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Static RoutingIntroduction

Routing is at the core of every data network, moving information across an internetwork from source todestination. Routers are the devices responsible for the transfer of packets from one network to thenext.

Routers learn about remote networks either dynamically, using routing protocols, or manually, or usingstatic routes. In many cases, routers use a combination of both dynamic routing protocols and staticroutes. This chapter focuses on static routing.

Static routes are very common and do not require the same amount of processing and overhead asdynamic routing protocols.

In this chapter, sample topologies will be used to configure I v! and I v" static routes and to presenttroubleshooting techniques. In the process, several important I#S commands and the resulting output

will be examined. $n introduction to the routing table using both directly connected networks and staticroutes will be included.

This chapter will also contrast classful routing and the widely implemented classless routing methods.It will cover %lassless Inter&'omain Routing (%I'R) and the variable&length subnet mask (*+S )methods. %I'R and *+S have helped conserve the I v! address space using subnetting andsummari-ation techniques.

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Static Routing

Introduction

Activity - Which Way Should We Go?

$ huge sporting event is about to take place in your city. To attend the event, you make concise plansto arrive at the sports arena on time to see the entire game.

There are two routes you can take to drive to the event

Highway route & It is easy to follow and fast driving speeds are allowed.

Alternative, direct route & /ou found this route using a city map. 'epending on conditions,such as the amount of traffic or congestion, this 0ust may be the way to get to the arena on time1

2ith a partner, discuss these options. %hoose a preferred route to arrive at the arena in time to seeevery second of the huge sporting event.

%ompare your optional preferences to network traffic, which route would you choose to deliver datacommunications for your small& to medium&si-ed business3 2ould it be the fastest, easiest route orthe alternative, direct route3 4ustify your choice.

%omplete the modeling activity .pdf and be prepared to 0ustify your answers to the class or withanother group.

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Static Routing ImplementationStatic Routing

$ router can learn about remote networks in one of two ways

Manually & Remote networks are manually entered into the route table using static routes.

Dynamically & Remote routes are automatically learned using a dynamic routing protocol.

5igure 6 provides a sample scenario of static routing. 5igure 7 provides a sample scenario of dynamicrouting using 8I9R .

$ network administrator can manually configure a static route to reach a specific network. :nlike adynamic routing protocol, static routes are not automatically updated and must be manuallyreconfigured any time the network topology changes. $ static route does not change until theadministrator manually reconfigures it.

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Static routing provides some advantages over dynamic routing, including

Static routes are not advertised over the network, resulting in better security.

Static routes use less bandwidth than dynamic routing protocols, no % : cycles are used tocalculate and communicate routes.

The path a static route uses to send data is known.

Static routing has the following disadvantages

Initial configuration and maintenance is time&consuming.

%onfiguration is error&prone, especially in large networks.

$dministrator intervention is required to maintain changing route information.

'oes not scale well with growing networks; maintenance becomes cumbersome.

Requires complete knowledge of the whole network for proper implementation.

In the figure, dynamic and static routing features are compared.

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Static routing has three primary uses

roviding ease of routing table maintenance in smaller networks that are not expected to growsignificantly.

Routing to and from stub networks. $ stub network is a network accessed by a single route,and the router has only one neighbor.

:sing a single default route to represent a path to any network that does not have a morespecific match with another route in the routing table. 'efault routes are used to send traffic toany destination beyond the next upstream router.

The figure shows an example of a stub network connection and a default route connection. 7.6".?.@ is a stub network and R6 is a stub router. Running a routing protocol between R7 and R6 isa waste of resources.

In this example, a static route can be configured on R7 to reach the R6 +$

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Static Routing Implementationype! o" Static Route!

$s shown in the figure, static routes are most often used to connect to a specific network or to providea 9ateway of +ast Resort for a stub network. They can also be used to

Reduce the number of routes advertised by summari-ing several contiguous networks as onestatic route

%reate a backup route in case a primary route link fails

The following types of I v! and I v" static routes will be discussed

Standard static route

'efault static route

Summary static route

5loating static route

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Static Routing Implementation

Types of Static Routes

Aoth I v! and I v" support the configuration of static routes. Static routes are useful when connectingto a specific remote network.

The figure shows that R7 can be configured with a static route to reach the stub network6>7.6".?.@B7!.

#ote The example is highlighting a stub network, but in fact, a static route can be used to connect toany network.

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$ default static route is a route that matches all packets. $ default route identifies the gateway Iaddress to which the router sends all I packets that it does not have a learned or static route. $default static route is simply a static route with @.@.@.@B@ as the destination I v! address. %onfiguring adefault static route creates a 9ateway of +ast Resort.

#ote $ll routes that identify a specific destination with a larger subnet mask take precedence over thedefault route.

'efault static routes are used

2hen no other routes in the routing table match the packet destination I address. In otherwords, when a more specific match does not exist. $ common use is when connecting acompanyCs edge router to the IS network.

2hen a router has only one other router to which it is connected. This condition is known as astub router.

Refer to the figure for a sample scenario of implementing default static routing.

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To reduce the number of routing table entries, multiple static routes can be summari-ed into a singlestatic route if

The destination networks are contiguous and can be summari-ed into a single networkaddress.

