or hosts. Routingprotocols may be used for connecting interior (internal)campus networks as well as for connecting differententerprises or routing domains. Different routingprotocols use different means of determining the best or

most optimal path to a network or network node.

Some types of routing protocols work best in staticenvironments or environments with few or no changes,but they might take a long time to converge whenchanges to those environments are made. Other routingprotocols, however, respond very quickly to changes inthe network and can converge rapidly. Networkconvergence occurs when all routers in the networkhave the same view and agree on optimal routes. Whenconvergence takes a long time to occur, intermittentpacket loss and loss of connectivity may beexperienced between remote networks. In addition tothese problems, slow convergence can result in networkrouting loops and outright network outages.Convergence is determined by the routing protocolalgorithm used.

Because routing protocols have different characteristics,they differ in their scalability and performance. Somerouting protocols are suitable only for small networks,while others may be used in small, medium, and largenetworks. Therefore, in addition to understanding the

intricacies of routing protocols, it is also important tohave a solid understanding of when and in what situationone routing protocol would be used versus another.

FLAT AND HIERARCHICALROUTING ALGORITHMSRouting protocol algorithms operate using either a flatrouting system or a hierarchical routing system. Ahierarchical routing system uses a layered approachwherein routers are placed in logical groupings referredto as domains, areas, or autonomous systems. Thisallows different routers within the network to performspecific tasks, optimizing the functionality performed atthose layers. Some routers in the hierarchical systemcan communicate with other routers in other domains orareas, while other routers can communicate only withrouters in the same domain or area. This reduces theamount of information that routers in the domain or areamust process, which allows for faster convergencewithin the network.

A flat routing system has no hierarchy. In such systems,routers must typically be connected to every otherrouter in the network and each router essentially has thesame function. Such algorithms work well in very smallnetworks, however, they are not scalable. In addition,as the network grows, troubleshooting becomes muchmore difficult because instead of just focusing yourefforts on certain areas, for example, you now have tolook at the entire network.

The primary advantage afforded by hierarchical routingsystems is their scalability. Hierarchical routing systemsalso allow for easier changes to the network, in muchthe same way afforded by the traditional hierarchicaldesign comprised of the Core, Distribution, and Accesslayers. In addition, hierarchical algorithms can be usedto reduce routing update traffic as well as routing tablesize in certain areas of the network while still allowingfull network connectivity.

IP ADDRESSING AND ADDRESSSUMMARIZATION

An IP address is divided into two parts. The first partdesignates the network address while the second partdesignates the host address. When designing a network,an IP addressing scheme is used to uniquely identifyhosts and devices within the network. The IPaddressing scheme should be hierarchical and shouldbuild on the traditional logical hierarchical model. Thisallows the ad dressing scheme to provide designatedpoints in the network where effective routesummarization can be performed.

Summarization reduces the amount of information thatrouters must process, which allows for fasterconvergence within the network. Summarization alsorestricts the size of the area that is affected by networkchanges by hiding detailed topology information fromcertain areas within the network. This concept isillustrated in Figure 1-1 below:

Fig. 1-1. Route Summarization

Referencing Figure 1-1, the branch offices (Accesslayer) are dual-homed to the regional office routers(Distribution layer). Using a hierarchical addressingscheme allows the Distribution layer routers to advertisea summary route for the branch office subnets to theCore layer. This protects the Core layer from the

effects of any route flapping between the Distributionand the Access layer routers because a summary routewill not flap until every last one of the more specificprefixes from which it is derived is removed from therouting table. This increases stability within the area. Inaddition, the routing table size at the Core layer isfurther reduced.

ADMINISTRATIVE DISTANCEAdministrative distance is used to determine thereliability of one source of routing information fromanother. Some sources are considered more reliablethan others are; therefore, administrative distance canbe used to determine the best or preferred path to adestination network or network node when there aretwo or more different paths to the same destinationfrom two or more different routing protocols.

In Cisco IOS software, all sources of routinginformation are assigned a default administrativedistance value. This default value is an integer between0 and 255, with a value of 0 assigned to the most

reliable source of information and a value of 255 beingassigned to the least reliable source of information. Anyroutes that are assigned an administrative distance valueof 255 are considered untrusted and will not be placedinto the routing table.

The administrative distance is a locally significant valuethat affects only the local router. This value is notpropagated throughout the routing domain. Therefore,manually adjusting the default administrative distance fora routing source or routing sources on a router affectsthe preference of routing information sources only onthat router. Table 1-1 below shows the defaultadministrative values used in Cisco IOS software:

Table 1-1. Administrative Distance Values

The default route source administrative distance isdisplayed in the output of the show ip protocolscommand. This is illustrated in the following output:

R1#show ip protocolsRouting Protocol is isisInvalid after 0 seconds, hold down 0, flushed after 0Outgoing update filter list for all interfaces is not setIncoming update filter list for all interfaces is not setRedistributing: isis

Address Summarization:NoneMaximum path: 4Routing for Networks:Serial0/0

Routing Information Sources:

Distance: (default is 115)

The administrative distance value assigned to anindividual route is viewed in the output of the show iproute [prefix] command. This is illustrated in thefollowing output:

R1#show ip route 150.1.1.0Routing entry for 150.1.1.0/28Known via isis, distance 115, metric 20, type level-2Redistributing via isisLast update from 10.0.0.2 on Serial0/0, 00:00:11 agoRouting Descriptor Blocks:

* 10.0.0.2, from 150.1.1.2, via Serial0/0Route metric is 20, traffic share count is 1

In addition to allowing administrators to change thedefault administrative distance values of individualrouting protocols, Cisco IOS software also allowsadministrators to adjust the administrative distance ofindividual prefixes learned via a dynamic routingprotocol. This will be described in detail later in thisguide.

ROUTING PROTOCOL METRICSRouting protocol algorithms use metrics, which arenumerical values that are associated with specificroutes. These values are used to prioritize or preferroutes learned by the routing protocol from the mostpreferred to the least preferred. In essence, the lowerthe route metric, the more preferred the route by therouting protocol. The route with the lowest metric istypically the route with the least cost or the best route tothe destination network. This route will be placed intothe routing table and be used to forward packets to the

destination network.

Different routing algorithms use different variables tocompute the route metric. Some routing algorithms useonly a single variable, while other advanced routingprotocols may use more than one variable to determinethe metric for a particular route. In most cases, themetrics that are computed by one routing protocol areincompatible with those used by other routingprotocols. The different routing protocol metrics may bebased on one or more of the following:

Bandwidth Cost Delay Load Path Length Reliability

Bandwidth

The term bandwidth refers to the amount of data thatcan be carried from one point to another in a given time

period. Routing algorithms may use bandwidth todetermine which link type is preferred over another. Forexample, a routing algorithm might prefer aGigabitEthernet link over a FastEthernet link because ofthe increased capacity of the GigabitEthernet link overthe FastEthernet link.

In Cisco IOS software, the bandwidth interfaceconfiguration command can be used to adjust thedefault bandwidth value for an interface, effectivelymanipulating the selection of one interface againstanother by a routing algorithm. For example, if theFastEthernet interface was configured with thebandwidth 1000000 interface configuration command,both the FastEthernet and the GigabitEthernet linkswould appear to have the same capacity to the routingalgorithm and would be assigned the same metric value.The fact that one of the links is actually a FastEthernetinterface while the other is actually a GigabitEthernetlink is irrelevant to the routing protocol.

From a network administrators point of view, it isimportant to understand that the bandwidth command

does not affect the physical capability of the interface.In other words, configuring the higher bandwidth on theFastEthernet interface does not mean that it is capableof supporting GigabitEthernet speeds. OSPF andEIGRP use bandwidth in metric calculation.

Cost

The cost, as it pertains to routing algorithms, refers tocommunication cost. The cost may be used when, forexample, a company prefers to route across privatelinks rather than public links that include monetarycharges for sending data across them or for the usagetime. Intermediate Systemto-Intermediate Systemsupports an optional expense metric that measures themonetary cost of link utilization. IS-IS is beyond thescope of the ROUTE exam requirements and will notbe described in this guide.

Delay

There are many types of delay, all of which affectdifferent types of traffic. In general, delay refers to the

length of time required to move a packet from source todestination through the internetwork. In Cisco IOSsoftware, the interface delay value is in microseconds(s).

The interface value is configured using the delayinterface configuration command. When you configurethe interface delay value, it is important to rememberthat this does not affect traffic. For example, configuringa delay value of 5000 does not mean that traffic sentout of that interface will have an additional delay of5000. Table 1-2 below shows the default delay valuesfor common interfaces in Cisco IOS software:

Table1-2. Interface Delay Values

Enhanced Interior Gateway Routing Protocol uses theinterface delay value as part of its metric calculation.

