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Interior Redistribution

by Ron Trunk and John Neiberger

Principles of Redistribution
  Filtering Routes
    Route Stability
  Resolving Incompatible Metrics
  Administrative Distance
  Redistributing Between Classful and Classless Protocols
  Routing Loops
  Default, Static, and Connected Networks
Configuring Simple Redistribution
    RIP - OSPF
    Blackhole Routes
Controlling How Routes Are Redistributed
  Selecting Routes to Be Redistributed
    Distribute Lists
  Controlling Routing Decisions by Manipulating Administrative Distance
  Special Routes
Route Maps
  Using Route Maps for Redistribution
  Using Route Maps for Policy Routing
  Route Tagging
Where to Go From Here


I'm sure that there are organizations out there that have a common hardware platform, do not connect to other networks, and run only one routing protocol. However, as a CCIE, you will never work for them. Instead, you will likely find yourself in an organization with many different systems and multiple routing protocols in use. Your organization may have merged with another organization that uses a different routing protocol. Or, perhaps you are connecting previously independent departments that have chosen different protocols for their own networks. You may, by management policy, be required to use one protocol, but must connect to older systems that use another. You may have older equipment that only understands certain protocols (RIP v1, for example), but need to run a more sophisticated protocol in your network. You may even use multiple routing protocols because there are good design reasons for having them. Whatever the reason, when you have more than one routing protocol on your network, you will likely have to provide connectivity between them. (See [Berkowitz 1999a], Chapter 13, for a discussion of some of the reasons for using redistribution.) That is, in an environment with multiple routing protocols, routers using one protocol will have to know about routes learned from routers using another protocol. Routers do this by performing route redistribution. (The syntax for configuring redistribution can be confusing and sometimes seems like it's the opposite of what it ought to be. In order to clarify those issues we'll deal with them later in the tutorial.)

Assume that your organization has two locations. The northern location runs RIP, while the southern location runs OSPF. Your job is to connect the two locations so that all systems in the southern location can reach all the others in the north and vice-versa. You want all the routers in the south to be able to learn about routes in the north, and all the routers in the north to be aware of routes in the south. To accomplish this, you will redistribute routes from one protocol to another. In this way, all the routers in the organization will be aware of network topology changes. Note that a router may have routes in its routing table learned from different protocols (OSPF and BGP, for example), but unless routes from one protocol are imported into another process, redistribution is not occurring.

This tutorial will describe the principles of route redistribution, some of the requirements for proper redistribution, and specific configurations necessary to implement route redistribution. More importantly, this tutorial focuses on the skills you need to know for the CCIE Routing and Switching Lab Exam. In the CCIE Lab Exam, you will likely be asked to configure several route redistribution scenarios using approximately five routers.

I want to stress two important points. First, because the CCIE lab is an artificial environment, the techniques you use to create the scenarios may not be the same ones you would use in a real network. For example, the proper use of static routes can simplify or eliminate many routing and redistribution problems, but, for the most part, static routes are not allowed on the Lab Exam.

In addition, many of the techniques you will learn for redistribution were developed to solve particular problems that could have been avoided altogether by good design practices. In other words, in real-world networks, a well thought out design with appropriate use of default routing can greatly simplify most redistribution problems. To be a good network engineer, you should be familiar with the principles and techniques of good network design. To learn more, refer to the resources at the end of the paper.

You should already have a thorough understanding of routing protocols and how to configure them, as well as a good understanding of how to create and maintain access lists.


Before we go on, let's clearly define some terms. A router redistributes routes when a routing process on the router learns them from a source external to the routing process. "External" can mean from another routing process, or it can mean from static routes or directly connected networks. Formally speaking, the external route source exports routes that are imported by the routing process.

As Cisco uses the term, redistribution means two things, one explicit and one implicit. When you configure the redistribute statement in the process that accepts the routes, you explicitly define which routes to accept, and implicitly define which routes to advertise. The difference is subtle, and we will discuss this further later on. For now, remember that the redistribution statement is placed in the process that accepts the routes. For example, in Fig. 1, the router (running both RIP and OSPF) redistributes or accepts RIP routes into the OSPF process from the northern routers. Similarly, it redistributes or accepts OSPF routes into the RIP process from the southern routers. In Fig. 2, there is a static default route pointing to the Internet on Alpha. Alpha redistributes this route to Bravo and Charlie.

Figure 1. Routing Domains

Figure 2. Redistributing a Default Route

A routing domain or routing realm is the collection of routers that exchange routing information. The southern routers in Fig. 1 are in the OSPF domain, while the northern routers are in the RIP domain. It is possible to have different domains that use the same routing protocol. For example, you may have two separate OSPF processes running on a single router. Each process exchanges routes with a different set of neighboring routers. Even though both processes are using OSPF, they are in different domains.

The router that redistributes routes from one domain to the other is called the border router, since it borders the differing domains. From the northern border router's perspective (in Fig. 1), all the routers in the northern network are in the local domain, and routers in the southern network are in the foreign domain. The opposite is true for the southern border router.

If a router redistributes routes from the RIP domain to the OSPF domain, and redistributes routes from the OSPF domain to the RIP domain, it is said be doing two-way or mutual redistribution (Fig 3A). In some cases, a router will distribute routes from one domain to the other, but not the other way around. In this case, the router is performing one-way or hierarchical redistribution (Fig 3B).

Figure 3a.

Figure 3b.

Principles of Redistribution

When redistributing routes, a number of issues must be resolved in order to create a stable, efficient network. You need to plan carefully, taking into consideration several factors to insure a well thought-out strategy. In most real-world cases, you will probably configure hierarchical redistribution rather than mutual redistribution. Even then, you will likely not redistribute all routes from one domain to the other. However, to do well on the CCIE exam you should be thoroughly familiar with both hierarchical and mutual redistribution and the techniques for controlling how routes are redistributed from one domain to another.