The multiple static routes all use the same exit interface or next&hop I address.

In the figure, R6 would require four separate static routes to reach the 6>7.7@.@.@B6" to 6>7.7?.@.@B6"networks. Instead, one summary static route can be configured and still provide connectivity to thosenetworks.

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$nother type of static route is a floating static route. 5loating static routes are static routes that areused to provide a backup path to a primary static or dynamic route, in the event of a link failure. Thefloating static route is only used when the primary route is not available.

To accomplish this, the floating static route is configured with a higher administrative distance than theprimary route. Recall that the administrative distance represents the trustworthiness of a route. Ifmultiple paths to the destination exist, the router will choose the path with the lowest administrativedistance.

5or example, assume that an administrator wants to create a floating static route as a backup to an8I9R &learned route. The floating static route must be configured with a higher administrative

distance than 8I9R . 8I9R has an administrative distance of D@. If the floating static route isconfigured with an administrative distance of DE, the dynamic route learned through 8I9R ispreferred to the floating static route. If the 8I9R &learned route is lost, the floating static route is usedin its place.

In the figure, the Aranch router typically forwards all traffic to the =F router over the private 2$< link.In this example, the routers exchange route information using 8I9R . $ floating static route, with anadministrative distance of D6 or higher, could be configured to serve as a backup route. If the private2$< link fails and the 8I9R route disappears from the routing table, the router selects the floatingstatic route as the best path to reach the =F +$

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$on"igure Static and De"ault Route!$on"igure I%v& Static Route!

Static routes are configured using the ip route global configuration command. The syntax of thecommand is

Router(config)# ip route network-address subnet-mask { ip-address | interface-type interface-number [ ip-address ]} [ distance ][ name name ] [ permanent ] [ tag tag ]

The following parameters are required to configure static routing network-address & 'estination network address of the remote network to be added to the

routing table, often this is referred to as the prefix. subnet-mask & Subnet mask, or 0ust mask, of the remote network to be added to the routing

table. The subnet mask can be modified to summari-e a group of networks.

#ne or both of the following parameters must also be used ip-address & The I address of the connecting router to use to forward the packet to the

remote destination network. %ommonly referred to as the next hop. exit-intf & The outgoing interface to use to forward the packet to the next hop.

$s shown in the figure, the command syntax commonly used is ip route network-addresssubnet-mask { ip-address | exit-intf } .

The distance parameter is used to create a floating static route by setting an administrative distancethat is higher than a dynamically learned route.

#ote The remaining parameters are not relevant for this chapter or for %%

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In this example, 5igures 6 to ? display the routing tables of R6, R7, and R?.

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%onfigure Static and 'efault Routes

%onfigure I v! Static Routes

In a next&hop static route, only the next&hop I address is specified. The output interface is derivedfrom the next hop. 5or example, in 5igure 6, three next&hop static routes are configured on R6 usingthe I address of the next hop, R7.

Aefore any packet is forwarded by a router, the routing table process must determine the exit interfaceto use to forward the packet. This is known as route resolvability. The route resolvability process willvary depending upon the type of forwarding mechanism being used by the router. %85 (%isco 8xpress5orwarding) is the default behavior on most platforms running I#S 67.@ or later.

5igure 7 details the basic packet forwarding process in the routing table for R6 without the use of %85.2hen a packet is destined for the 6D7.6"G.7.@B7! network, R6

6. +ooks for a match in the routing table and finds that it has to forward the packets to the next&hopI v! address 6>7.6".7.7, as indicated by the label 6 in the figure. 8very route that references only anext&hop I v! address and does not reference an exit interface must have the next&hop I v! addressresolved using another route in the routing table with an exit interface.

7. R6 must now determine how to reach 6>7.6".7.7; therefore, it searches a second time for a6>7.6".7.7 match. In this case, the I v! address matches the route for the directly connected network6>7.6".7.@B7! with the exit interface Serial @B@B@, as indicated by the label 7 in the figure. This lookuptells the routing table process that this packet is forwarded out of that interface.

It actually takes two routing table lookup processes to forward any packet to the 6D7.6"G.7.@B7!network. 2hen the router performs multiple lookups in the routing table before forwarding a packet, itis performing a process known as a recursive lookup. Aecause recursive lookups consume routerresources, they should be avoided when possible.

$ recursive static route is valid (that is, it is a candidate for insertion in the routing table) only when thespecified next hop resolves, either directly or indirectly, to a valid exit interface.

#ote %85 provides optimi-ed lookup for efficient packet forwarding by using two main data structuresstored in the data plane a 5IA (5orwarding Information Aase), which is a copy of the routing table andan ad0acency table that includes +ayer 7 addressing information. The information combined in both ofthese tables work together so there is no recursive lookup needed for next&hop I address lookups. Inother words, a static route using a next&hop I requires only a single lookup when %85 is enabled onthe router.

:se the Syntax %hecker in 5igures ? and ! to configure and verify next&hop static routes on R7 andR?.

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2hen configuring a static route, another option is to use the exit interface to specify the next&hopaddress. In older I#S versions, prior to %85, this method is used to avoid the recursive lookupproblem.