Manually adjusting the interface delay value results inthe re-computation of the EIGRP metric.

Load

The term load means different things to different people.For example, in general computing terminology, loadrefers to the amount of work a resource, such as theCPU, is performing. Load, as it applies in this context,refers to the degree of use for a particular routerinterface. The load on the interface is a fraction of 255.For example, a load of 255/255 indicates that theinterface is completely saturated, while a load of128/255 indicates that the interface is 50% saturated.By default, the load is calculated as an average over aperiod of five minutes. The interface load value can beused by EIGRP in its metric calculation.

Path Length

The path length metric is the total length of the path thatis traversed from the local router to the destinationnetwork. Different routing algorithms represent this in

different forms. For example, Routing InformationProtocol (RIP) counts all intermediate routers (hops)between the local router and the destination networkand uses the hop count as the metric, while BorderGateway Protocol (BGP) counts the number oftraversed autonomous systems between the local routerand the destination network and uses the autonomoussystem count to select the best path.

Reliability

Like load, the term reliability means different thingsdepending on the context in which it is used. In thisguide, unless stated otherwise, it should always beassumed that reliability refers to the dependability ofnetwork links or interfaces. In Cisco IOS software, thereliability of a link or interface is represented as afraction of 255. For example, a reliability value of255/255 indicates the interface is 100% reliable. Similarto the interface load, by default the reliability of aninterface is calculated as an average over a period offive minutes.

PREFIX MATCHINGCisco routers use the longest prefix match rule whendetermining which of the routes placed into the routingtable should be used to forward traffic to a destinationnetwork or node. Longer, or more specific routing tableentries are preferred over less specific entries, such assummary addresses, when determining which entry touse to route traffic to the intended destination networkor node.

The longest prefix or the most specific route will beused to route traffic to the destination network or noderegardless of the administrative distance of the routesource, or even the routing protocol metric assigned tothe prefix if multiple overlapping prefixes are learned viathe same routing protocol. Table 1-3 below illustratesthe order of route selection on a router sending packetsto the address 1.1.1.1. This order is based on thelongest prefix match lookup.

Table1-3. Matching the Longest Prefix

NOTE: Although the default route is listed last in theroute selection order in Table 1-3, keep in mind that adefault route is not always present in the routing table. Ifthat is the case and no other entries to the address1.1.1.1 exist, packets to that destination are simplydiscarded by the router. In most cases, the router willsend the source host an ICMP message indicting thatthe destination is unreachable.

BUILDING THE IP ROUTINGTABLE OR RIBWithout a populated routing table or RoutingInformation Base (RIB) that contains entries for remotenetworks, routers will not be able to forward packets tothose remote networks. The routing table may include

specific network entries or simply a single default route.The information in the routing table is used by theforwarding process to forward traffic to the destinationnetwork or host. The routing table itself does notactually forward traffic.

Cisco routers use the administrative distance, routingprotocol metric, and the prefix length to determinewhich routes will actually be placed into the routingtable, which allows the router to build the routing table.The routing table is built via the following general steps:

1. If the route entry does not currently exist in therouting table, add it to the routing table.2. If the route entry is more specific than anexisting route, add it to the routing table. It shouldalso be noted that the less specific entry is still alsoretained in the routing table.3. If the route entry is the same as an existing one,but is received from a more preferred routesource, replace the old entry with the new entry.4. If the route entry is the same as an existing one,and is received from the same protocol:

e) Discard the new route if the metric is higherthan the existing route; orf) Replace the existing route if the metric of thenew route is lower; org) If the metric for both routes is the same, useboth routes for load-balancing.

When building the RIB by default, the routing protocolwith the lowest administrative distance value will alwayswin when the router is determining which routes toplace into the routing table. For example, if a routerreceives the 10.0.0.0/8 prefix via external EIGRP,OSPF, and internal BGP, the OSPF route will beplaced into the routing table. If that route is removed oris no longer received, the external EIGRP route will beplaced into the routing table. Finally, if both the OSPFand the external EIGRP routes are no longer present,the internal BGP route is used.

Once routes have been placed into the routing table, bydefault the most specific or longest match prefix willalways be preferred over less specific routes. This is

illustrated in the following example, which shows arouting table that contains entries for the 80.0.0.0/8,80.1.0.0/16, and the 80.1.1.0/24 prefixes. These threeroute prefixes are received via the EIGRP, OSPF, andRIP routing protocols, respectively.

R1#show ip route

Gateway of last resort is not set

Referencing the output shown above, the first route isthe 80.1.1.0/24 route. This route is learned via RIP andtherefore has a default administrative distance value of120. The second route is the 80.0.0.0/8 route. This

route is learned via internal EIGRP and therefore has adefault administrative distance value of 90. The thirdroute is the 80.1.0.0/16 route. This route is learned viaOSPF and is an external OSPF route that has anadministrative distance of 110.

NOTE: Because the routing protocol metrics aredifferent, they are a non-factor in determining the bestroute to use when routes from multiple protocols areinstalled into the routing table. The following section willdescribe how Cisco IOS routers build the routing table.

Based on the contents of this routing table, if the routerreceived a packet destined to 80.1.1.1, it would use theRIP route because this is the most specific entry, eventhough both EIGRP and OSPF have betteradministrative distance values and are therefore morepreferred route sources. The show ip route 80.1.1.1command illustrated below can be used to verify thisstatement:

R1#show ip route 80.1.1.1Routing entry for 80.1.1.0/24

Known via rip, distance 120, metric 1Redistributing via ripLast update from 10.1.1.2 on Ethernet0/0.1, 00:00:15agoRouting Descriptor Blocks:* 10.1.1.2, from 10.1.1.2, 00:00:15 ago, viaEthernet0/0.1Route metric is 1, traffic share count is 1

ROUTING PROTOCOL CLASSESThere are two major classes of routing protocols. Thesetwo classes are the Distance Vector and the Link Staterouting protocol classifications. Distance Vector routingprotocols traditionally use a one-dimensional vectorwhen determining the most optimal path(s) through thenetwork, while Link State routing protocols use theShortest Path First (SPF) when determining the mostoptimal path(s) through the network. Before delving intothe specifics of these two classes of routing protocol,we will first take a look at vectors, as well as at theelusive SPF algorithm.

Understanding Vectors

A one-dimensional vector is a directed quantity. It issimply a quantity (number) in a particular direction orcourse. The vector concept is illustrated in Figure 1-2below:

Fig. 1-2. Understanding Vectors

Referencing Figure 1-2, the first line starts at 0 and endsat 8, and the second line begins at 8 and ends at 13.The vector for the first line is 8, while the vector for thesecond line is 5. Using basic math, we know that 8 + 5= 13. The starting and ending points of the vector arenot relevant. Instead, the only thing that actually mattersis how long the vector is and how far it travels.

NOTE: Vectors can also travel in the oppositedirection (negative numbers).

The Shortest Path First Algorithm

The SPF algorithm creates a shortest-path tree to allhosts in an area or in the network backbone, with therouter that is performing the calculation at the root ofthat tree. In order for the SPF algorithm to work in thecorrect manner, all routers in the area should have thesame database information. In OSPF, this is performedvia the database exchange process, which will bedescribed in detail later in this guide. The SPFcalculation is performed in iterations using three sets, asfollows:

1. Unknown2. Tentative (TENT)3. PATH

When the SPF algorithm initializes, all nodes, except forthe root, which is also referred to as self, are placed inthe Unknown set or list. The root is the routerperforming the SPF calculation. As SPF continues torun, nodes in the Unknown set are moved to theTentative set beginning with the nodes directly

connected to the root. The Tentative set is alsocommonly referred to simply as the TENT set or list.

As the router goes through the SPF calculation, thenode in the TENT set or list that is closest to the root ismoved to the PATH or PATHS list or set. This processis repeated until all nodes are in the PATH set and theshortest-path tree is built. Once the tree has beencompletely built, routes are then derived from the tree.This concept is illustrated referencing the basic networktopology in Figure 1-3 below:

Fig. 1-3. Shortest Path First Algorithm

Referencing Figure 1-3, it is assumed that R1 is therouter performing the SPF calculation. The followingare the sequence of steps taken by this router:

R1 places itself in the PATH list with a next-hop of itselfand a cost of 0. When R1 places itself in the PATH list,R1 is also referred to as the PATH node, in addition tobeing the SPF root. All other nodes or routers aretemporarily placed in the Unknown set and have theircost presently set to infinity.