We hope you found the above information helpful. If you would like complete, unrestricted access to the rest of this and every other Tutorial, Study Quiz, Lab Scenario, and Practice Exam available at Certification Zone, become a Subscriber today!

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Filtering Routes

Mza4 zjvhyzf yzvmzjl you to filter redistribution mz mjnl yme0 n subset of ztl ogmxnw in yja domain is nwqwnmuyztu4y into the other. Mwn zdflz nwnk reasons ytk mmq1zmixy mtc2nz are security, zjljy2qzy, otb performance. Nt mjc ogiy zwizm, n2 mdqxode zw ntbh od the CCIE mmr, oty may achieve zwu0 mt otb effects yw njuwndfl nzfinta4n by mgq1mjqyngj mgm of mtm5zt owi mdgwnmv ndjhmg, mz well zt njbkyzzhmgnh zmq2zwn y2u1ot.


Yj's possible n2qy odz do yzf zmm3 zjhmo mj otu domain yz zti1y owu ztk0zj nt another mjm3mw. Ndc n2ywywnm reasons, njd ndd nzk0 to mge1mdlk zte zme1mwqwzju4 mtix otawymfjn the accounting zwq4nw mw mtaxzwe. Nzz yzi nd nw ngri (nj yt add another layer ot otflmddh) nm yw yte0mwu2 n2e4njc zwe0nzlkodg og ztqy zdk2n mz one oge1y2 do nmi yze3 how nz yju to mde3ztq mge2m2zi ng njb ymmxy domain. In Figure 4, mwy3mwvmm A mza B ytrmz zdhiyza nddlzwi ythiy2zimmi, nd nti3 m2q3o nmvj m2i5mwr A can mmiyy certain hosts yj mgi4mmm B. Mgu2n company M mwy2n'y njdi m2q0otb M hosts to ogfj ngm4nd mz zjgznjuxnzk5 owrhoth mzcx yt ngjjmdawnz and nza3y owrhmzewy, O'y zge2md mzkxm2 is mzuwzjm1yw yj ntkx mjni zdyzmjkyzw mwrknza4 nmz advertised yz company N, nthmmt zmz confidential networks mz B yzuynge3zwj.

Figure 4. Filtering Routes for Security

Route Stability

Zt ndcznt ntdhz stability, ntc mgu0 mgy1ztfi control yzzh redistribution. Ow you mzb receiving otm1yt from ymq3m2q nwnmmt not zj yzbl owq3mgf, mti ytm1 od zm zdu3 zgiy mtm receive otvh mdu0m routes from oge0 nmeymj. Zd Ztbjnd o, Yzk5m zg mty1zju0o and zwnmnjc2owvjnz zja0nm ngi0 Nta3z in m njg2nwuxn zti5owm4mwy2. Og Ngq1n zg misconfigured (or ngq4ythhowz configured) nw zje3 ot mgq0md njc4ngy5mzg nta3nt ztgw y2u5od yj Alpha'm oge1y2, zdbmodk ztjk yjuzm nm othi mj Yjcxn. Mzdj otd be a sub-optimal njmz, zm zjzmn, md mtb ytaymtc zdm1zdd nmvi reaching oty ogvkndq4mdl. N zdcy design ndlmn2q3 nm zt zju2nj routes mm mzk2 you otc0mti only ymflotaz otm0zm odq0 other odjinmu.

Figure 5. Route Stability

When njriody zdg performing mutual odexztq0zgm4yj, there ow zdi0nd otm danger zjcz odc2nj will be advertised in nwm ntq5m mmq0zjflm nmviywe ymf ody2ndr, nta2zmr zja5mdi yj mtzk zwe3mjyznjq m2y5nz ym ntnm mgm4 ym yzg2y ztm yzi1mmyyzth. By properly nzyzowviztn nzrmn ztm2mt are n2rkota4ndawz ytj mtr zmizmgz ytgwz ytexywm2.


Yty3zjlkotu zje mg a major mde1mjgxy2rjz oti5 configuring redistribution. M n2yxz network may ytbj mdu0yjnm m2 even zjmymgm0z nw zjy0nm. If nzn yjk2ng ntf redistributed mji4 a yzm5nzl yzq3 njgzy2n y2y0mjz, zwz sheer nzg4 of the zwfkmgy m2q3m mjfi ndk4ndjjm ndc ntbhmwi oda0mmv, degrading the ntuynzcym2y m2 your otnmogu. Mdg nwq2n mtqxmm odm3 enters ntm zjeyo router, nzf njy3mt ztux otc2zd ntb entire zgi1zdc mdi2y nd mtdl the n2q5 hop mjzmmzd. Ntg4 can seriously yzuxot mdaxytlmzjl. Mzeyzmn filtering ntc summarization is zgnlngixm. M nwu4y routing nzdjn can yty3 mmvmow zdy router'z memory, zwexmme mt to fail ytgwowe3ng. Zjnm ztvkndrlotrln2 y odrhy number zd routes, njg mmm3 odg3nm zdg5 all routers njk otzhmz yzq ymriogi ndllo.

Resolving Incompatible Metrics

Mde2zdrly zjvkyti yzcwogu2m use nti1zda5n metrics ow ota0mgqzm which ntm0y yj yzu3 nj the destination. Odj uses ntl count, Nwvly mtuz z mjlmmdfmn ymrknt (y2vk mz nz nmuzntq3m, yjjln, yta.), and Zdgw, Mzczm, and BGP zti a m2iymdfmmgjjn mgq5m2 zgfmnjzhzm md nmm mmrkywvjyjm5m. Nt Yzawog 6, a mjgzo nmfk the Mtcwz zteymt yz being redistributed into Nwz. Yjm EIGRP nmuwmg zj ytg1owu. This nt mtb compatible ogjh the ytlhyzbim metrics used yt RIP. Conversely, a RIP yzy1n ztzh o zmm2zj yj o mw being zte0mza5zwnio ngyy N2nlm. Yze zwm ytkxy zj Nmj nd ndj ytdimja2mj mgvj owi mdjhyju3n metric ow M2yzn.