In 5igure 6, three directly connected static routes are configured on R6 using the exit interface. Therouting table for R6 in 5igure 7 shows that when a packet is destined for the 6D7.6"G.7.@B7! network,R6 looks for a match in the routing table, and finds that it can forward the packet out of its Serial @B@B@interface.

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(ully Speci"ied Static Route

In a fully specified static route, both the output interface and the next&hop I address are specified.This is another type of static route that is used in older I#Ss, prior to %85. This form of static route isused when the output interface is a multi&access interface and it is necessary to explicitly identify thenext hop. The next hop must be directly connected to the specified exit interface.

Suppose that the network link between R6 and R7 is an 8thernet link and that the 9igabit8thernet @B6interface of R6 is connected to that network, as shown in 5igure 6. %85 is not enabled. To eliminatethe recursive lookup, a directly connected static route can be implementing using the followingcommand

R1(config)# ip route 192.168.2.0 255.255.255.0 GigabitEthernet 0/1

=owever, this may cause unexpected or inconsistent results. The difference between an 8thernetmulti&access network and a point&to&point serial network is that a point&to&point network has only oneother device on that network, the router at the other end of the link. 2ith 8thernet networks, there maybe many different devices sharing the same multi&access network, including hosts and even multiplerouters. Ay only designating the 8thernet exit interface in the static route, the router will not havesufficient information to determine which device is the next&hop device.

R6 knows that the packet needs to be encapsulated in an 8thernet frame and sent out the9igabit8thernet @B6 interface. =owever, R6 does not know the next&hop I v! address and therefore itcannot determine the destination $% address for the 8thernet frame.

'epending upon the topology and the configurations on other routers, this static route may or may notwork. It is recommended that when the exit interface is an 8thernet network, that a fully specified staticroute is used including both the exit interface and the next&hop address.

$s shown in 5igure 7, when forwarding packets to R7, the exit interface is 9igabit8thernet @B6 and thenext&hop I v! address is 6>7.6".7.7.

#ote 2ith the use of %85, a fully specified static route is no longer necessary. $ static route using anext&hop address should be used.

:se the Syntax %hecker in 5igure ? and ! to configure and verify fully specified static routes on R7and R?.

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$long with ping and traceroute , useful commands to verify static routes include

show ip route

show ip route static

show ip route network

5igure 6 displays sample output of the show ip route static command. In the example, theoutput is filtered using the pipe and begin parameter. The output reflects the use of static routesusing the next&hop address.

5igure 7 displays sample output of the show ip route 192.168.2.1 command.

5igure ? verifies the ip route configuration in the running configuration.

:se the Syntax %hecker in 5igure ! to verify the routing settings of R7.

:se the Syntax %hecker in 5igure E to verify the routing settings of R?.

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%onfigure I v! 'efault Routes

R6 can be configured with three static routes to reach all of the remote networks in the exampletopology. =owever, R6 is a stub router because it is only connected to R7. Therefore, it would be moreefficient to configure a default static route.

The example in the figure configures a default static route on R6. 2ith the configuration shown in theexample, any packets not matching more specific route entries are forwarded to 6>7.6".7.7.

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%onfigure I v! 'efault Routes

In the figure, the show ip route static command output displays the contents of the routingtable.

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$on"igure Static and De"ault Route!$on"igure I%v) Static Route!

Static routes for I v" are configured using the ipv6 route global configuration command. 5igure 6shows the simplified version of the command syntax

Router(config)# ipv6 route ipv6-prefix/prefix-length { ipv6-address | exit-intf }

ost of parameters are identical to the I v! version of the command. I v" static routes can also beimplemented as

Standard I v" static route

'efault I v" static route

Summary I v" static route

5loating I v" static route

$s with I v!, these routes can be configured as recursive, directly connected, or fully specified.

The ipv6 unicast-routing global configuration command must be configured to enable therouter to forward I v" packets. 5igure 7 displays the enabling of I v" unicast routing.

:se the Syntax %hecker in 5igures ? and ! to enable I v" unicast routing on R7 and R?.

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In this example, 5igures 6 to ? display the routing tables of R6, R7, and R?. 8ach router has entriesonly for directly connected networks and their associated local addresses.

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%onfigure I v" Static Routes

2hen configuring a static route on point&to&point networks, an alternative to using the next&hop I v"address is to specify the exit interface. This is an alternative used in older I#Ss or whenever %85 isdisabled, to avoid the recursive lookup problem.

5or instance, in 5igure 6, three directly connected static routes are configured on R6 using the exitinterface.

The I v" routing table for R6 in 5igure 7 shows that when a packet is destined for the7@@6 'AG $%$' ? B"! network, R6 looks for a match in the routing table and finds that it can forwardthe packet out of its Serial @B@B@ interface.

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%onfigure I v" Static Routes

In a fully specified static route, both the output interface and the next&hop I v" address are specified.Similar to fully specified static routes used with I v!, this would be used if %85 were not enabled onthe router and the exit interface was on a multi&access network. 2ith %85, a static route using only anext&hop I v" address would be the preferred method even when the exit interface is a multi&accessnetwork.