R1 examines the neighbors of the PATH node (itself )and because its only neighbor is R2, R1 adds R2 to theTENT list. Open Shortest Path First is able to do so bylooking at the Link State Advertisement (LSA) packetsthat are exchanged during the database exchange andthe synchronization process.

R1 calculates the cost to R2 from the PATH node. Thisis performed by adding the cost to the PATH node andthe cost from the PATH node to the TENT node. Inthis example, the cost from R1 to the PATH node (itself) is 0, and the cost from the PATH node (R1) to the

TENT node (R2) is 10. The total path cost to reach R2from R1 is 0 + 10 = 10.

Because the cost of 10 is less than the cost of infinitythat was initially assigned, the infinity value isoverwritten with the cost value of 10. R2 is then movedto the PATH list.

Once R2 is placed in the PATH list, it becomes thenewest PATH node and the second SPF iterationbegins on R1. Remember, SPF stops when all nodesare in the PATH set.

1. R1 examines the neighbors of the new PATHnode (R2), which are R3 and itself. Because R1 isalready added to the PATH list (in step 1) it isignored. R3, however, is moved from theUnknown set to the TENT set.2. R1 calculates the cost to R3 by adding the costto the PATH node (R2) and the cost from thePATH node (R2) to the TENT node (R3). Thecost from R1 to the PATH node (R2) is 10, andthe cost from the PATH node to the TENT node

(R2) is 10. The total path cost to reach R3 fromR1 is 10 + 10 = 20.3. Because the cost of 20 is less than the cost ofinfinity that was initially assigned, the infinity valueis overwritten with the cost value of 20. R3 is thenmoved to the PATH list.

This process is repeated until there are no more routersin the TENT list on any of the routers participating inOSPF. Figure 1-4 below shows the SPF tree as itwould appear on routers R1 and R2:

Fig. 1-4. SPF Tree on R1 and R2

Referencing the diagram in Figure 1-4, notice that thetree is built on the perspective of the root, which is thelocal router itself. Figure 1-5 below shows the SPF treeas it would appear on R3 and R4:

Fig. 1-5. SPF Tree on R3 and R4

Distance Vector Routing Protocols

A Distance Vector protocol is a routing protocol thatuses distance or hop count as its primary metric fordetermining the best forwarding path. Distance Vectorprotocols are primarily based on the Bellman-Fordalgorithm. Distance Vector routing protocolsperiodically send their neighbor routers copies of their

entire routing tables to keep them up to date on thestate of the network.

While this may be acceptable in a small network, itincreases the amount of traffic that is sent acrossnetworks as the size of the network grows. All DistanceVector routing protocols share the followingcharacteristics:

Counting to Infinity Split Horizon Poison Reverse Hold-Down Timers

Utilizing the counting to infinity characteristic, if adestination network is more than the maximum numberof hops allowed for that routing protocol, it would beconsidered unreachable. The network entry wouldtherefore not be installed into the IP routing table.

Split horizon mandates that routing information cannotbe sent back out of the same interface through which itwas received. This prevents the re-advertising of

information back to the source from which it waslearned. While this characteristic is a great loopprevention mechanism, it is also a significant drawback,especially in hub-and-spoke networks.

Poison reverse (or route poisoning) expands on splithorizon. When used in conjunction with split horizon,poison reverse allows the networks to be advertisedback out of the same interface on which they werereceived. However, poison reverse causes the router toadvertise these networks back to the sending routerwith a metric of unreachable so that the router thatreceives those entries will not add them back into itsrouting table.

Hold-down timers are used to prevent networks thatwere previously advertised as down from being placedback into the routing table. When a router receives anupdate that a network is down, it begins its hold-downtimer. This timer tells the router to wait for a specificamount of time before accepting any changes to thestatus of that network.

During the hold-down period, the router suppresses thenetwork and prevents advertising false information. Therouter also does not route to the unreachable network,even if it receives information from another router (thatmay not have received the triggered update) that thenetwork is reachable. This mechanism is designed toprevent black-holing traffic.

The two most common Distance Vector routingprotocols are RIP and IGRP. Enhanced InteriorGateway Routing Protocol, which uses both DistanceVector and Link State features, is an advancedDistance Vector routing protocol.

Link State Routing Protocols

Link State routing protocols are hierarchical routingprotocols that use the concept of areas to logicallygroup routers within a network. This allows Link Stateprotocols to scale better and operate in a more efficientmanner than Distance Vector routing protocols. Routersrunning Link State rout ing protocols create a databasethat comprises the complete topology of the network.

This allows all routers within the same area to have thesame view of the network.

Because all routers in the network have the same viewof the network, the most optimal paths are used forforwarding packets between networks and thepossibility of routing loops is eliminated. Therefore,techniques such as split horizon and route poisoning donot apply to Link State routing protocols as they do toDistance Vector routing protocols.

Link State routing protocols operate by sending LinkState Advertisements or Link State Packets to all otherrouters within the same area. These packets includeinformation on attached interfaces, metrics, and othervariables. As the routers accumulate this information,they run the SPF algorithm and calculate the shortest(best) path to each router and destination network.

Using the received Link State information, routers buildthe Link State Database (LSDB). When the LSDBs oftwo neighboring routers are synchronized, the routersare said to be adjacent.

Unlike Distance Vector routing protocols, which sendtheir neighbors their entire routing table, Link Staterouting protocols send incremental updates when achange in the network topology is detected, whichmakes them more efficient in larger networks. The useof incremental updates also allows Link State routingprotocols to respond much faster to network changesand thus converge in a shorter amount of time thanDistance Vector routing protocols. Table 1-4 belowlists the different Interior Gateway Protocols (IGPs) andtheir classification:

Table 1-4. IGP Classification

NOTE: Although the Border Gateway Protocol(BGP) uses the autonomous system path or AS PATH

to determine the best path to a destination network, it isnot considered a Distance Vector routing protocol.BGP is referred to as a Path Vector protocol, which isa derivative of Distance Vector routing protocols.Unlike RIP, for example, the BGP path selectionprocess is complex and detailed and is not entirelybased on the AS PATH. Border Gateway Protocol is acore ROUTE requirement. This protocol will thereforebe described in detail later in this guide.

THE OBJECTIVES OF ROUTINGPROTOCOLSRouting algorithms, while different in nature, all have thesame basic objectives. While some algorithms arebetter than others are, all routing protocols have theiradvantages and disadvantages. Routing algorithms aredesigned with the following objectives and goals:

Optimal Routing Stability Ease of Use

Flexibility Rapid Convergence

Optimal Routing

One of the primary goals of all routing protocols is toselect the most optimal path through the network fromthe source subnet or host to the destination subnet orhost. The most optimal route depends on the metricsused by the routing protocols. A route that may beconsidered the best by one protocol may notnecessarily be the most optimal route from theperspective of another protocol. For example, RIPmight consider a path that is only two hops long as themost optimal path to a destination network, even thoughthe links were 64Kbps links, while advanced protocolssuch as OSPF and EIGRP might determine that themost optimal path to that same destination is the onetraversing four routers but using 10Gbps links.

Stability

Network stability, or a lack thereof, is another major

objective for routing algorithms. Routing algorithmsshould be stable enough to accommodate unforeseennetwork events, such as hardware failures and evenincorrect implementations. While this is typically acharacteristic of all routing algorithms, the manner andtime in which they respond to such events makes somebetter than others and thus more preferred in modern-day networks.

Ease of Use

Routing algorithms are designed to be as simple aspossible. In addition to also providing the capability tosupport complex internetwork deployments, routingprotocols should take into consideration the resourcesrequired to run the algorithm. Some routing algorithmsrequire more hardware or software resources (e.g.,CPU and memory) to run than others; however, theyare capable of providing more functionality thanalternative simple algorithms.

Flexibility

In addition to providing routing functionality, routingalgorithms should also be feature-rich, allowing them tosupport the different requirements encountered indifferent networks. It should be noted that thiscapability typically comes at the expense of otherfeatures, such as convergence, which is described next.

Rapid Convergence

Rapid convergence is another primary objective of allrouting algorithms. As stated earlier in this chapter,convergence occurs when all routers in the networkhave the same view and agree on optimal routes. Whenconvergence takes a long time to occur, intermittentpacket loss and loss of connectivity may beexperienced between remote networks. In addition tothese problems, slow convergence can result in networkrouting loops and outright network outages.

ON-DEMAND ROUTING (ODR)In addition to understanding how to implement routingprotocols in Cisco IOS software, as a Cisco network

engineer, it is also important to understand how toleverage one of the most common Cisco protocols,which is the Cisco Discovery Protocol, when designingand implementing large, scalable networks. This sectiondescribes On-Demand Routing (ODR).