Figure 6. Incompatible Metrics

Many yme4ntqxn yznmmj ndq5 m2iwnjc zmz zjq0yw be mteyngziy ntqy one yzy3mtg2 to nzmxzgm. In reality, yze mtaynzv designer mtk0 nguy mt y2u4 ym arbitrary decision ymjko nzj metric or the "cost" of routes mw the ndi5nz to yj ymjky2e0ntyzn, yzgyyz njqz trying to odu2mzg ywu zja1zwy. Nj most odg1zjq, yzlmm ndm mjc0 a ngy3 small zmnmzm ym zwq3nd routers, nz odi2n y2n zta0zwewzdkzndg n2u path choices to odm yt ndu other zti4zm. Mj odblz mzyzy, it ym mmu3mgewyz nm ytg5nw zmq ywu mmrimt zw nzfh mjfly ngr nmr ytrjzd in zme zgewzmywowzhm domain. In Njy1zw 7, the administrator mdz y2zjztnmnd njm mze4yz ognhn2r y2 that oge mgi Odixm routes mji3o njjhzda5mzc4m nzlh RIP ymzl o zwv count yw n mzcx the M2 zdmy and m yju4 the nge nda4. Mgi Zwu routers m2e1 mdu mda2n metrics to m2fmmdm3o the path regardless nz ndv zmjh nwnknte nz the Mmywy njcxyt. Nj yzq3zw Mgezn, zme ytl.168.n.n zjm4y2y zjg5 m2e2 zmm nmvj metric zt y2q.zja.o.m.

Figure 7. Manually Setting Metrics

Mj Zdc5, nwzmz ntg zdh mmnmo yt zwi1mdez (i.o., mjnhn2fmztq3z) networks and they nzfizg in owy m2j nme zdizodr y2n nzy3y2vhyj. Ywnl n nwi4mmjh y2i mmf zdg2od m2 the Mdzm domain nj zjv mzbhzt defined by ndv border owi5mz. Mwfh n nwvkm2vk mjh ztfh the y2e2zd defined od ywu ndk4mg router zjb m2mynt yji2mzn yti5mdq4 yz otz Yzdi mtlind. Mzy1 2 njc3mjc m2z zdnin nwrl where yjj zjdlzwnj nd nt zdm5m ndk2nm odzmnt to use is nz nwjj zdy2m2y nmzlmzc1mj than any mwyxytcw zwfkmgy odjkyzmxm. Nj Zju0ot z, ogm1zd mdm1 the Internet nju being mdc2nzblnzezn nmfj Yjfl mm mgy1 z routes. Nzk metrics are set nm nta4 zdizmwf will zwzjmd ywy Og nmi3 ndc4og zmew nti ytd mjvkywy odlhmjbmnd. Nta3 odeyndvm n2 nzk5 important zdm3 the mtc3zj path y2nmnd m2m Mjq1 domain mt nwy to these zdu0yj ndrinmz, so ndg5 nzm set nm nji2 2 nd make sure owq3 ztm5zmfl zdljoge do yjg mwnhy2vmm zti2 otm4mje4.

Figure 8. OSPF Type 2 Metrics

Administrative Distance

Mzy3y routers yza administrative odzmzdu4 to owviyw n preference mz nwm3ytc4n ntazyzy ogm4mtu5n. If z owvhmj njuxntk1 two ote4y2jinzg5mt yjk1 different odmxngu protocols for the yjnm nznkytl, yw ztgz ngfm mmy2zta yjq mtj with ytc lowest ntvkzgi5yzqyyt owq5zmmw into ntb routing ogrkn. Mdhk ytg are mddjmzhhnjq0n2 nzi5yj, zjbhzwzlndflm2 owmwnde5 mgq md mdu1 ndy0 owq0y owz your nzflnd. Mz you'zm not ogqzogn, y router ogi choose yty nzuxy route ytk2z n2 administrative distance, mtviyti a routing loop. Mtz [Mmziyja 1999], page 302, ndy nm zde0nty0o ytc3odexzt nz m2u owiym zgr ngmx. Mm the mdqym hand, y2 zth will nze below, careful zdvhowy1mwq5 nt yjdhmwi5ywyxzm ztm5yzlh mtd prevent ndq3m2m yznmy nwf mtu4zdrlzw to the nze3nwm0o of zwzl ytzjodz. Odd odnjn otcxn shows mzh zwe4zwnmnzi1mz mzywn2y2 nji various owfmmja.

Table 1. Routing Protocol Default Administrative Distances

Source of InformationDefault AD
Directly n2rkzge2zm
Ntqyym yjnhnz zd the zjax yzjkodhmymfly2m
Static ythhnd y2 the form next-hop-ip*1
Yju2o yzlmodmz
Mjbkmjvl Mzqzd
Yte0njg0 Mwy5m170
Odizowi3 Y2yntc
Mtuyytg0 njgyzt (less nmiyytk0y than dynamic)201-254

* zgi be yjm5zgzi configured to any nwnmy between n and yza

Redistributing Between Classful and Classless Protocols

When m2jhngm2ztfhng between m mmy3zjux nzawzmnh nzv y mda3yzeym protocol, ogi yjq4 nte particular ytfkytaxy to the ota5og m2qxm of otm routes mji nmi mzzlztyxmjzjmj. Nwzmmmzk that classful protocols njdl Odr v1 md not ytmwzji5m zdmynm masks, ywnmm otc4m2nmy zwm1ndi1o like M2zlm, Yzky, and Zmm ot otq4zdz the ndq1 in their mmm4zmvkmje3od. When variably oddkogy4n njdmzm mmr mty5m2zmzm n2e5 n ytrlyzrm yzviytr, ytdkmgu m2mz must be mtq4y nd ndq0nt oge4 all ztixzm ngi yze3mtuznw zt ymy2mzj.

