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Review Routing Review Routing fundamentalfundamental

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Review Routing Review Routing FundamentalFundamental

• VLSM VLSM

• Route SummarizationRoute Summarization

• Static & Dynamic Routing Static & Dynamic Routing

• Routing algorithm conceptRouting algorithm concept

• RIP V2RIP V2

VLSM VLSM

• Variable Length Subnet MaskVariable Length Subnet Mask

• VLSM allows an organization to VLSM allows an organization to use use more than one subnet maskmore than one subnet mask within within the same network address space the same network address space

• VLSM implementation maximizes VLSM implementation maximizes address efficiency, and is often address efficiency, and is often referred to as subnetting a subnet referred to as subnetting a subnet

• Main reason – Main reason – addressing crisisaddressing crisis

Supporting protocols for Supporting protocols for classless routing and VLSMclassless routing and VLSM

• OSPFOSPF

• Integrated IS-ISIntegrated IS-IS

• EIGRPEIGRP

• RIP V2RIP V2

• Static Routing Static Routing

• Subnet informationSubnet information will be exchanged as will be exchanged as well as routing informationwell as routing information– 172.16.10.0 /172.16.10.0 /255.255.255.0255.255.255.0 – 10.5.2.0 /10.5.2.0 /255.255.255.0 255.255.255.0

VLSM - exampleVLSM - example

Calculating VLSMCalculating VLSM

Subnet MaskSubnet Mask

• 255.255.255.252 - /30255.255.255.252 - /30

• 255.255.255.248 - /29255.255.255.248 - /29

• 255.255.255.240 - /28255.255.255.240 - /28

• 255.255.255.224 - /27255.255.255.224 - /27

• 255.255.255.192 - /26255.255.255.192 - /26

• 255.255.255.128 - /25255.255.255.128 - /25

• 255.255.255.0 - /24255.255.255.0 - /24

• 255.255.254.0 - /23255.255.254.0 - /23

• 255.255.252.0 - /22255.255.252.0 - /22

• 255.255.248.0 - /21255.255.248.0 - /21

Waste of Space (1/2)Waste of Space (1/2)

• All one subnet and all zero subnet can be All one subnet and all zero subnet can be used to reduce the waste of spaceused to reduce the waste of space

Waste of space (2/2)Waste of space (2/2)

Sub-subnet (1/2)Sub-subnet (1/2)

Sub-subnet (2/2)Sub-subnet (2/2)

Calculating VLSM (1/6)Calculating VLSM (1/6)

Problem 1- 192.168.10.0/24Problem 1- 192.168.10.0/24

Route Aggregation (Route Route Aggregation (Route Summarization) Summarization)

• The use of classless interdomain The use of classless interdomain routing (CIDR) and VLSM prevents routing (CIDR) and VLSM prevents address waste and promotes route address waste and promotes route aggregation, or summarizationaggregation, or summarization

• Aka. Route SummarizationAka. Route Summarization

• Save routing table spaceSave routing table space

Route summarization (1/3)Route summarization (1/3)

Static Routing VS Dynamic Static Routing VS Dynamic RoutingRouting

AS / IGP and EGPAS / IGP and EGP

• An autonomous system (AS) - otherwise An autonomous system (AS) - otherwise known as a routing domain - is a collection known as a routing domain - is a collection of routers under a common administration. of routers under a common administration.

• Interior Gateway Protocols (IGP) are used Interior Gateway Protocols (IGP) are used for intra-autonomous system routing - for intra-autonomous system routing - routing inside an autonomous system.routing inside an autonomous system.

• Exterior Gateway Protocols (EGP) are used Exterior Gateway Protocols (EGP) are used for inter-autonomous system routing - for inter-autonomous system routing - routing between autonomous systems.routing between autonomous systems.

AS /IGP and EGP AS /IGP and EGP

Class of routing protocol Class of routing protocol

• Most routing algorithms can be Most routing algorithms can be classified into one of two categories: classified into one of two categories: – Distance vector Distance vector – Link-state Link-state

• The distance vectorThe distance vector routing approach routing approach determines the direction, or vector, and determines the direction, or vector, and distance to any link in an internetwork. distance to any link in an internetwork.

• The link-stateThe link-state approach recreates the approach recreates the exact topology of an entire exact topology of an entire internetwork.internetwork.

Distance Vector Routing Distance Vector Routing

• The distance vector routing algorithm The distance vector routing algorithm passes periodic copies of a routing passes periodic copies of a routing tabletable from router to router. from router to router.