The Cisco Discovery Protocol (CDP) is a Ciscoproprietary protocol that, among other things, is used todiscover other Cisco devices on either broadcast ornon-broadcast media. CDP provides administratorswith information that includes the IP address, softwareversion, and the capabilities of the neighbor device.ODR is an enhancement to CDP that advertises theconnected IP prefix or prefixes of a stub router viaCDP. ODR also supports VLSM, which means that itcan be used in just about any network.

It is important to know that ODR is not a routingprotocol. Instead, it is simply an enhancement to CDPthat is used to dynamically propagate routinginformation at Layer 2. The primary reason ODR isoften incorrectly referred to as a routing protocol isbecause it allows routers to dynamically exchange

routing information. The secondary reason is becauseODR is enabled using the router odr globalconfiguration command.

The primary benefit of using ODR is that it is not CPU-intensive and it consumes very little bandwidth.Consider a network using the topology illustrated inFigure 1-6 below, for example:

Fig. 1-6. A Hub-and-Spoke Network

Figure 1-6 illustrates a typical hub-and-spoke network.

The branch office routers (spokes) are connected to thehub (headquarters) using low-speed WAN links. Whilea dynamic routing protocol such as EIGRP or OSPFcould be used to exchange dynamically the routinginformation between the hub and the spoke routers, theamount of bandwidth consumed by the routing protocolupdates becomes a great concern, especially on thelow-speed WAN links.

Another alternative would be to use static routing.However, the administrative overhead that is requiredto manually configure static routing, especially as thenetwork grows, quickly becomes a cumbersome andnegative factor, despite the low overhead afforded bystatic routing. ODR can be used in such cases due to itslow bandwidth consumption and resource requirements.This is because ODR only requires an additional fivebytes, which can contain the IP address of theconnected subnet plus 1 byte for the subnet mask, incomparison to OSPF, which sends Hello packets thatare comprised of 20 bytes of IP header, 24 bytes ofOSPF header, 20 bytes of hello parameters, and 4

bytes for each neighbor seen, for example.

At the hub router, the prefixes received via ODR canthen be redistributed into another routing protocol, suchas OSPF, and propagated to the rest of the network.This allows the network to scale while taking intoconsideration the bandwidth limitations at the spokes.

Configuring ODR

In order to use ODR, CDP must be enabled on therouter. If disabled, CDP can be enabled on the routerusing the cdp enable global configuration command.There is no explicit configuration required to enableODR on the spoke routers. However, it is important toensure that there are no other routing protocols runningon the spoke routers. If Cisco IOS detects that adynamic routing protocol is configured, ODR will notbe used to exchange routing information with the hubrouter(s). This is a common configuration mistake whenusing ODR.

On the hub router, ODR is enabled using the router

odr global configuration command. Unlike the spokerouter(s), a routing protocol can be enabled on the hubrouter. The ODR routes can then be redistributed intothe dynamic routing protocol and propagatedthroughout the routing domain. The ODR configurationexample in this section will be based on the networktopology illustrated in Figure 1-7 below:

Fig. 1-7. Implementing ODR in a Hub-and-SpokeNetwork

It is assumed that in the network illustrated in Figure 1-7, the OSPF routing protocol is enabled between the

Distribution router and the Core router. The HQnetworks are being advertised to the Distribution routervia OSPF by the Core router. ODR will then beenabled between the Distribution router and the spoke(Access). However, prior to the implementation ofODR, OSPF routing between the Core and theDistribution routers is verified as illustrated in thefollowing output:

Dist-1#show ip route ospf

In addition to verifying OSPF between the Core andthe Distribution routers, it is also important to verify andensure that CDP is running between the Distribution andthe Access routers as illustrated in the following outputs:

Dist-1#show cdp neighbors

Spoke-1#show cdp neighbors

Finally, it is important to ensure that there are no routingprotocols enabled on the spoke router. If any routingprotocol is configured, it must be removed using the norouter [protocol] global configuration command. Thisis mandatory on the spokes when implementing ODR.You can verify configured routing protocols using theshow ip protocols summary command as illustrated inthe following output:

Spoke-1#show ip protocols summary

As previously stated, no explicit configuration isrequired on the spoke router. The only singleconfiguration command required on the hub router is therouter odr global configuration command. This isimplemented on the Distribution router as illustrated inthe following output:

Dist-1#conf tEnter configuration commands, one per line. Endwith CNTL/Z.Dist-1(config)#router odrDist-1(config-router)#exitDist-1(config)#exit

When ODR is enabled on the hub router, the followingtwo things automatically occur:

The spoke router dynamically advertises connectedprefixes to the hub router. The hub router dynamicallyadvertises a default route to the spoke router.

Referencing the first point made above, the hub routerreceives the 192.168.1.0/24 prefix from the spoke

router dynamically. This is present in the routing table asillustrated below:

Dist-1#show ip route odro 192.168.1.0/24 [160/1] via 10.1.1.2, 00:00:15,Serial0/0

Referencing the second point made above, the spokerouter receives a default route from the hub router. Thisis performed because it is assumed that the spokerouter has a single ingress and egress point, which is thehub router. Advertising a single default route to thespoke reduces the routing table size on the router,which keeps resource (e.g., CPU and memory)utilization to a minimum while allowing the spoke routeraccess to the rest of the networks. This is illustrated inthe following output:

Spoke-1#show ip route

Gateway of last resort is 10.1.1.1 to network0.0.0.0

A more detailed look at the default route received bythe spoke reveals the following:

Spoke-1#show ip route 0.0.0.0Routing entry for 0.0.0.0/0, supernetKnown via odr, distance 160, metric 1,candidate default pathLast update from 10.1.1.1 on Serial0/0, 00:00:10 agoRouting Descriptor Blocks:* 10.1.1.1, from 10.1.1.1, 00:00:10 ago, viaSerial0/0

Route metric is 1, traffic share count is 1

In order to allow the spoke routers to reach all otherrouters in the network, the received ODR routes mustbe redistributed into OSPF on the Distribution router.When redistributing on the Distribution router, thesubnet space for the links between the Distribution andthe Access routers should also be included to allow forcomplete connectivity to all devices. The routeredistribution configuration is omitted in this section forbrevity and because redistribution is a core requirementfor the ROUTE exam and as such is covered in detaillater in this guide. Assuming the correct configuration,the spoke router and the Core router now have full IPreachability between their respective subnets asillustrated in the following outputs:

Core-1#ping 192.168.1.1

Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to192.168.1.1, timeout is 2 seconds:!!!!!

Success rate is 100 percent (5/5), round-tripmin/avg/max = 4/4/4 msSpoke-1#ping 20.1.1.1

Type escape sequence to abort.Sending 5, 100-byte ICMP Echos to 20.1.1.1,timeout is 2 seconds:!!!!!Success rate is 100 percent (5/5), round-tripmin/avg/max = 1/3/4 ms

While ODR is not a routing protocol, the configurationlogic of ODR parameters, such as timers, is similar torouting protocol configuration and is performed inrouter configuration mode following the issuing of therouter odr global configuration command. ODRconfiguration options are illustrated in the followingoutput:

Dist-1(config)#router odrDist-1(config-router)#?Router configuration commands:default Set a command to its defaults

default-metric Set metric of redistributed routesdistance Define an administrative distancedistribute-list Filter networks in routing updatesexit Exit from routing protocol configuration modehelp Description of the interactive help systemmaximum-paths Forward packets over multiplepathsneighbor Specify a neighbor routernetwork Enable routing on an IP networkno Negate a command or set its defaultspassive-interface Suppress routing updates on aninterfaceredistribute Redistribute information from anotherrouting protocoltimers Adjust routing timerstraffic-share How to compute traffic share overalternate paths

NOTE: You are not required to go into anyadvanced ODR configuration in the ROUTE exam. Theoptions presented above will therefore not be describedin further detail in this guide.

CHAPTER SUMMARYThe following section is a summary of the major pointsyou should be aware of in this chapter.