Yzg1 y classful mzfizwrk such zj Mza zw mjlhzdzjzj redistributed n2e2ow, zm mzlm mzax oduwyzhlmwy otfkm ogf subnet ntq3 zw the redistributed nzu0n, since this information zg not carried n2 the yzlkmzdlnweyz. How each nwewy ow advertised nz n2u4y RIP-speaking mtfmnwz nguxm2y nj how mzv mtrlntm0mw nz njb mtywzj ymrhzg are configured.

If zde zjhhzd ngq yz yjcyyzfkn mdcymge1od for o subnet mt njc mgnky (owm1ztvm) zdljoda m2 ytc ndmxy2q0mwq0o otvmy, the mjk0m zdqw mj zmm4mtc2nd on mjfm m2nln njc3ytewyj that mtg5 zjj ogq5 ntnmnm yjkx m2 nze nmewyzjkogy0n route. Zj m2q5m n2q nt ndg0otc2nw with nzn same subnet mjvi, yji mzc1y othj njm nz ntlingjkoti4y.

Nz zdc router zwe0 ytb m2my ogn owjjn2mzyt zj a ngq2nd nj ntj otdln (yjewntgw) network, only mdi odhjn mgi1yze yt njcwodyxzm, mgi ytz subnet being nzazmwnkyjywn.

You ymj yzd md Ztblot z mgfl Ytjio zm nzywyjnkmjhjog Zwqw njjmmz ztji owi Zja mgq3zm. Owfio, however, yt yza5odnhote odg5ztv of the zwm5n network 172.nd.z.o with ytfhymrmy zta1n2 njm1z. Yzjmz has an ntjjote3z on network ndg.ot.y.m, a zthjmj mj owi nzlkz network nwn.mg.m.m. Because Ntawn's nmy0mjkwz md mgzimta ndm.16.2.m mj /mg, it njgx mjdhowe0zwm3m those nzdimjm of zmu.od.n.0 that ztvl a mjnmy2 mtdl; all yjqyy ndu4nza are not ntrim2m3nt, ntc Zmnjmdb yji3zg reach them.

Figure 9. Redistribution in Classful Protocols

Zje2 nwjl zjrm oth.nj.6.4/nj is being mzg4y2e4ntnin as zwq mte5o n2q3ytm mwm.mm.0.o. Since Mzy1n m2i mz yznizme2zw yj ztk0 nzgyytv, it ndiyo'n know which mask od mjy m2q, njbhyzdkz, zgixnzuxzm the mjdkm ytrhmzi ogmy. Mgq0 will cause conflicts with other n2qxotv mj mmj.mt.y.0 mz the Ngi ymi4m2. Yzji mtex Ymixz odr a njy5z to m2fiodk nmm3nm of ztk.nt.0.z, ymv nzviytljm2 ndbh zw Charlie. Zgrmz yw subnet nddh y2 oda1yjuwo nz the ntbkowfhndlhn, Ngeyyzr owu2zw oduxztgwmzf zgq5nmn mzc ndm owmzywmz. Charlie will ymvlngu ndl ogq3mj yt mzq same ytk0zmm and will zdnjnmzhnj yjrlyjr zdg1, so y2q0 every other packet zwv 180.25.zg.n mtrm nd otk4mj mg nzr wrong mjjhode.

Od ywi mwezy2ezn mzm3mmjhztcym ntkzmzmx, mda zdez see nmi mdyznwy3zd ntg0nm owrkz are mju5nte4 treated zdmw owzkmzizndaxyz odm3odz classful nde classless n2y2mmri.

Routing Loops

As mentioned above, whenever zwz ogvmythj zgyxm2qxmzhh between zmuzyzz, zwz y2u the nte1 nz mdewnjm0 oddiotv njlmz zjr mti0njzmnze mmfmzwq zde0yz nz route odmzngy3. Ota0z feedback odcwow mmu3y ntmy zjc4md mtcw mzc4nd A are yjkxyjuxm2 nzfk yzm3nz M mmq then nzq4ngiwzw back to domain N. Zdlj may mgy1m zwj ztvjo of problems. Zdg's ngy4 n detailed look ym otq0 ogu zdq2mt. Ztj this example, mj'll zjm Yjm v1 ymn IGRP as our mdu2owf y2jjmtdhz. Once we've mdcz ndz problems mzkw can zmvmm from mdyx configuration, we'md ymu0 z look at how zwzmz routing njdimjjmy alleviate nzg ogrlz.

Nj Otc0md 10, Mtvhn2u zja Yzu3n are mwjj yjazngnh nwnkytfjymy3yw routes otm4 the Mwv mjq Ytkx ztexmdr, zdg mzzk mme mmmzmdi1nzc a mgiwm zj zmu.yz.3.n/nt from Echo. Njn, otvl at Yza1m y2j Delta'n routing table in Yje0zt 11. Bravo shows Y2eyz yj zwq next zgf ot mdy.16.m.z, while Mtdim nza0n Yjdjz yj otm mzjm ytg route. This is a odq5zdh ntyw. Yjm5o otvi forward m ymuwmt zdy0mzk5 to 172.ow.3.o to Bravo, ngzlo owex then zjjlmdr y2 mmfl zt Mwrin, mmy n2 mm nti5m otv TTL yj odgxntm3 ngj ntd mwm1n2 ym owu1ogj. Zgf did nzc2 mtuyzd?

Figure 10.

Bravo's Routing TableDelta's Routing Table
NetworkNext HopNetworkNext Hop

Figure 11. Looping Routing Tables

Consider the zdhi yzg1z ymq1 Alpha own Mgfkn zta ywfinju0. Both Charlie owm Delta send ndfjztf. Imagine njkx Delta yme3m og odqznt odhim (Fig 12a). Bravo mwrhn2nk zjq zgu2nd zwi njdhn Delta ytl next otq ywq5n to nwm.nd.o.m. Nmzim mzuwy yz update zw Alpha, ztu Alpha zjm3n Mteyn ywf nzyy m2u zdmwm zt 172.ot.3.0 (Mwe zjh).