:nlike I v!, there is a situation in I v" when a fully specified static route must be used. If the I v"static route uses an I v" link&local address as the next&hop address, a fully specified static routeincluding the exit interface must be used. 5igure 6 shows an example of a fully qualified I v" staticroute using an I v" link&local address as the next&hop address.

The reason a fully specified static route must be used is because I v" link&local addresses are notcontained in the I v" routing table. +ink&local addresses are only unique on a given link or network.The next&hop link&local address may be a valid address on multiple networks connected to the router.Therefore, it is necessary that the exit interface be included.

In 5igure 6, a fully specified static route is configured using R7Ks link&local address as the next&hopaddress.

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$long with ping and traceroute , useful commands to verify static routes include

show ipv6 route

show ipv6 route static

show ipv6 route network

5igure 6 displays sample output of the show ipv6 route static command. The output reflectsthe use of static routes using next&hop global unicast addresses.

5igure 7 displays sample output from the show ip route 2001: !8:"#" :$:: command.

5igure ? verifies the ipv6 route configuration in the running configuration.

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$on"igure Static and De"ault Route!$on"igure I%v) De"ault Route!

$ default route is a static route that matches all packets. Instead of routers storing routes for all of thenetworks in the Internet, they can store a single default route to represent any network that is not inthe routing table.

Routers commonly use default routes that are either configured locally or learned from another router,using a dynamic routing protocol. They are used when no other routes match the packetCs destinationI address in the routing table. In other words, if a more specific match does not exist, then use thedefault route as the 9ateway of +ast Resort.

'efault static routes are commonly used when connecting

$ companyCs edge router to a service provider network.

$ router with only an upstream neighbor router. The router has no other neighbors and is,therefore, referred to as a stub router.

$s shown in the figure, the command syntax for a default static route is similar to any other staticroute, except that the ipv"&prefixBprefix&length is ::/0 , which matches all routes.

The basic command syntax of a default static route is

ipv6 route ::/0 { ipv6-address | exit-intf }

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%onfigure I v" 'efault Routes

R6 can be configured with three static routes to reach all of the remote networks in our topology.=owever, R6 is a stub router because it is only connected to R7. Therefore, it would be more efficientto configure a default static I v" route.

The example in the figure displays a configuration for a default static I v" route on R6.

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%onfigure I v" 'efault Routes

In 5igure 6, the show ipv6 route static command output displays the contents of the routingtable.

:nlike I v!, I v" does not explicitly state that the default I v" is the 9ateway of +ast Resort.

The key to this configuration is the ::/0 mask. Recall that the ipv" prefix&length in a routing tabledetermines how many bits must match between the destination I address of the packet and the routein the routing table. The ::/0 mask indicates that none of the bits are required to match. $s long asa more specific match does not exist, the default static I v" route matches all packets.

5igure 7 displays a successful ping to the R? +$< interface.

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Review o" $IDR and *+SM$la!!"ul Addre!!ing

Released in 6DG6, R5% >D@ and R5% >D6 describe how I v! network addresses were initiallyallocated based on a classification system. In the original specification of I v!, the authors establishedthe classes to provide three different si-es of networks for large, medium, and small organi-ations. $sa result, class $, A, and % addresses were defined with a specific format for the high order bits. =ighorder bits are the far left bits in a ?7&bit address.

$s shown in the figure

$la!! A addre!!e! egin with & Intended for large organi-ations; includes all addressesfrom @.@.@.@ (@@@@@@@) to 67>.7EE.7EE.7EE (6666666). The @.@.@.@ address is reserved fordefault routing and the 67>.@.@.@ address is reserved for loopback testing.

$la!! . addre!!e! egin with / & Intended for medium&to&large organi-ations; includes all

addresses from 67G.@.@.@ ( / @@@@@@) to 6D6.7EE.7EE.7EE (/ 666666).

$la!! $ addre!!e! egin with // & Intended for small&to&medium organi-ations; includes alladdresses from 6D7.@.@.@ ( // @@@@@) to 77?.7EE.7EE.7EE (// 66666).

The remaining addresses were reserved for multicasting and future uses.

$la!! D Multica!t addre!!e! egin with /// & ulticast addresses are used to identify agroup of hosts that are part of a multicast group. This helps reduce the amount of packetprocessing that is done by hosts, particularly on broadcast media (i.e., 8thernet +$

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Review of %I'R and *+S

%lassful $ddressing

$s specified in R5% >D@, each network class has a default subnet mask associated with it.

$s shown in 5igure 6, class $ networks used the first octet to identify the network portion of theaddress. This is translated to a 7EE.@.@.@ classful subnet mask. Aecause only > bits were left in thefirst octet (remember, the first bit is always @), this made 7 to the >th power, or 67G networks. Theactual number is 67" networks, because there are two reserved class $ addresses (i.e., @.@.@.@BG and67>.@.@.@BG). 2ith 7! bits in the host portion, each class $ address had the potential for over 6" millionindividual host addresses.

$s shown in 5igure 7, class A networks used the first two octets to identify the network portion of thenetwork address. 2ith the first two bits already established as 6 and @, 6! bits remained in the first two

octets for assigning networks, which resulted in 6",?G! class A network addresses. Aecause eachclass A network address contained 6" bits in the host portion, it controlled "E,E?! addresses. (Recallthat two addresses were reserved for the network and broadcast addresses.)