• These regular updates between routers These regular updates between routers communicate topology changes. communicate topology changes.

• The distance vector routing algorithm The distance vector routing algorithm is also known as the is also known as the Bellman-Ford Bellman-Ford algorithmalgorithm. .

Distance Vector Operation Distance Vector Operation

Distance Vector Network Distance Vector Network DiscoveryDiscovery

Routing Metric ComponentRouting Metric Component

Work best situation for Work best situation for Distance VectorDistance Vector

• Distance vector protocols work best in Distance vector protocols work best in situations where:situations where:– The network is simple and flat and does The network is simple and flat and does

not require a special hierarchical design.not require a special hierarchical design.– The administrators do not have enough The administrators do not have enough

knowledge to configure and troubleshoot knowledge to configure and troubleshoot link-state protocols.link-state protocols.

– Specific types of networks, such as hub-Specific types of networks, such as hub-and-spoke networks, are being and-spoke networks, are being implemented.implemented.

– Worst-case convergence times in a Worst-case convergence times in a network are not a concern.network are not a concern.

Link State ProtocolLink State Protocol

• The link-state algorithm is also known The link-state algorithm is also known as Dijkstra's algorithm or as the as Dijkstra's algorithm or as the shortest path first (SPF) algorithm. shortest path first (SPF) algorithm.

• The link-state routing algorithm The link-state routing algorithm maintains a complex database of maintains a complex database of topology information topology information

• It also maintain full knowledge of It also maintain full knowledge of distant routers and how they distant routers and how they interconnect interconnect

Link State ConceptLink State Concept

Link state Network Link state Network discoverydiscovery

Link State ConcernLink State Concern

Work best situation for Link Work best situation for Link statestate

• Link-state protocols work best in Link-state protocols work best in situations where:situations where:– The network design is hierarchical, The network design is hierarchical,

usually occurring in large networks.usually occurring in large networks.– The administrators have a good The administrators have a good

knowledge of the implemented link-state knowledge of the implemented link-state routing protocol.routing protocol.

– Fast convergence of the network is Fast convergence of the network is crucial.crucial.

Classful routing protocols Classful routing protocols (1/3)(1/3)• Classful routing protocols Classful routing protocols do not send subnet do not send subnet

mask informationmask information in routing updates. in routing updates.

• This was at a time when network addresses This was at a time when network addresses were allocated based on classes, class A, B, were allocated based on classes, class A, B, or C. or C.

• A routing protocol did not need to include the A routing protocol did not need to include the subnet mask in the routing update because subnet mask in the routing update because the network mask could be determined based the network mask could be determined based on the on the first octet of the network addressfirst octet of the network address. .

Classful routing Classful routing protocols(2/3)protocols(2/3)• Classful routing protocols cannot be used Classful routing protocols cannot be used

when a network is subnetted using more when a network is subnetted using more than one subnet mask,than one subnet mask,– do not support variable length subnet masks do not support variable length subnet masks

(VLSM). (VLSM).

• There are other limitations to classful routing There are other limitations to classful routing protocols including their inability to support protocols including their inability to support discontiguous networks.discontiguous networks.

• Classful routing protocols include RIPv1 and Classful routing protocols include RIPv1 and IGRP.IGRP.

Classful routing Classful routing protocols(3/3)protocols(3/3)

Classless Routing Protocols Classless Routing Protocols (1/3)(1/3)

• Classless routing protocols include the subnet Classless routing protocols include the subnet mask with the network address in routing mask with the network address in routing updates. updates.

• Today's networks are no longer allocated Today's networks are no longer allocated based on classesbased on classes and the subnet mask cannot and the subnet mask cannot be determined by the value of the first octet. be determined by the value of the first octet.

• Classless routing protocols are required in Classless routing protocols are required in most networks today because of their support most networks today because of their support for VLSMfor VLSM


• In the figure, notice that the classless In the figure, notice that the classless version of the network is using both /30 version of the network is using both /30 and /27 subnet masks in the same and /27 subnet masks in the same topology. topology. – Also notice that this topology is using a Also notice that this topology is using a

discontiguous design.discontiguous design.

• Classless routing protocols are RIPv2, Classless routing protocols are RIPv2, EIGRP, OSPF, IS-IS, BGP. EIGRP, OSPF, IS-IS, BGP.