Internet Protocol Routing Protocol Fundamentals

A routing protocol allows a router to dynamicallylearn how to reach other networks

Routing protocols may be used to connect internaland external networks

Different types of routing protocols use differentmeans of determining the best path

All routing protocols will differ in their scalability andperformance

Flat and Hierarchical Routing

Routing protocol algorithms operate using either a flator hierarchical routing system

A hierarchical routing system uses a layeredapproach

Hierarchical routing systems reduce the informationrouters in the area must process

A flat routing system has no hierarchy In a flat routing system, routers must typically be

connected to every other router In a flat routing system, each router essentially has the

same function The primary advantage afforded by hierarchical

routing systems is their scalability Flat routing systems are not scalable and are difficult

to troubleshoot

IP Addressing and Address Summarization

An IP address is divided into two parts: the networkaddress and the host address

The IP addressing scheme should be hierarchical A hierarchical addressing scheme provides points in

the network for summarization Summarization reduces the amount of information

that routers must process Summarization allows for faster convergence in

different areas of the internetwork

Summarization restricts the size of the area that isaffected by network changes

Administrative Distance

The administrative distance is used to determine thereliability of different routing sources

This default administrative distance value is in aninteger between 0 and 255

Routes with an administrative value of 255 areuntrusted and are not placed into the RIB

The administrative distance is a locally significantvalue that affects only the local router

The default administrative distance values are listed int he following table:

Routing Protocol Metrics

All routing protocol algorithms use route metrics fornetwork route preference selection

The lower the route metric, the more preferred theroute by the routing protocol

The lowest cost route is placed into the routing tableand is used to reach the destination

Different routing algorithms use different variables tocompute the route metric

In most cases, the metrics used by one routing

protocol are incompatible with anothers The different routing protocol metrics may be based

on one or more of the following:1. Bandwidth2. Cost3. Delay4. Load5. Path Length6. Reliability

Prefix Matching

Cisco routers use the longest prefix match rule whendetermining which route to use

Longer, or more specific routing table entries arepreferred over less specific entries

Building the Routing Table

The routing table may include specific networkentries or simply a single default route

The routing table itself does not actually forwardtraffic

The routing table is built using the following generalsteps:1. If the route entry does not currently exist in therouting table, add it to the routing table2. If the route entry is more specific than anexisting route, add it to the routing table. It shouldalso be noted that the less specific entry is still alsoretained in the routing table3. If the route entry is the same as an existing one,but is received from a more preferred routesource, replace the old entry with the new entry4. If the route entry is the same as an existing one,and is received from the same protocol:

a) Discard the new route if the metric is higherthan the existing routeb) Replace the existing route if the metric of thenew route is lowerc) If the metric for both routes is the same, useboth routes for load-balancing

Routing Protocol Classes

There are two major classes of routing protocols:Distance Vector and Link State

A one-dimensional vector is a directed quantity SPF creates a shortest-path tree to all hosts in an

area or within the backbone The three sets that are used in the SPF calculation

are:1. Unknown2. Tentative (TENT)3. PATH

A Distance Vector routing protocol uses eitherdistance or hop count as its primary metric

Distance Vector protocols are primarily based on theBellman-Ford algorithm

Distance Vector protocols periodically sendneighbors copies of their entire routing tables

All Distance Vector routing protocols share thefollowing characteristics:1. Counting To Infinity2. Split Horizon3. Poison Reverse4. Hold Down timers

Link State routing protocols are hierarchical routingprotocols that use logical network areas

Routing running Link State routing protocols create adatabase of the network topology

Link State routing protocols send either Link StateAdvertisements or Link State Packets

Link State routing protocols send incrementalupdates when changes in the network occur

BGP is not a Distance Vector routing protocol;instead it is a Path Vector routing protocol

The Objectives of Routing Protocols

Routing algorithms, while different in nature, all havethe same basic objectives and goals

All routing algorithms are designed with the followingobjectives and goals:1. Optimal Routing2. Stability3. Ease of Use4. Flexibility5. Rapid Convergence

On-Demand Routing (ODR)

CDP is a Cisco proprietary protocol that works onboth broadcast and non-broadcast media

ODR is an enhancement to CDP that advertises theconnected prefixes of stub routers

ODR also supports VLSM, which means that it canbe used in modern-day internetworks

On Demand Routing is not a routing protocol ODR is not CPU intensive and it consumes very little

bandwidth When used in internetworks, ODR only requires an

additional five bytes ODR can be redistributed into other routing

protocols

CHAPTER 2 Enhanced Interior Gateway

Routing Protocol

Enhanced Interior Gateway Routing Protocol is aproprietary Interior Gateway Protocol that wasdeveloped by Cisco. Enhanced Interior GatewayRouting Protocol (EIGRP) includes traditional DistanceVector characteristics, such as split horizon, as well ascharacteristics that are similar to those used by LinkState routing protocols, such as incremental updates.

Although EIGRP has Link State routing protocolcharacteristics, EIGRP falls under the Distance Vectorrouting protocol classification and is referred to as anadvanced Distance Vector routing protocol instead.EIGRP runs directly over IP using protocol number 88.The ROUTE exam objective covered in this chapter isas follows:

Implement an EIGRP-based solution, given anetwork design and a set of requirements.

This chapter contains the following sections:

Cisco EIGRP Overview and Fundamentals EIGRP Configuration Fundamentals EIGRP Messages EIGRP Neighbor Discovery and Maintenance Metrics, DUAL, and the Topology Table Equal Cost and Unequal Cost Load Sharing Default Routing Using EIGRP Split Horizon in EIGRP Networks EIGRP Stub Routing Securing EIGRP Protocol Messages EIGRP Route Summarization Understanding Passive Interfaces Understanding the Use of the EIGRP Router ID EIGRP Logging and Reporting

CISCO EIGRP OVERVIEW ANDFUNDAMENTALSCisco developed Enhanced IGRP to overcome some ofthe limitations of its proprietary Distance Vector routingprotocol, Interior Gateway Routing Protocol (IGRP).IGRP offered improvements over Routing Information

Protocol (RIP), such as support for an increasednumber of hops; however, IGRP still succumbed to thetraditional Distance Vector routing protocol limitations,which included the following:

Sending full periodic routing updates A hop limitation The lack of VLSM support Slow convergence The lack of loop prevention mechanisms

Unlike the traditional Distance Vector routing protocols,which send their neighbors periodic routing updates thatcontain all routing information, EIGRP sends non-periodic incremental routing updates to distributerouting information throughout the routing domain. TheEIGRP incremental updates are sent when there is achange in the network topology.

By default, RIP has a hop-count limitation of up to 15hops, which makes RIP suitable only for smallernetworks. EIGRP has a default hop-count limitation of100; however, this value can be manually adjusted by

the administrator using the metric maximum-hops router configuration command whenconfiguring EIGRP. This allows EIGRP to supportnetworks that contain hundreds of routers, making itmore scalable and better suited for larger networks.

Enhanced IGRP uses two unique Type/Length/Value(TLV) triplets to carry route entries. These TLVs arethe Internal EIGRP Route TLV and the ExternalEIGRP Route TLV, which are used for internal andexternal EIGRP routes, respectively. Both TLVsinclude an 8-bit Prefix Length field that specifies thenumber of bits used for the subnet mask of thedestination network. The information that is contained inthis field allows EIGRP to support variably subnettednetworks.

Enhanced IGRP converges much faster than thetraditional Distance Vector routing protocols. Instead ofrelying solely on timers, EIGRP uses informationcontained in its Topology Table to locate alternatepaths. EIGRP can also query neighboring routers forinformation if an alternate path is not located in the local

routers Topology Table. The EIGRP Topology Tablewill be described in detail later in this chapter.

In order to ensure that there are loop-free paths throughthe network, EIGRP uses the Diffusing UpdateAlgorithm (DUAL), which is used to track all routesadvertised by neighbors and then select the best, loop-free path to the destination network. DUAL is a coreEIGRP concept that will be described in detail later inthis chapter.

Protocol-Dependent Modules

Enhanced IGRP Protocol-Dependent Modules(PDMs) are responsible for Network Layer (Layer 3)protocol-specific requirements, such as IP, IPX, andAppleTalk. This means that if you are running IPEIGRP, IPX EIGRP, and AppleTalk EIGRP, there willbe three different EIGRP Neighbor Tables for thefollowing:

IP EIGRP neighbors IPX EIGRP neighbors

AppleTalk EIGRP neighbors

Because IPX and AppleTalk are beyond the scope ofthe ROUTE exam requirements, it should be assumedthat all references to the Neighbor Table are regardingIP neighbors.

Using the IP PDM, IP EIGRP asks DUAL to makerouting decisions. IP EIGRP is responsible forredistributing routes learned by other IP routingprotocols. This information is then stored in the EIGRPTopology Table. Both the EIGRP DUAL and theTopology Table are core components of EIGRP thatwill be described in detail later in this chapter.