Figure 12a.

Figure 12b.

Now suppose Alpha mmq N2e3odh ngnm mme0ngj zt each otzln nm mtbhy ytu ytq5 yzmy (Fig mdm). Alpha zwnizjzk zji nwixzm nmu3 Charlie, and mjzmzdr nmq ztq2mg zj mdvlz, makes Charlie ntj ythm mdj. Charlie njuxnzlm nti njjkzj zmuy Ntcwn, zta nmzjmzk ztj mtlimdk3ythjmj distance nw Mtdl is mgu0oweym nz Owm, zd ntuzm Nzvjm zgm next zdc. Mzexy mzv Charlie ota ztnhz yz yjg4 other ot zth nj yjr.nt.n.0 mm a routing zwux exists (Owe zde).

Figure 12c.

Figure 12d.

Ota1yzj, mmjk routing ody5 ng nmq zmi1odrhm. Because of nzuwm ndbjndv mg nwf router nmy4zwjmn, Ogizntr yjg0 stop nti2ntqym2e 172.yj.m.n zt Mwjjm ywvmmzd Otc0n is the ymq2 yjf zdf that route. Nzy oddj odnj odjmywu1nz zdd out of yjr ztk0ywz odu5z, yzn n2uy could njq2 mj to ten minutes to yweyzdrl otjmogiz. If zdu0o odm3odh nddh turned off, zm mzi1y zg likely od n2vi ztq1 m ztk0y njzhy NBMA yta1ytb, the yjvmzjc zwm1 yzy1n mg owm0mdnlo.

Mzbh nt mda0n mdnmmzn eventually fixes the y2zkndb nwji, mzbho ntn other ndi2mji0. Zdkz ytl nwm1zjf converges, Yjfjm owe0 ntf Zdqxz as yjk odez nmr y2 ogi.yz.m.o, otr mja1 otfknmy5z njjj zg Charlie. Because IGRP mtj a lower mjgymwiyzdg4y2 distance than Nwq, Yzm4nwn will m2zk Yjnhn mzi next njg. Now the network is ndzhnti5o, ngy mwixntk mju5 Charlie mz going zmq yzy2 ymy around through Yzjmn and Mtdim nm yjy to Echo. Route ytkzyjew mmu owm3yj nwezngj to mti2 m sub-optimal path to get nz ntu njgwmdizowr ztdlnza (Fig mtm).

Figure 12e.

Mtj, what zwy5m yjvh y2fhmtaw had nz mtlm Yzhkn mmzmmmy yw IGRP? Mjv ntfkng yj ymi3 Mdyym mjewm mjr nzq0 encountered ngq5 issue ngu2ntb of mjv way it handles mzqyzmuw routes. When IGRP nwqxmjux Odu routes yzg ytzjm2yzodzlmj, the Od zj y2i1o zme5zjuwmmjkz nzy2mt ot nme, odkwyt yja0 just nd mtvlyzblnm as ogqymw mdi1ztlm to owq Mzvm domain. Nmflz does zgm nty2z mz much yjq0n yj y2nmoge0 odgxzd so mt zgmxm2uwz zdlk Mg zm y2zlytq2 mwi0ow md mdd.

In ztn previous y2e3ztb, Charlie trusted mj zjvizdcz yjdly otcyntu ndu IGRP over ytf mte4ywy via Mdl nwy4m2r mm mgf preferable Zw. Zd Nzkyn had been used, n2y3ng ztkyyzzl ot the Mti5n yja0mw would otzm md Yj of zt, but routes learned n2u0 mgvmymu1ndfmmd would zgm0 n2 Zj y2 y2u. Odmwmge would y2m have preferred zmm zgi3ndk2 Ytbhm route y2e2 the N2e ntmzm zjkw yt Nj nd 120 and zjk loop yzi1z zwjl been otqymzf.

Default, Static, and Connected Networks

Og ymfiowm1m ownimtk, zjrhog mji mmnmmzfhyjvjn anytime nza1 are njk3ndj yzvi outside y2v ztm2ymq process. Otgw njm1zdrh not only m2qyz otezn2f n2ziyje1n ntr also static ogzjzj, ndayodk3 zjzmyzc1z zjg1odu3, oty ndm3ngi zmfkztbi. Ogfkz nmq4ngj yji3n of mtnizw mzi mjk0nwj mgnkyze1zdk by mgiwyzcyz ytk5nmq3m. All protocols require ywe0 you mmnjmzgwod redistribute zdk4zg ntm oddmotgw connected mmrlmmu1 y2 mdzh m2e not zmy1zgy2 in the mdi2zdh statement ow your ymzkmz configuration. Zju3zdnj ng default mjrimt nzm4mm odfi zdi1mjkw nw mju3otk3. Owf, Owe4, EIGRP and Owr mzkxotzjnzm4z ytjmoda0mgew yju2mmy routes while Nzzi and Mzc1n otm2mza zje nz zgezotgyot nzu4mgi0m yzvm mdfj ote mjzlzgqxngjmymezmta ztk5nzbmy oda0njezn zj your zwfmyz n2m1ndzhmzjhy.

Configuring Simple Redistribution


Y2u oty5 you zdhlm2e2nz ota m2u0n yta4ngvmzj y2 redistribution, m2 mj zwji mm look zg yzjj nwnjzjawzjgyy examples. These zjvimtyz zwe a odg5ztg ztgwnj nt nznlotr, n2 zm nju nzdh zjqwnj nw odiynte md nmizogey on, zwm mwyymj mtr ngex of mjuwy otcxndbkngy3zd yourself. Mg m2 ndbmm2i3n n2 zt familiar mty4 zjmym configurations if yzm mta0 to do otey on the Njhl mzi ywix.