Convergence Convergence

• Convergence time is the time it takes Convergence time is the time it takes routers to share information, routers to share information, calculate best paths, and update their calculate best paths, and update their routing tables. routing tables.

• Most networks require short Most networks require short convergence times.convergence times.

• Generally, RIP and IGRP are slow to Generally, RIP and IGRP are slow to converge, whereas EIGRP and OSPF converge, whereas EIGRP and OSPF are faster to converge.are faster to converge.

Metrics (1/4)Metrics (1/4)

• A metric is a value used by routing A metric is a value used by routing protocols to assign costs to reach protocols to assign costs to reach remote networks. remote networks.

• The metric is used to determine The metric is used to determine which path is most preferable when which path is most preferable when there are multiple paths to the same there are multiple paths to the same remote networkremote network. .

Metrics (3/4)Metrics (3/4)• Metrics used in IP routing protocols include:Metrics used in IP routing protocols include:

– Hop countHop count - A simple metric that counts the - A simple metric that counts the number of routers a packet must traversenumber of routers a packet must traverse

– BandwidthBandwidth - Influences path selection by - Influences path selection by preferring the path with the highest bandwidthpreferring the path with the highest bandwidth

– LoadLoad - Considers the traffic utilization of a certain - Considers the traffic utilization of a certain linklink

– DelayDelay - Considers the time a packet takes to - Considers the time a packet takes to traverse a pathtraverse a path

– ReliabilityReliability - Assesses the probability of a link - Assesses the probability of a link failure, calculated from the interface error count failure, calculated from the interface error count or previous link failuresor previous link failures

– Cost Cost - A value determined either by the IOS or by - A value determined either by the IOS or by the network administrator to indicate preference the network administrator to indicate preference for a route. for a route.


• The metric for each routing protocol is:The metric for each routing protocol is:– RIPRIP: Hop count - Best path is chosen by the : Hop count - Best path is chosen by the

route with the lowest route with the lowest hop counthop count..– IGRP and EIGRPIGRP and EIGRP: Bandwidth, Delay, : Bandwidth, Delay,

Reliability, and Load - Best path is chosen Reliability, and Load - Best path is chosen by the route with the smallest by the route with the smallest composite composite metric valuemetric value calculated from these calculated from these multiple parameters. By default, only multiple parameters. By default, only bandwidth and delay are used. bandwidth and delay are used.

– IS-IS and OSPFIS-IS and OSPF: Cost - Best path is chosen : Cost - Best path is chosen by the route with the lowest by the route with the lowest costcost. . . .

Metric in routing tableMetric in routing table

Load BalancingLoad Balancing

• When two or more routes to the same When two or more routes to the same destination have identical metric valuesdestination have identical metric values

• The router does not choose only one route. The router does not choose only one route. • Instead, the router "load balances" between Instead, the router "load balances" between

these equal cost paths. The packets are these equal cost paths. The packets are forwarded using all equal-cost paths. forwarded using all equal-cost paths.

• Note: Load balancing can be done either per Note: Load balancing can be done either per packet or per destination. packet or per destination.

Administrative Distance (AD)Administrative Distance (AD)(1/3)(1/3)• Administrative distance (AD) defines the preference Administrative distance (AD) defines the preference

of a routing source. of a routing source. • Each routing source - including specific routing Each routing source - including specific routing

protocols, static routes, and even directly connected protocols, static routes, and even directly connected networks - is prioritized in order of most- to least-networks - is prioritized in order of most- to least-preferable using an administrative distance value. preferable using an administrative distance value.

• Administrative distance is an integer value from 0 to Administrative distance is an integer value from 0 to 255. 255. The lower the value the more preferred the The lower the value the more preferred the route source. route source.

• An administrative distance of 0 is the most preferred. An administrative distance of 0 is the most preferred. – Only a directly connected network has an administrative Only a directly connected network has an administrative

distance of 0, which cannot be changed. distance of 0, which cannot be changed.

Administrative Distance (AD) Administrative Distance (AD) (2/3)(2/3)

Administrative Distance (AD) Administrative Distance (AD) (3/3)(3/3)

RIP V2 RIP V2

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RIP V2 RIP V2

• RFC 2453 (obsoletes –RFC 1723 /1388)RFC 2453 (obsoletes –RFC 1723 /1388)

• Extension of RIP v1 (Classful routing protocol)Extension of RIP v1 (Classful routing protocol)

• Classless routing protocolClassless routing protocol– VLSM is supportedVLSM is supported

• Subnet mask included in the routing updatesSubnet mask included in the routing updates

• Next-hop addresses included in the routing Next-hop addresses included in the routing updatesupdates

• Use of multicast addresses in sending updatesUse of multicast addresses in sending updates

• Authentication option availableAuthentication option available

RIP V2 & V1RIP V2 & V1

• Use of holddown and other timers to help Use of holddown and other timers to help prevent routing loops.prevent routing loops.