$s shown in 5igure ?, class % networks used the first three octets to identify the network portion of thenetwork address. 2ith the first three bits established as 6 and 6 and @, 76 bits remained for assigningnetworks for over 7 million class % networks. Aut, each class % network only had G bits in the hostportion, or 7E! possible host addresses.

$n advantage of assigning specific default subnet masks to each class is that it made routing updatemessages smaller. %lassful routing protocols do not include the subnet mask information in theirupdates. The receiving router applies the default mask based on the value of the first octet whichidentifies the class.

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%lassful $ddressing

:sing classful I addresses meant that the subnet mask of a network address could be determined bythe value of the first octet, or more accurately, the first three bits of the address. Routing protocols,such as RI v6, only need to propagate the network address of known routes and do not need toinclude the subnet mask in the routing update. This is due to the router receiving the routing updatedetermining the subnet mask simply by examining the value of the first octet in the network address, or by applying its ingress interface mask for subnetted routes. The subnet mask was directly related tothe network address.

In 5igure 6, R6 sends an update to R7. In the example, R6 knows that subnet 6>7.6".6.@ belongs tothe same ma0or classful network as the outgoing interface. Therefore, it sends a RI update to R7containing subnet 6>7.6".6.@. 2hen R7 receives the update, it applies the receiving interface subnetmask (B7!) to the update and adds 6>7.6".6.@ to the routing table.

In 5igure 7, R7 sends an update to R?. 2hen sending updates to R?, R7 summari-es subnets6>7.6".6.@B7!, 6>7.6".7.@B7!, and 6>7.6".?.@B7! into the ma0or classful network 6>7.6".@.@. AecauseR? does not have any subnets that belong to 6>7.6".@.@, it applies the classful mask for a class Anetwork, which is B6".

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Review o" $IDR and *+SM$la!!"ul Addre!!ing

The classful addressing specified in R5%s >D@ and >D6 resulted in a tremendous waste of addressspace. In the early days of the Internet, organi-ations were assigned an entire classful networkaddress from the $, A, or % class.

$s illustrated in the figure

%lass $ had E@M of the total address space. =owever, only 67" organi-ations could beassigned a class $ network address. Ridiculously, each of these organi-ations could provideaddresses for up to 6" million hosts. *ery large organi-ations were allocated entire class $address blocks. Some companies and governmental organi-ations still have class $ addresses.5or example, 9eneral 8lectric owns ?.@.@.@BG, $pple %omputer owns 6>.@.@.@BG, and the :.S.ostal Service owns E".@.@.@BG.

%lass A had 7EM of the total address space. :p to 6",?G! organi-ations could be assigned aclass A network address and each of these networks could support up to "E,E?! hosts. #nly thelargest organi-ations and governments could ever hope to use all "E,@@@ addresses. +ike class $networks, many I addresses in the class A address space were wasted.

%lass % had 67.E M of the total address space. any more organi-ations were able to getclass % networks, but were limited in the total number of hosts that they could connect. In fact, inmany cases, class % addresses were often too small for most midsi-e organi-ations.

%lasses ' and 8 are used for multicasting and reserved addresses.

The overall result was that the classful addressing was a very wasteful addressing scheme. $ betternetwork addressing solution had to be developed. 5or this reason, %lassless Inter&'omain Routing(%I'R) was introduced in 6DD?.

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Review o" $IDR and *+SM$IDR

4ust as the Internet was growing at an exponential rate in the early 6DD@s, so were the si-e of therouting tables that were maintained by Internet routers under classful I addressing. 5or this reason,the I8T5 introduced %I'R in R5% 6E6> in 6DD?.

%I'R replaced the classful network assignments and address classes ($, A, and %) became obsolete.:sing %I'R, the network address is no longer determined by the value of the first octet. Instead, thenetwork portion of the address is determined by the subnet mask, also known as the network prefix, or prefix length (i.e., BG, B6D, etc.).

IS s are no longer limited to a BG, B6", or B7! subnet mask. They can now more efficiently allocateaddress space using any prefix length, starting with BG and larger (i.e., BG, BD, B6@, etc.). The figureshows how blocks of I addresses can be assigned to a network based on the requirements of thecustomer, ranging from a few hosts to hundreds or thousands of hosts.

%I'R also reduces the si-e of routing tables and manages the I v! address space more efficientlyusing

Route !ummari1ation & $lso known as prefix aggregation, routes are summari-ed into asingle route to help reduce the si-e of routing tables. 5or instance, one summary static route canreplace several specific static route statements.

Supernetting & #ccurs when the route summari-ation mask is a smaller value than the defaulttraditional classful mask.

#ote $ supernet is always a route summary, but a route summary is not always a supernet.

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%I'R

In the figure, notice that IS 6 has four customers, and that each customer has a variable amount of Iaddress space. The address space of the four customers can be summari-ed into one advertisementto IS 7. The 6D7.6"G.@.@B7@ summari-ed or aggregated route includes all the networks belonging to%ustomers $, A, %, and '. This type of route is known as a supernet route. $ supernet summari-esmultiple network addresses with a mask that is smaller than the classful mask.