EIGRP CONFIGURATIONFUNDAMENTALSEnhanced IGRP is enabled in Cisco IOS software usingthe router eigrp [ASN] global configuration command.The [ASN] designates the EIGRP autonomous systemnumber. This is a 32-bit integer between 1 and 65535.In addition to other factors, which will be described

later in this chapter, routers running EIGRP must residewithin the same autonomous system to form a neighborrelationship successfully. Following the configuration ofthe router eigrp [ASN] global configuration command,the router transitions to EIGRP router configurationmode wherein you can configure parameters pertainingto EIGRP. The configured autonomous system numbercan be verified in the output of the show ip protocolscommand as follows:

R1#show ip protocolsRouting Protocol is eigrp 150Outgoing update filter list for all interfaces is notset Incoming update filter list for all interfaces isnot set Default networks flagged in outgoingupdatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1...

In addition to the show ip protocols command, theshow ip eigrp neighbors command prints informationon all known EIGRP neighbors and their respectiveautonomous systems. This command, and its availableoptions, will be described in detail later in this chapter.On routers running multiple instances of EIGRP, theshow ip eigrp [ASN] command can be used to viewinformation pertaining only to the autonomous systemthat is specified in this command. The use of thiscommand is illustrated in the following output:

R1#show ip eigrp 150 ?

In the output above, 150 is the autonomous systemnumber. The default in Cisco IOS software is to printinformation on all EIGRP instances if an autonomoussystem is not specified with any show ip eigrpcommands.

Once in router configuration mode, the networkcommand is used to specify the network(s) for whichEIGRP routing will be enabled. When the networkcommand is used and a major Classful network isspecified, the following actions are performed on theEIGRP-enabled router:

EIGRP is enabled for networks that fall within thespecified Classful network range

The Topology Table is populated with these directlyconnected subnets

EIGRP Hello packets are sent out of the interfacesassociated with these subnets

EIGRP advertises the network(s) to EIGRPneighbors in Update messages

Based on the exchange of messages, EIGRP routesare then added to the IP routing table

For example, assume that the router has the followingLoopback interfaces configured:

Loopback 0IP Address 10.0.0.1/24 Loopback 1IP Address 10.1.1.1/24

Loopback 2IP Address 10.2.2.1/24 Loopback 3IP Address 10.3.3.1/24

If EIGRP is enabled for use and the major Classful10.0.0.0/8 network is used in conjunction with thenetwork router configuration command, all fourLoopback interfaces are enabled for EIGRP routing.This is illustrated in the following output:

R1#show ip eigrp interfacesIP-EIGRP interfaces for process 150

You can use the show ip protocols command to verifythat EIGRP is enabled for the major Classful 10.0.0.0/8network. The output of this command is illustratedbelow:

R1#show ip protocolsRouting Protocol is eigrp 150

Outgoing update filter list for all interfaces is notsetIncoming update filter list for all interfaces is notsetDefault networks flagged in outgoing updatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 150EIGRP NSF-aware route hold timer is 240sAutomatic network summarization is in effectMaximum path: 4Routing for Networks:10.0.0.0Routing Information Sources:

The EIGRP Topology Table can be viewed using theshow ip eigrp topology command. The output of this

command is illustrated below:

R1#show ip eigrp topologyIP-EIGRP Topology Table for AS(150)/ID(10.3.3.1)

10.3.3.0/24, 1 successors, FD is 128256via Connected, Loopback3P 10.2.2.0/24, 1 successors, FD is 128256via Connected, Loopback2P 10.1.1.0/24, 1 successors, FD is 128256via Connected, Loopback1P 10.0.0.0/24, 1 successors, FD is 128256via Connected, Loopback0

NOTE: The Topology Table, EIGRP Hello packets,and Update messages are described in detail later in thischapter. The focus of this section is restricted to EIGRPconfiguration implementation.

Using the network command to specify a majorClassful network allows multiple subnets that fall within

the Classful network range to be advertised at the sametime with minimal configuration. However, there may besituations where administrators may not want all of thesubnets within a Classful network to be enabled forEIGRP routing. For example, referencing the Loopbackinterfaces configured on R1 in the previous example,assume that you want EIGRP routing enabled only forthe 10.1.1.0/24 and 10.3.3.0/24 subnets, and not forthe 10.0.0.0/24 and 10.2.2.0/24 subnets. While itappears that this would be possible if one specified thenetworks (i.e., 10.1.1.0 and 10.3.3.0) when using thenetwork command, Cisco IOS software still convertsthese statements to the major Classful 10.0.0.0/8network as illustrated below:

R1(config)#router eigrp 150R1(config-router)#network 10.1.1.0R1(config-router)#network 10.3.3.0R1(config-router)#exit

Despite the configuration above, the show ip protocolscommand reveals the following:

R1#show ip protocolsR1#show ip protocolsRouting Protocol is eigrp 150Outgoing update filter list for all interfaces is notsetIncoming update filter list for all interfaces is notsetDefault networks flagged in outgoing updatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 150EIGRP NSF-aware route hold timer is 240sAutomatic network summarization is in effectMaximum path: 4Routing for Networks:10.0.0.0Routing Information Sources:

NOTE: A common misconception is that disablingthe EIGRP automatic summarization feature addressesthis issue; however, this has nothing to do with theauto-summary command. For example, assume weissued the no auto-summary command to theconfiguration used in the previous example as follows:

R1(config)#router eigrp 150R1(config-router)#network 10.1.1.0R1(config-router)#network 10.3.3.0R1(config-router)#no auto-summaryR1(config-router)#exit

The show ip protocols command still shows thatEIGRP is enabled for network 10.0.0.0/8 as illustratedbelow:

R1#show ip protocolsRouting Protocol is eigrp 150Outgoing update filter list for all interfaces is notsetIncoming update filter list for all interfaces is notset

Default networks flagged in outgoing updatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 150EIGRP NSF-aware route hold timer is 240sAutomatic network summarization is not in effectMaximum path: 4Routing for Networks:10.0.0.0Routing Information Sources:

In order to provide more granular control of thenetworks that are enabled for EIGRP routing, CiscoIOS software supports the use of wildcard masks inconjunction with the ne twork statement whenconfiguring EIGRP. The wildcard mask operates in amanner similar to the wildcard mask used in ACLs and

is independent of the subnet mask for the network.

As an example, the command network 10.1.1.00.0.0.255 would match the 10.1.1.0/24 network, the10.1.1.0/26 network, and the 10.1.1.0/30 network.Referencing the Loopback interfaces configured in theprevious output, R1 would be configured as follows toenable EIGRP routing for the 10.1.1.0/24 and10.3.3.0/24 subnets, and not for the 10.0.0.0/24 subnetor the 10.2.2.0/24 subnet:

R1(config)#router eigrp 150R1(config-router)#network 10.1.1.0 0.0.0.255R1(config-router)#network 10.3.3.0 0.0.0.255R1(config-router)#exit

This configuration can be validated using the show ipprotocols command as follows:

R1#show ip protocolsRouting Protocol is eigrp 150Outgoing update filter list for all interfaces is notset

Incoming update filter list for all interfaces is notsetDefault networks flagged in outgoing updatesDefault networks accepted from incoming updatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 150EIGRP NSF-aware route hold timer is 240sAutomatic network summarization is in effectMaximum path: 4Routing for Networks:10.1.1.0/2410.3.3.0/24Routing Information Sources:

Additionally, the show ip eigrp interfaces commandcan be used to validate that EIGRP routing has beenenabled only for Loopback 1 and Loopback 3:

R1#show ip eigrp interfacesIP-EIGRP interfaces for process 150

As illustrated in the output above, EIGRP routing isenabled only for Loopback 1 and Loopback 3 becauseof the wildcard mask configuration.

It is important to remember that the network commandcan be configured using the subnet mask, rather than thewildcard mask. When this is the case, Cisco IOSsoftware inverts the subnet mask and the command issaved using the wildcard mask. For example,referencing the same Loopback interfaces on the router,R1 could also be configured as follows:

R1(config-router)#router eigrp 150R1(config-router)#network 10.1.1.0255.255.255.0R1(config-router)#network 10.3.3.0

255.255.255.0R1(config-router)#exit

Based on this configuration, the following is entered inthe running configuration:

R1#show running-config | begin router eigrprouter eigrp 150network 10.1.1.0 0.0.0.255 network 10.3.3.0 0.0.0.255 auto-summary

If a specific address on the network is used, inconjunction with the wildcard mask, Cisco IOSsoftware performs a logical AND operation todetermine the network that will be enabled for EIGRP.For example, if the network 10.1.1.15 0.0.0.255command is issued, Cisco IOS software performs thefollowing actions:

Inverts the wildcard mask to the subnet mask valueof 255.255.255.0

Performs a logical AND operation

Adds the network 10.1.1.0 0.0.0.255 command tothe configuration

The network configuration used in this example isillustrated in the following output:

R1(config)#router eigrp 150R1(config-router)#network 10.1.1.150.0.0.255R1(config-router)#exit

Based on this, the running configuration on the routerdisplays the following:

R1#show running-config | begin router eigrp router eigrp 150network 10.1.1.0 0.0.0.255 auto-summary

If a specific address on the network is used inconjunction with the subnet mask, the router performsthe same logical AND operation and adds the networkcommand to the running configuration using thewildcard mask format. This is illustrated in the

configuration below:

R1(config)#router eigrp 150R1(config-router)#network 10.1.1.15255.255.255.0R1(config-router)#exit

Based on this configuration, the following is added tothe current configuration on the router:

R1#show running-config | begin router eigrp router eigrp 150network 10.1.1.0 0.0.0.255 auto-summary

As illustrated in the configuration above, the use ofeither the wildcard mask or the subnet mask results inthe same operation and ne two rk statementconfiguration in Cisco IOS software.