• Use of split horizon or split horizon with poison Use of split horizon or split horizon with poison reverse to also help prevent routing loops.reverse to also help prevent routing loops.

• Use of triggered updates when there is a Use of triggered updates when there is a change in the topology for faster convergence.change in the topology for faster convergence.

• Maximum hop count limit of 15 hops, with the Maximum hop count limit of 15 hops, with the hop count of 16 signifying an unreachable hop count of 16 signifying an unreachable network.network.

RIP v1 LimitationRIP v1 Limitation(Discontiguous Address)(Discontiguous Address)

Addressing schemeAddressing scheme

VLSMVLSM

Private IP Private IP

ProblemsProblems• R1 cannot ping to network 172.30.100.0R1 cannot ping to network 172.30.100.0

• R3 cannot ping to network 172.30.1.0R3 cannot ping to network 172.30.1.0

• R2 can partially ping to network 172.30.1.0 and 172.30.100.0R2 can partially ping to network 172.30.1.0 and 172.30.100.0

R2 installs both paths in routing table

R2 routing tableR2 routing table

NO VLSM supportedNO VLSM supported• RIPv1 either summarizes RIPv1 either summarizes the subnets to the subnets to

the classful boundary or uses the subnet the classful boundary or uses the subnet mask of the outgoing interfacemask of the outgoing interface to to determine which subnets to advertise. determine which subnets to advertise.

No CIDR supportedNo CIDR supported

Static Routing configuration and Routing Table on R2

Because …Because …

• RIPv1 and other classful routing protocols RIPv1 and other classful routing protocols cannot support CIDR routes that are cannot support CIDR routes that are summarized routes with a smaller subnet summarized routes with a smaller subnet mask than the classful mask of the route. mask than the classful mask of the route.

• RIPv1 ignores these supernets in the routing RIPv1 ignores these supernets in the routing table and does not include them in updates table and does not include them in updates to other routers.to other routers.

• This is because the receiving router would This is because the receiving router would only be able to apply the larger classful mask only be able to apply the larger classful mask to the update and not the shorter /16 mask.to the update and not the shorter /16 mask.

RIP V2RIP V2

• RFC 1723RFC 1723

• RIPv2 is encapsulated in a UDP segment RIPv2 is encapsulated in a UDP segment using port 520 and can carry up to 25 using port 520 and can carry up to 25 routes. routes.

• 3 extensions are added. 3 extensions are added. – The subnet mask fieldThe subnet mask field– The Next Hop addressThe Next Hop address– The Route TagThe Route Tag

RIP V2 configurationRIP V2 configuration

Auto-Summary and RIP V2 Auto-Summary and RIP V2 (1)(1)


Auto-summary

Auto summary


Redistribute StaticRedistribute Static

Disabling Auto-summaryDisabling Auto-summary

RIP V2 and VLSMRIP V2 and VLSM

RIP V2 and CIDRRIP V2 and CIDR

Verifying RIPVerifying RIP

AuthenticationAuthentication

• RIPv2, EIGRP, OSPF, IS-IS, and BGP can be RIPv2, EIGRP, OSPF, IS-IS, and BGP can be configured to authenticate routing configured to authenticate routing information. information.

• This practice ensures routers will only This practice ensures routers will only accept routing information from other accept routing information from other routers that have been configured with routers that have been configured with the same password or authentication the same password or authentication information. information.

• Note: Authentication does not encrypt the Note: Authentication does not encrypt the routing table. routing table.

RIPV2 Authentication RIPV2 Authentication

•The authentication scheme for RIP version 2 will use the space of an entire RIP entry.

• If the Address Family Identifier of the first (and only the first) entry in the message is 0xFFFF, then the remainder of the entry contains the authentication.

•This means that there can be at most, 24 RIP entries in the remainder of the message.

LAB – RIP V2LAB – RIP V2

• CCNA2 – LAB7.5.1 / 7.5.2CCNA2 – LAB7.5.1 / 7.5.2

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