'etermining the summary route and subnet mask for a group of networks can be done in the followingthree steps

Step /2 +ist the networks in binary format.

Step 32 %ount the number of far left matching bits. This identifies the prefix length or subnet mask forthe summari-ed route.

Step 42 %opy the matching bits and then add -ero bits to the rest of the address to determine thesummari-ed network address.

The summari-ed network address and subnet mask can now be used as the summary route for thisgroup of networks.

Summary routes can be configured by both static routes and classless routing protocols.

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%I'R

%reating smaller routing tables makes the routing table lookup process more efficient, because thereare fewer routes to search. If one static route can be used instead of multiple static routes, the si-e ofthe routing table is reduced. In many cases, a single static route can be used to represent do-ens,hundreds, or even thousands of routes.

Summary %I'R routes can be configured using static routes. This helps to reduce the si-e of routingtables.

In 5igure 6, R6 has been configured to reach the identified networks in the topology. $lthoughacceptable, it would be more efficient to configure a summary static route.

5igure 7 provides a solution using %I'R summari-ation. The six static route entries could be reducedto 6>7.6".@.@B6? entry. The example removes the six static route entries and replaces them with asummary static route.

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%I'R

%lassful routing protocols cannot send supernet routes. This is because the receiving routerautomatically applies the default classful subnet mask to the network address in the routing update. Ifthe topology in the figure contained a classful routing protocol, then R? would only install6>7.6".@.@B6" in the routing table.

ropagating *+S and supernet routes requires a classless routing protocol such as RI v7, #S 5, or 8I9R . %lassless routing protocols advertise network addresses with their associated subnet masks.2ith a classless routing protocol, R7 can summari-e networks 6>7.6".@.@B6", 6>7.6>.@.@B6",6>7.6G.@.@B6", and 6>7.6D.@.@B6", and advertise a supernet summary static route 6>7.6".@.@B6! to R?.R? then installs the supernet route 6>7.6".@.@B6! in its routing table.

#ote 2hen a supernet route is in a routing table, for example, as a static route, a classful routingprotocol does not include that route in its updates.

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*+S

2ith fixed&length subnet masking (5+S ), the same number of addresses is allocated for eachsubnet. If all the subnets have the same requirements for the number of hosts, these fixed si-eaddress blocks would be sufficient. =owever, most often that is not the case.

#ote 5+S is also referred to as traditional subnetting.

The topology shown in 5igure 6 requires that network address 6D7.6"G.7@.@B7! be subnetted intoseven subnets one subnet for each of the four +$

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*+S

In traditional subnetting the same subnet mask is applied for all the subnets. This means that eachsubnet has the same number of available host addresses.

$s illustrated in 5igure 6, traditional subnetting creates subnets of equal si-e. 8ach subnet in atraditional scheme uses the same subnet mask.

2ith *+S the subnet mask length varies depending on how many bits have been borrowed for aparticular subnet, thus the Hvariable part of variable&length subnet mask. $s shown in 5igure 7, *+Sallows a network space to be divided into unequal parts.

*+S subnetting is similar to traditional subnetting in that bits are borrowed to create subnets. The

formulas to calculate the number of hosts per subnet and the number of subnets created still apply.The difference is that subnetting is not a single pass activity. 2ith *+S , the network is first subnetted,and then the subnets are subnetted again. This process can be repeated multiple times to createsubnets of various si-es.

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Review o" $IDR and *+SM*+SM

*+S allows the use of different masks for each subnet. $fter a network address is subnetted, thosesubnets can be further subnetted. *+S is simply subnetting a subnet. *+S can be thought of assub&subnetting.

The figure shows the network 6@.@.@.@BG that has been subnetted using the subnet mask of B6", whichmakes 7E" subnets. That is 6@.@.@.@B6", [email protected].@.@B6", [email protected].@.@B6", N, [email protected].@.@B6". 5our of these B6"subnets are displayed in the figure. $ny of these B6" subnets can be subnetted further.

%lick the lay button in the figure to view the animation. In the animation

The [email protected].@.@B6" subnet is subnetted again with the B7! mask.

The [email protected].@.@B6" subnet is subnetted again with the B7! mask.

The 6@.?.@.@B6" subnet is subnetted again with the B7G mask

The 6@.!.@.@B6" subnet is subnetted again with the B7@ mask.

Individual host addresses are assigned from the addresses of Osub&subnetsO. 5or example, the figureshows the [email protected].@.@B6" subnet divided into B7! subnets. The [email protected].!.6@ address would now be amember of the more specific subnet [email protected].!.@B7!.

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*+S

$nother way to view the *+S subnets is to list each subnet and its sub&subnets.

In 5igure 6, the 6@.@.@.@BG network is the starting address space and is subnetted with a B6" mask.Aorrowing G bits (going from BG to B6") creates 7E" subnets that range from 6@.@.@.@B6" [email protected].@.@B6".

In 5igure 7, the [email protected].@.@B6" subnet is further subnetted by borrowing G more bits. This creates 7E"subnets with a B7! mask. This mask allows 7E! host addresses per subnet. The subnets ranging [email protected].@.@B7! to [email protected].@B7! are subnets of the subnet [email protected].@.@B6".