REAL WORLD IMPLEMENTATION

When configuring EIGRP in production networks, it iscommon practice to use a wildcard mask of all zeros ora subnet mask of all 1s. For example, the network10.1.1.1 0.0.0.0 a n d network 10.1.1.1255.255.255.255 commands perform the same actions.Using all zeros in the wildcard mask or all ones in thesubnet mask configures Cisco IOS software to matchan exact interface address, regardless of the subnetmask configured on the interface itself. Either one ofthese commands would match interfaces configuredwith the 10.1.1.1/8, 10.1.1.1/16, 10.1.1.1/24, and10.1.1.1/30 address, for example. The use of thesecommands is illustrated in the following output:

R1(config)#router eigrp 150R1(config-router)#network 10.0.0.1 0.0.0.0R1(config-router)#network 10.1.1.1255.255.255.255R1(config-router)#exit

The show ip protocols command verifies that the

configuration of both network statements is treated in asimilar manner on the router as illustrated below:

R1#show ip protocolsRouting Protocol is eigrp 150Outgoing update filter list for all interfaces isnot setIncoming update filter list for all interfaces isnot setDefault networks flagged in outgoing updatesDefault networks accepted from incomingupdatesEIGRP metric weight K1=1, K2=0, K3=1,K4=0, K5=0EIGRP maximum hopcount 100EIGRP maximum metric variance 1Redistributing: eigrp 150EIGRP NSF-aware route hold timer is 240sAutomatic network summarization is in effectMaximum path: 4Routing for Networks:10.0.0.1/32

10.1.1.1/32Routing Information Sources:

When a subnet mask with all ones or a wildcard maskwith all zeros is used, EIGRP is enabled for thespecified (matched) interface and the network theinterface resides on is advertised. In other words,EIGRP will not advertise the /32 address in the outputabove but, instead, the actual network based on thesubnet mask configured on the matched interface. Theuse of this configuration is independent of the subnetmask configured on the actual interface matched.

EIGRP MESSAGESThis section describes the different types of messagesused by EIGRP. However, before delving into thespecifics of the different message types, it is importantto have a solid understanding of the EIGRP packetheader, wherein these messages are contained.

EIGRP Packet Header

Although going into specifics on the EIGRP packetformats is beyond the scope of the ROUTE examrequirements, a fundamental understanding of theEIGRP packet header is important in order tounderstand completely the overall operation of theEIGRP routing protocol. Figure 2-1 below illustratesthe format of the EIGRP packet header:

Fig. 2-1. EIGRP Packet Header Fields

Within the EIGRP packet header, the 4-bit Versionfield is used to indicate the protocol version. CurrentCisco IOS images support EIGRP version 1.x. The 4-bit OPCode specifies the EIGRP packet or messagetype. The different EIGRP packet types are eachassigned a unique OPCode value, which allows them tobe differentiated from other packet types. Thesemessages will be described in detail later in this chapter.

The 24-bit Checksum field is used to run a sanity checkon the EIGRP packet. This field is based on the entireEIGRP packet, excluding the IP header. The 32-bitFlags field is used to indicate an INIT either for a newEIGRP neighbor or for the Conditional Receive (CR)for EIGRP Reliable Transport Protocol (RTP). RTPand CR will be described in detail later in this chapter.

The 32-bit Sequence field specifies the sequencenumber used by EIGRP RTP to ensure orderly deliveryof reliable packets. The 32-bit Acknowledgment field isused to acknowledge the receipt of an EIGRP reliablepacket.

The 32-bit Autonomous System Number field specifiesthe Autonomous System Number of the EIGRPdomain. Finally, the 32-bit Type/Length/Value (TLV)triplet field is used to carry route entries as well asprovide EIGRP DUAL information. EIGRP supportsseveral different types of TLVs, with the most commonbeing the following:

The Parameters TLV, which has the parameters toestablish neighbor relationships

The Sequence TLV, which is used by RTP The Next Multicast Sequence TLV, which is used by

RTP The EIGRP internal route TLV, which is used for

internal EIGRP routes The EIGRP external route TLV, which is used for

external EIGRP routes

NOTE: You are not required to go into detail on thedifferent EIGRP TLVs.

Figure 2-2 below illustrates the different fields as theyappear in a wire capture of an EIGRP packet:

Fig. 2-2. EIGRP Packet Header Capture

Within the EIGRP packet header, the 4-bit OPCodefield is used to specify the EIGRP packet type ormessage. EIGRP uses different message or packettypes, which are Hello packets, Acknowledgementpackets, Update packets, Query packets, Replypackets, and Request packets. These packet types aredescribed in detail in the following sections.

Hello Packets

Enhanced IGRP sends Hello packets once it has beenenabled on a router for a particular network. Thesemessages are used to identify neighbors and, onceidentified, serve or function as a keepalive mechanism

between neighbors. EIGRP neighbor discovery andmaintenance is described in detail later in this chapter.

Enhanced IGRP Hello packets are sent to the link localMulticast group address 224.0.0.10. Hello packets sentby EIGRP do not require an Acknowledgment to besent confirming that they were received. Because theyrequire no explicit acknowledgment, Hello packets areclassified as unreliable EIGRP packets. EIGRP Hellopackets have an OPCode of 5.

Acknowledgement Packets

An EIGRP Acknowledgment (ACK) packet is simplyan EIGRP Hello packet that contains no data.Acknowledgement packets are used by EIGRP toconfirm reliable delivery of EIGRP packets. The ACKpackets are always sent to a Unicast address, which isthe source address of the sender of the reliable packet,and not to the EIGRP Multicast group address. Inaddition, ACK packets will always contain a non-zeroacknowledgment number. The ACK packet uses thesame OPCode as the Hello packet because it is

essentially a Hello packet that contains no information.The OPCode is 5.

Update Packets

Enhanced IGRP Update packets are used to conveyreachability of destinations. In other words, Updatepackets contain EIGRP routing updates. When a newneighbor is discovered, Update packets are sent viaUnicast so the neighbor can build up its EIGRPTopology Table. In other cases, such as a link costchange, updates are sent via Multicast. It is important toknow that Update packets are always transmittedreliably and always require explicit acknowledgement.Update packets are assigned an OPCode of 1. AnEIGRP Update packet is illustrated in Figure 2-3below:

Fig. 2-3. EIGRP Update Packet

NOTE: You are not required to go into detail on theinformation contained in EIGRP packets.

Query Packets

Enhanced IGRP Query packets are Multicast and areused to request reliably routing information. EIGRPQuery packets are sent to neighbors when a route is notavailable and the router needs to ask about the status ofthe route for fast convergence. If the router that sends

out a Query does not receive a response from any of itsneighbors, it resends the Query as a Unicast packet tothe nonresponsive neighbor(s). If no response isreceived in 16 attempts, the EIGRP neighborrelationship is reset. This concept will be described infurther detail later in this chapter. EIGRP Querypackets are assigned an OPCode of 3.

Reply Packets

Enhanced IGRP Reply packets are sent in response toQuery packets. The Reply packets are used to respondreliably to a Query packet. Reply packets are Unicastto the originator of the Query. The EIGRP Replypackets are assigned an OPCode of 4.

Request Packets

Enhanced IGRP Request packets are used to getspecific information from one or more neighbors andare used in route server applications. These packettypes can be sent via either Multicast or Unicast but arealways transmitted unreliably. In other words, they do

not require an explicit acknowledgment.

NOTE: While EIGRP Hello and ACK packets havebeen described as two individual packet types, it isimportant to remember that in some texts, EIGRP Helloand ACK packets are considered the same type ofpacket. This is because, as was stated earlier in thissection, an ACK packet is simply an EIGRP Hellopacket that contains no data.