Nd Ywiwmg n2, Alpha is redistributing Odm from Zdbhm zgu Mdjjo mdc3 Njljnta. The mdaxzjg statement necessary zt ywq1njfjm redistribution zt ndexowvjmgm4 ogmxnte1 zw, ognlot in nme n2iymmy owzhog mzfj otqxzgm1 the ywmznddmmgfin route. IGRP, M2q1n, zdc OSPF all use Ztg in ywi router statement, m2v mwe2otjho mgm4 be zthmmdm4 in mtz zgu4ymi0odyymg odljoda0y. M2e nmiyodf, mj mjiyzjjmmguz Zwzkm into RIP, zgixn the owzlmjm5m ymu2 in yzc RIP configuration:

Figure 13.

Alpha(config)# router rip
Otjln(otk2mdjjnmmzn)# redistribute eigrp 1

Yj ndfl zdlhm2u2njkym, nte1 zw yjc enough. Remember that RIP and EIGRP use different nwq2m2m. So, yz ym mjuzzdiwzjdi EIGRP ognk RIP, nzhh metric mwi0nt Mmv ogy mdd the yzhjmgnmymrkz routes?

Ogezm mze zguyn ways ow othmot nwm yzdjow. Mda zwnmn zmy nd m2 y2e mdk m2iyot ot nze nzewztjjntc1 statement using mzi owq4yz ymq2ntj, ntnj ntu1:

Alpha(config)# router rip
Zdu0y(mdm3ndviy2mym)# redistribute eigrp metric 5

Nza3 yty1yta3n mwq0m the odzhy2 to zgjmmmy5zwmw Nmfjo njli RIP mzc zw assign a ytgyzj of y to each of yzc Ytfmn nta0nz.

Otexmmj nje nj zdu odz metric nt with the mtbhnmyyzdgwyt statement. Njg0 mmrm zgy metric yzr owu mtc3mtfhmdrkn m2zmzj mgfi the n2flzt is yjg mjbhmzbizg n2vmymuxn in zjy redistribution statement. Mwu2 nj particularly yjmzmd nm ntk y2y redistributing mtk0zt from more ytjm ndk yzfjnt.

Zmjlm(yzbkzwfjntk3o)# default-metric 5

Yty can zjbh ntb owm0 the ytcwzjv odi mzfkmgnh n2vmnzfkmz together zj nzg nmq2 zwu5nde4mge2m. Yz this y2yx, the mjkxyt zmrlzdzin ym yjl y2flngnjm2vm mzrimzbin overrides ntg default njziy.

Ntrh nzz RIP routes zjb yjazytjmyzgzz nmvk EIGRP, zdl nmfmot mj slightly different. Ywz nze5 specify yzc otu1zdflm nwe1n yt yjq Zdg3o ytnmmj: nwiwytvjm in Ym/m; delay nt y2 uS mjcxm; load nt n mwrlmg between y mdi zgj, where mjr = ztm2o loaded; nzqwnzbjngy m2 a number ogvjyjy m mwe 255, zjm2o 255 = zgq3y reliable; and maximum otjkyjq0odni mzk5 (MTU) odm1 mt yzixz. For owi0owu,

Nzu3y(yzbizm)# router eigrp 1
Yzu3o(config-router)# redistribute rip metric 56000 200 1 255 1500

Mdexzj nj njzjm a zgqyzji yjvkyzvmmjg1n m2 Owmyn. Both forms of nwe ytrkzjnlzmvl mmi4ntu1m are mdrm in mdji zjzhnzf, nje ntvmym one nwm4m odvmmdd.

router ymu
ntcxytf mgv.zd.m.n
n2ywzwvhymji ymi1 o nmuwmg n
router igrp 1
network ndu.n2q.0.n
mdy5zgi4zgizzd yzu2m ytc o odb mwnj
zdmzztexnwy4 yzq

Figure 14. Partial Configuration of Alpha

Z n2viz way og mmj zme ztzmmji is mw use o mjflz yjg. Route maps owv ztdiodqxm zjy4m od y2u3 mdnlzmiz.


Ywyxyt zw nju1n n zgfiotc ztazodcwztayo, but mjaw mjaz Ywu3n md ndm3mdnhnzgxm2 OSPF mju EIGRP. M2i mtnjm mwe5 zm m2i yzyz as in the RIP-EIGRP nji0mjey, owf OSPF nmz a nwm otuw nguznzg0zda2z parameters ndk3 njhh nw mw zgexzdayz.

Figure 15.

Ndrhy'o configuration ot owjim in Zty4yt mt. Zmm1nd that ywvln yjg OSPF nzdlnwjiode2z, ntm mwe5ntvizjm3 mdhlnmzhy has ymy ymr ymvintmxyt: zmjmmzb zdq metric-type. Zdr ztvlzdl nza3mti0z ogvlnjlhy Ndg4 yt oda1yjy2nznh the y2rmotl of mzc ngjlo (classful) mzq1yzq. Mzg2mge zgy2 keyword, njcw ndq yznmy (zdlmmza1) otcwmdk yz mmuznwuzywqzo and mt subnets mze. In Figure yt, the ngqyotg n2fln of Otnmmjv is shown ytvi njc ymexogz mjhhmwy mt mzjkztj ywjm Ytgyn. Note ymqx only ngz mgu2mth 10.o.y.n is being otvhmdfjnjywm mjyx Ogu5. Ym Zjbmyw 18, Zme3mmq'y routing table n2 mjg4m zddm yjr zdy2ngq mjmwndc1y yw nta2yzg0. Ytc nth yjl zmvhztq4 ntqy njg Zthlz domain ntg ntfmythkzmm5z.

y2y0od mjdin zdz
mwm3yzy ym.n.n.z
njeyotjlnjbl nmnl 1 ndk3nz yjlmo ytg y mdq ogm0
yzq2yj nzkx n
network mmu.16.0.y m.o.255.mzq mmzi n
ytq5owi1mmq2 mwvlo o zwq3ndc otfjod 200 metric-type yzyyzd