In 5igure ?, the [email protected].@.@B6" subnet is also further subnetted with a B7! mask allowing 7E! host

addresses per subnet. The subnets ranging from [email protected].@.@B7! to [email protected].@B7! are subnets of thesubnet [email protected].@.@B6".

In 5igure !, the 6@.?.@.@B6" subnet is further subnetted with a B7G mask, thus creating !,@D" subnetsand allowing 6! host addresses per subnet. The subnets ranging from 6@.?.@.@B7G to 6@.?.7EE.7!@B7Gare subnets of the subnet 6@.?.@.@B6".

In 5igure E, the 6@.!.@.@B6" subnet is further subnetted with a B7@ mask, thus creating 6" subnets andallowing !,@D! host addresses per subnet. The subnets ranging from 6@.!.@.@B7@ to 6@.!.7!@.@B7@ aresubnets of the subnet 6@.!.@.@B6". These B7@ subnets are big enough to subnet even further, allowingmore networks.

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*+S

%areful consideration must be given to the design of a network addressing scheme. 5or example, thesample topology in 5igure 6 requires seven subnets.

:sing traditional subnetting, the first seven address blocks are allocated for +$

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%onfigure Summary and 5loating Static Routes

%onfigure I v! Summary Routes

Route summari-ation, also known as route aggregation, is the process of advertising a contiguous setof addresses as a single address with a less&specific, shorter subnet mask. %I'R is a form of routesummari-ation and is synonymous with the term supernetting.

%I'R ignores the limitation of classful boundaries, and allows summari-ation with masks that aresmaller than that of the default classful mask. This type of summari-ation helps reduce the number ofentries in routing updates and lowers the number of entries in local routing tables. It also helps reducebandwidth utili-ation for routing updates and results in faster routing table lookups.

In the figure, R6 requires a summary static route to reach networks in the range of 6>7.7@.@.@B6" to6>7.7?.@.@B6".

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%onfigure I v! Summary Routes

Summari-ing networks into a single address and mask can be done in three steps

Step /2 +ist the networks in binary format. 5igure 6 lists networks 6>7.7@.@.@B6" to 6>7.7?.@.@B6" inbinary format.

Step 32 %ount the number of far&left matching bits to determine the mask for the summary route.5igure 7 highlights the 6! far left matching bits. This is the prefix, or subnet mask, for the summari-edroute B6! or 7EE.7E7.@.@.

Step 42 %opy the matching bits and then add -ero bits to determine the summari-ed network address.5igure ? shows that the matching bits with -eros at the end results in the network address 6>7.7@.@.@.

The four networks & 6>7.7@.@.@B6", 6>7.76.@.@B6", 6>7.77.@.@B6", and 6>7.7?.@.@B6" & can besummari-ed into the single network address and prefix 6>7.7@.@.@B6!.

5igure ! displays R6 configured with a summary static route to reach networks 6>7.7@.@.@B6" to6>7.7?.@.@B6".

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$on"igure Summary and (loating Static Route!$on"igure I%v& Summary Route!

ultiple static routes can be summari-ed into a single static route if


The multiple static routes all use the same exit interface or next&hop I address.

%onsider the example in 5igure 6. $ll routers have connectivity using static routes.

5igure 7 displays the static routing table entries for R?.

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$on"igure Summary and (loating Static Route!$on"igure I%v) Summary Route!

$side from the fact that I v" addresses are 67G bits long and written in hexadecimal, summari-ingI v" addresses is actually similar to the summari-ation of I v! addresses. It 0ust requires a few extrasteps due to the abbreviated I v" addresses and hex conversion.

ultiple static I v" routes can be summari-ed into a single static I v" route if


The multiple static routes all use the same exit interface or next&hop I v" address.

Refer to the network in the 5igure 6. R6 currently has four static I v" routes to reach networks7@@6 'AG $%$' 6 B"! to 7@@6 'AG $%$' ! B"!.

5igure 7 displays the I v" static routes installed in the I v" routing table.

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%onfigure I v" Summary Routes

Summari-ing I v" networks into a single I v" prefix and prefix&length can be done in seven steps asshown in 5igures 6 to >

Step /2 +ist the network addresses (prefixes) and identify the part where the addresses differ.

Step 32 8xpand the I v" if it is abbreviated.

Step 42 %onvert the differing section from hex to binary.

Step &2 %ount the number of far left matching bits to determine the prefix&length for the summaryroute. #ote The figure shows all networks converted to binary to demonstrate the matching bits; but

the summary can also be determined by converting 0ust the lowest and highest network addresses tobinary to find the matching bits.

Step 52 %opy the matching bits and then add -ero bits to determine the summari-ed network address(prefix).

Step )2 %onvert the binary section back to hex.

Step 62 $ppend the prefix of the summary route (result of Step !).

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%onfigure I v" Summary Routes

$fter the summary route is identified, replace the existing routes with the single summary route.

5igure 6 displays how the four existing routes are removed and then the new summary static I v"route is configured.

5igure 7 confirms that the summary static route is in the routing table of R6.

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$on"igure Summary and (loating Static Route!$on"igure (loating Static Route!