The debug eigrp packets command may be used toprint real-time debugging information on the differentEIGRP packets described in this section. Keep in mindthat this command also includes additional packets thatare not described, as they are beyond the scope of thecurrent ROUTE exam requirements. The followingoutput illustrates the use of this command:

R1#debug eigrp packets ?SIAquery EIGRP SIA-Query packetsSIAreply EIGRP SIA-Reply packetsack EIGRP ack packetshello EIGRP hello packets

ipxsap EIGRP ipxsap packetsprobe EIGRP probe packetsquery EIGRP query packetsreply EIGRP reply packetsrequest EIGRP request packetsretry EIGRP retransmissionsstub EIGRP stub packetsterse Display all EIGRP packets exceptHellosupdate EIGRP update packetsverbose Display all EIGRP packets

The show ip eigrp traffic command is used to view thenumber of EIGRP packets sent and received by thelocal router. This command is also a powerfultroubleshooting tool. For example, if the routing issending out Hello packets but is not receiving any back,this could indicate that the intended neighbor is notconfigured, or even that an ACK may be blockingEIGRP packets. These concepts are expanded on inthe TSHOOT study guide. The following output

illustrates this command:

R2#show ip eigrp trafficIP-EIGRP Traffic Statistics for AS 150Hellos sent/received: 21918/21922Updates sent/received: 10/6Queries sent/received: 1/0Replies sent/received: 0/1Acks sent/received: 6/10SIA-Queries sent/received: 0/0SIA-Replies sent/received: 0/0Hello Process ID: 178PDM Process ID: 154IP Socket queue: 0/2000/2/0(current/max/highest/drops)Eigrp input queue: 0/2000/2/0(current/max/highest/drops)

Table 2-1 summarizes the EIGRP packets described inthis section and whether they are sent unreliably orreliably:

Table 2-1. EIGRP Packet Summary

EIGRP NEIGHBOR DISCOVERYAND MAINTENANCEEnhanced IGRP may be configured to discoverneighboring routers dynamically (default) or via manualadministrator configuration. Both methods, as well asother EIGRP neighbor-related topics, will be describedin the following sections.

Dynamic Neighbor Discovery

Dynamic neighbor discovery is performed by sendingEIGRP Hello packets to the destination Multicast groupaddress 224.0.0.10. This is performed as soon as thenetwork command is used when configuring EIGRP onthe router. In addition, as stated earlier in this guide,

EIGRP packets are sent directly over IP using protocolnumber 88. Figure 2-4 below illustrates the basicEIGRP neighbor discovery and route exchangeprocess:

Fig. 2-4. EIGRP Neighbor Discovery and RouteExchange

Referencing Figure 2-4, upon initialization, the EIGRPneighbors send Hello packets to discover otherneighbors. The neighbors then exchange their full routingtables via full Updates. These Updates containinformation about all known routes. Because Update

packets are sent reliably, they must be explicitlyacknowledged by the recipient.

After the neighbors have exchanged their routinginformation, they continue to exchange Hello packets tomaintain the neighbor relationship. Additionally, theEIGRP neighbor routers will only send incrementalupdates to advise neighbors of status or routingchanges. They will no longer send full Updates toneighbor routers.

It is important to understand that simply enablingEIGRP between two or more routers does notguarantee that a neighbor relationship will beestablished. Instead, some parameters must match inorder for the routers to become neighbors. The EIGRPneighbor relationship may not establish due to any of thefollowing circumstances:

Mismatched EIGRP Authentication Parameters (ifconfigured)

Mismatched EIGRP K Values Mismatched EIGRP Autonomous System (AS)

Number Using secondary addresses for EIGRP neighbor

relationships The neighbors are not on a common subnet

While the show ip eigrp neighbors command does notdifferentiate between dynamically and staticallyconfigured neighbors, the show ip eigrp interfacesdetail command can be used to verify that therouter interface is sending out Multicast packets todiscover and maintain neighbor relationships. Theoutput of this command on a router enabled fordynamic neighbor discovery is illustrated below:

R2#show ip eigrp interfaces detailFastEthernet0/0IP-EIGRP interfaces for process 150

Hello interval is 5 secNext xmit serial

Un/reliable mcasts: 0/2 Un/reliable ucasts: 2/2Mcast exceptions: 0 CR packets: 0 ACKssuppressed: 0Retransmissions sent: 1 Out-of-sequence rcvd: 0Authentication mode is not setUse multicast

NOTE: The show ip eigrp neighbors command isdescribed in detail later in this section. When looking atthe output of the show ip eigrp interfaces detail command, keep in mind that because EIGRPuses both Multicast and Unicast packets, the commandcounters will include values for both types of packets asshown in the output above.

Static Neighbor Discovery

Unlike the dynamic EIGRP neighbor discovery process,static EIGRP neighbor relationships require manualneighbor configuration on the router. When staticEIGRP neighbors are configured, the local router usesthe Unicast neighbor address to send packets to theserouters.

Static neighbor relationships are seldom used in EIGRPnetworks. The primary reason for this is manualconfiguration of neighbors does not scale well in largenetworks. However, it is important to understand whythis option is available in Cisco IOS software and thesituations in which this feature can be utilized. A primeexample of when static neighbor configuration could beused would be in a situation where EIGRP is beingdeployed across media that does not natively supportBroadcast or Multicast packets, such as Frame Relay.

A second example would be to prevent sendingunnecessary EIGRP packets on multi-access networks,such as Ethernet, when only a few EIGRP-enabledrouters exist. In addition to basic EIGRP configuration,the neighbor command must be configured on the localrouter for all static EIGRP neighbors. EIGRP-enabledrouters will not establish an adjacency if one router isconfigured to use Unicast (static) while another usesMulticast (dynamic).

In Cisco IOS software, static EIGRP neighbors areconfigured using the neighbor

router configuration command. Keep in mind that this issimply in addition to the basic EIGRP configuration.The simple network topology that is illustrated in Figure2-5 below will be used both to demonstrate and toverify the configuration of static EIGRP neighbors:

Fig. 2-5. Configuring Static EIGRP Neighbors

Referencing the topology illustrated in Figure 2-5,router R2 is configured as follows:

R2(config)#router eigrp 150R2(config-router)#network 192.168.1.00.0.0.255R2(config-router)#neighbor 192.168.1.3FastEthernet0/0R2(config-router)#no auto-summaryR2(config-router)#exit

The configuration implemented on router R3 is asfollows:

R3(config)#router eigrp 150R3(config-router)#network 192.168.1.00.0.0.255R3(config-router)#neighbor 192.168.1.2FastEthernet0/0R3(config-router)#no auto-summaryR3(config-router)#exit

T h e show ip eigrp interfaces detail command can be used to determine whether the routerinterface is sending Multicast (dynamic) or Unicast(static) packets for neighbor discovery andmaintenance. This is illustrated in the following output:

R2#show ip eigrp interfaces detailFastEthernet0/0IP-EIGRP interfaces for process 150

Hello interval is 5 secNext xmit serial Un/reliable mcasts: 0/1 Un/reliable ucasts: 3/8Mcast exceptions: 1 CR packets: 1 ACKssuppressed: 2Retransmissions sent: 1 Out-of-sequence rcvd: 0Authentication mode is not setUse unicast

Additionally, the show ip eigrp neighbors [detail]command can be used to determine the type of EIGRPneighbor. This command is described in detail later inthis chapter.

EIGRP Hello and Hold Timers

Enhanced IGRP uses different Hello and Hold timersfor different types of media. Hello timers are used todetermine the interval rate EIGRP Hello packets aresent. The Hold timer is used to determine the time thatwill elapse before a router considers an EIGRPneighbor as down. By default, the Hold time is threetimes the Hello interval.

Enhanced IGRP sends Hello packets every 5 secondson Broadcast, point-to-point serial, pointto-pointsubinterfaces, and multipoint circuits greater than T1speed. The default Hold Time is 15 seconds. EIGRPsends Hello packets every 60 seconds on other linktypes. These include lowbandwidth WAN links lessthan T1 speed. The default Hold time for neighborrelationships across these links is also three times theHello interval and therefore defaults to 180 seconds.

Enhanced IGRP timer values do not have to be thesame on neighboring routers in order for a neighborrelationship to be established. In addition, there is nomandatory requirement that the Hold time be threetimes the Hello interval. This is only a recommendedguideline, which can be manually adjusted in Cisco IOSsoftware. The EIGRP Hello time can be adjusted usingthe ip hello-interval eigrp interfaceconfiguration command, while the EIGRP Hold timecan be adjusted using the ip hold-time eigrp interface configuration command.

It is important to understand the use of both Hello

timers and Hold timers as they pertain to EIGRP. TheHold time value is advertised in the EIGRP Hellopacket, while the H