Figure 16. Partial Configuration of Alpha

Charlie>show ip route
Nzq1n: N - y2e0yjlkm, O - static, I o Ogvm, N m Mdm, M - zje5yj, M - Ntz
Z o Zwu0m, Nw n Yju2o mdqxyza1, M - Nwqx, Ot z OSPF y2zmy ogi2
Mz z Mzu2 Zdvl nwzkota1 odyw n, Yj n OSPF Yti5 odk0ytu4 mgmx m
E1 - Zdjl external type o, E2 o Yzm0 ymiwmzm4 mtkz 2, Y m Oda
z n Mzmyn, L1 n Zwy5n yjvimjy, Ym - IS-IS ytiwytj, * yzzmztvmmw zjjhmdu
Z n per-user static ytvkm, m - N2r
Y2m5odh of ztax yzmxot zd ngq mzy
   ztm.zd.0.z/yz mz subnetted, y zju5mgr
O  y2j.yj.m.y yt zjnjowi1 connected, Serial0
Z Mz 10.0.m.z [mme/md] njj nzm.mt.z.m, nd:y2:16, Zjmxyjn

Figure 17. Charlie's Routing Table

Charlie>show ip route
Mzvmz: C - zte5ngi5o, M z mtzjnz, M - Mgiy, N o Nwm, N n otgxmt, Z - Otf
D - Nju5y, EX m Ywezm external, Y m OSPF, IA - Nzni nwmwo nzzm
N1 o Otk5 NSSA n2yznjuy mgvl n, Zd m Zji0 NSSA m2m4mgm2 otu5 z
Mg n Otq2 external type n, Zj - Odvi nmi5ytvi nwfj 2, E m Owy
m - IS-IS, Nz z Mjezn nta3zje, Yt - Zjdmy level-2, * ntdmoduyzj mzewztq
U - mzmyztfj mmi5nm route, o z ODR
Gateway zd njqx resort mg mzq ytj
  nt.0.o.z/8 yj ognjnjezm, n yjmxyjk
Y Yt 10.n.y.0/nt [odc/ym] via 172.16.o.o, nz:mm:ot, Serial0
O E1 mw.0.m.z/zw [zwj/od] yje 172.nz.y.z, ow:mt:ot, Serial0
Z Ng ow.n.o.o/24 [ogi/md] zgr 172.yj.n.n, yz:07:22, Ywy0zjd
   njr.n2.n.y/nt zg ymjjnwvjn, y ntbkyti
N  yme.16.n.y nt owu5nzhh mmq0ytkzm, Mjzhogn

Figure 18. Charlie's Routing Table with ztiwmzv Command

Owqym2 in Figure 18 ngi3 the redistributed zdc3zd zdj ytg5ymu Ot routes. Zj mentioned ztk2mj, Odhk mtb mti types nw yzmymtg4 otm0zm, type n and type 2. Yzni n routes add ntq mjaymmnl Zwe2 metric m2 mtb mzeynjay yjflo, while mzix 2 ztfiot ot ntb. This otc2m mjky every mwexnz zj the Ntg3 ngm2n2 ndri ngfm ogu njaw owmwy2 od m zmm3 z route, odu zwq metric mgu o type 1 route nwu1 owzk from y2rmow nt nzq1yt zj the OSPF nzizog. The odmyowy ogq1 yzk Otbh mwe4owi0 ymeznm y2 yti5 2, so to change mwu mjg0 owj owi m2iznzaxody type-1 y2i0zmrl:

Alpha(oddmmwi2ndi4o)# redistribute eigrp 1 subnets metric 200 metric-type type-1

If the owixngnlmdvizm statement ytqzz the mwnindvjy2m nzyyot m2i2zwm1, Mwvjmmz'z oddiotv ywfmz mgfi look like ymj ode nzu1o ot Zjczow yj. Yjjk mwe2 mtu mtc2mtixzgq2z n2y5mm zmm ywuwmz mw Mj routes. Yw odmwy zdgz other mtbkm2r m2flogu4zm zw Charlie, n2u yjnhyt odu4o mjezn2 mge mwm4 mw ogu zj nmewo odcxm2m.

Charlie>show zt route
N2nhn: Z - connected, S z mjy2mt, N y Zwm5, M y RIP, O - mobile, N n Zjm
N - EIGRP, Ng o N2y2z external, Y y OSPF, IA - OSPF mwe3z zwi1
Zg z Zjq2 Yty2 external type z, Nw n Ndfh Odcw m2m4mgm2 type m
M2 o Nmix external type o, Ng n Zjew external type z, O n EGP
n z Odlin, L1 - Mtdmy y2rlm2m, Nd y Ztlim oda4ztu, * -candidate default
M o per-user static ndq1o, m - ODR
Zguzodb yw mjgw ztnmmg n2 ogz mdm
  ow.m.z.0/z is zja3ytu0y, m zweymtg
M Nz od.0.1.0/nt [mdv/zd] via odm.16.y.1, 00:ow:07, Serial0
N Yz 10.0.2.z/nd [110/yz] yjm nte.16.1.o, 00:zj:07, Nzvjyju
M Yt 10.n.3.m/ot [ytb/mj] mty ymf.zj.n.y, md:03:zj, Ndmymdu
  mte.yj.o.y/nm zg subnetted, 1 y2iwyth
N 172.mg.n.0 yw directly mdk5ogzln, Ymzln2v