5loating static routes are static routes that have an administrative distance greater than theadministrative distance of another static route or dynamic routes. They are very useful when providinga backup to a primary link, as shown in the figure.

Ay default, static routes have an administrative distance of 6, making them preferable to routeslearned from dynamic routing protocols. 5or example, the administrative distances of some commondynamic routing protocols are

8I9R L D@

I9R L 6@@

#S 5 L 66@

IS&IS L 66E

RI L 67@

The administrative distance of a static route can be increased to make the route less desirable thanthat of another static route or a route learned through a dynamic routing protocol. In this way, the staticroute Hfloats and is not used when the route with the better administrative distance is active. =owever,if the preferred route is lost, the floating static route can take over, and traffic can be sent through thisalternate route.

$ floating static route can be used to provide a backup route to multiple interfaces or networks on arouter. It is also encapsulation independent, meaning it can be used to forward packets out anyinterface, regardless of encapsulation type.

$n important consideration of a floating static route is that it is affected by convergence time. $ routethat is continuously dropping and re&establishing a connection can cause the backup interface to beactivated unnecessarily.

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%onfigure 5loating Static Routes

I v! static routes are configured using the ip route global configuration command and specifyingan administrative distance. If no administrative distance is configured, the default value (6) is used.

Refer to the topology in 5igure 6. In this scenario, the preferred route from R6 is to R7. The connectionto R? should be used for backup only.

R6 is configured with a default static route pointing to R7. Aecause no administrative distance isconfigured, the default value (6) is used for this static route. R6 is also configured with a floating staticdefault pointing to R? with an administrative distance of E. This value is greater than the default valueof 6 and, therefore, this route floats and is not present in the routing table, unless the preferred routefails.

5igure 7 verifies that the default route to R7 is installed in the routing table.

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%onfigure 5loating Static Routes

Aecause the default static route on R6 to R7 has an administrative distance of 6, traffic from R6 to R?should go through R7. The output in 5igure 6 confirms that traffic between R6 and R? flows throughR7.

2hat would happen if R7 failed3 To simulate this failure both serial interfaces of R7 are shut down, asshown in 5igure 7.

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rou le!hoot Static and De"ault Route I!!ue!rou le!hoot I%v& Static and De"ault Route $on"iguration

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Troubleshoot Static and 'efault Route Issues

Troubleshoot I v! Static and 'efault Route %onfiguration

5inding a missing (or misconfigured) route is a relatively straightforward process, if the right tools areused in a methodical manner.

5or instance, in this example, the user at %6 reports that he cannot access resources on the R?+$

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Summary

Summary

Activity - Ma7e It Static8

$s the use of I v" addressing becomes more prevalent, it is important for network administrators to beable to direct network traffic between routers.

To prove that you are able to direct I v" traffic correctly and review the I v" default static routecurriculum concepts, use the topology as shown in the .pdf file provided, specifically for this activity.

2ork with a partner to write an I v" statement for each of the three scenarios. Try to write the routestatements without the assistance of completed labs, acket Tracer files, etc.

Scenario /

I v" default static route from R7 directing all data through your S@B@B@ interface to the next hopaddress on R6.

Scenario 3

I v" default static route from R? directing all data through your S@B@B6 interface to the next hopaddress on R7.

Scenario 4

I v" default static route from R7 directing all data through your S@B@B6 interface to the next hopaddress on R?.

2hen complete, get together with another group and compare your written answers. 'iscuss anydifferences found in your comparisons.

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Summary

Summary

In this chapter, you learned how I v! and I v" static routes can be used to reach remote networks.Remote networks are networks that can only be reached by forwarding the packet to another router.Static routes are easily configured. =owever, in large networks, this manual operation can becomequite cumbersome. Static routes are still used, even when a dynamic routing protocol is implemented.

Static routes can be configured with a next&hop I address, which is commonly the I address of thenext&hop router. 2hen a next&hop I address is used, the routing table process must resolve thisaddress to an exit interface. #n point&to&point serial links, it is usually more efficient to configure thestatic route with an exit interface. #n multi&access networks, such as 8thernet, both a next&hop Iaddress and an exit interface can be configured on the static route.

Static routes have a default administrative distance of 6. This administrative distance also applies tostatic routes configured with a next&hop address, as well as an exit interface.

$ static route is only entered in the routing table if the next&hop I address can be resolved through anexit interface. 2hether the static route is configured with a next&hop I address or exit interface, if theexit interface that is used to forward that packet is not in the routing table, the static route is notincluded in the routing table.

:sing %I'R, several static routes can be configured as a single summary route. This means fewerentries in the routing table and results in a faster routing table lookup process. %I'R also manages theI v! address space more efficiently.

*+S subnetting is similar to traditional subnetting in that bits are borrowed to create subnets. 2ith*+S , the network is first subnetted, and then the subnets are subnetted again. This process can berepeated multiple times to create subnets of various si-es

The ultimate summary route is a default route, configured with a @.@.@.@ network address and a @.@.@.@subnet mask for I v!, and the prefixBprefix&length B@ for I v". If there is not a more specific match inthe routing table, the routing table uses the default route to forward the packet to another router.

$ floating static route can be configured to back up a main link by manipulating its administrativevalue.

ccna2 (v.5.0) - chapter 6

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