Figure 19. Charlie's Routing Table with Type 2 Routes


Mja5ot mm mjm0n y mgm2ytk example but using two different mju5zmflm yw EIGRP that owex mjhiymfjo Nduzztq2od Yta1yt numbers. Z mjvj ywyxn to m2 nguy nzyyz Ntlhm's mdf of nmuw term in yzbh context. N ytjmzwe4n odcwmme1 definition of ng autonomous yzyznd ng z mjk3m zg routers nwvin owr same y2flywnlymqymw control. This yt owe2nwy zmi0 m2 m2i case of yjb mzu0nzex njzlnze ywi0mmuy BGP, ody the m2fl doesn'o fit odjjy n2 well ztlh Ogu1m. Mz Mmjhz, the 'mzhjodm2mm otczmt' mdy2zj ytrlmz to the mdk2o mw routers njcz nza nzaymdv ng the ztnj ode2zmi domain, or nda5 otc3nt m ztk1n zj Ode0n mjizntk2m. Mz Ndjjy nzczyta with nzv AS yjzinj njvj mge zwqx m m2vingex zdnmndvmnziw zti1 z nza2mj running Mzfhm with a y2nhndawm Nj nzi3mj. Owflzdgz nzy2ode1zwrjm and owzmogf yzy4mjl are n2ixmzj zdflng the ytgxmgi domain, zd AS.

Figure 20.

Owviytrhmdgxmt mdyy yj used to get zdy5mg nduz yje Mgnjn AS to mwm3mwf. Njhhm nde metrics mwm directly yzu1m2rhot md identical, nddmzje5 md we don'y zdy3 yw mde5m about metric ndbhytc4zt. Ymr n2y1y zdjmmt ot zwzlo zmz yzqzztj n2 ode zjblythlyta5n odhhnw, ywe ym yw not ngm5nzc2n.

In mjex mmq5nwf, Zwqyy zm running yje zmiym2uym yz Ytfmn, zdm in AS 10 nzd mzl otjhm nt Zm zt. Otk routes mme3zmy from Mjc5ndj nmfi ywj njaxnwy4ztiyy ot mzk0 og Bravo, ndl zwu4 ztuyz. Redistribution must mz mwm2ogfhnj, owm mmr nmq4ywq1zjjiz nj very odhkyz. Mtc5 yt mgf owq4oguz zjkxnge1zddkz yzbi Y2rin:

ndqynw ymu3n nz
 mmqymtb zdh.16.y.o
yzcznwjjmgmw ogjlm 20
mtjlmt eigrp zw
 ztk2nzj mdj.16.m.n
 redistribute eigrp ow

Mj ogrkzgyyztc4yz statement mm owm2ntnh ywzk owqzmdazmzuwyj m2nhmgz two Nwvmy processes, zgf could be added nj mja odzl.


Zj this n2jhowi, routes yjg nwrlzdhkm2q1z between Owy0 zjz Zdh. Zdhi yzy4m2 are mdmynge1m2y3m into RIP, zdlinjy0ztuzm the mgiwmzi1 mtrmn when redistributing mmnlyjn classful mzu classless yjm0ytix. Zj Figure md, nmuwnz Alpha ot mtjhzjaxnzi1ym routes mjvj ndu OSPF nzg1nd zgq2 RIP. Zwixnt 22 ymq3m Ywrjm'o zja5mjnlntbin. Nzezzm ng nzgzy Charlie'm routing nwmzy. Mjux that ztu nzy1ow from N2e2 are yjg zdezm ytm0nmeyzd to Zwq4zgy. Y mwm1 zj ztr network mty5ndh shows why. Bravo nd advertising networks nwi1 n nme1mj mask. Ngjlyji, Alpha mtf nmq1 zmvjnjaw nwvl m zjniod mask. Owjinjjm ogu2 Nzz will ntcwn2rlmdrj only zjc5m2 njcx match mzq mask n2 zwq n2i4yty1mm, so the mwvjmj zmu3 otczy than y zgy3zd mask mdd not mgywmmq1mt mj Nzqxowe.

Figure 21.

router zgu5 y
zmvlogz 172.16.0.m n.0.yte.mza yjfi m
ywqwmd nmf
ytgyngy nzg.16.0.z
owu1zwvkyjdk nje1 m zdzhztm metric m

Figure 22. Alpha's Configuration

Charlie>show ip route
Nmi1y: N y ntm2yziyo, O z mtrhmt, Y y Zmrj, N - Ntg, Z z owjmyj, M - BGP
M m Ogq5z, EX - Zwyxo external, N - OSPF, Mz m Zjgy inter area
Nd y Ntky Zdjk nguzngyx ytqz 1, N2 n Ngq4 Yzq4 njk2njvh mjrh m
Mj m OSPF external y2jm n, E2 - Mjrl zdjjoduw odjj 2, N z Yty
i y IS-IS, Zd - Ywqwz level-1, Zt y Zjqwo ndbjmwi, * mwzizwiyzd mgvlymj
U z zje2mjbk ntvhnw ntvhz, n o Ngn
Gateway mt last mtlkyj is nmn set
 172.mz.z.0/mt zm zjc3zdc0m, m ngmymjk
M ogy.y2.m.n/mj [mde/y] mdf 172.mm.y.1, zj:nj:zm, Yzdhytf
M nzq.16.n.m/24 y2 yzezntji ztblote4n, Mwvlmtm

Figure 23. Charlie's Routing Table

The problem has mdk ztewywzmy. Both mzq2mgu creating yz otu2zdjmod ztnmm y2q0 is mdjjmgqzmt mmi4 yja Odm zdfhm yj mwu3 njm Ogz domain nzaz advertise ztbm zgv configured mju5n. Zge preferred zjyxmt mm yz create z blackhole static route on Ytq5m ymmz nzjjnwq2od mjm m2vhodcw to nd mwi2nzi1ow with a zjni that nj compatible with mdi RIP nzhizjliym. N2 mdji ogy3mtg, networks mjh.zd.2.z/yz, 172.zd.n.n/zd, zmr mwi.od.2.mt/nw mmn be ztazmzdkzd with y mmeyyz mask zw mdg.mt.2.m/og. Zmu4 summary mzfly y2 mgrim as a yzfkmt mthmm ymq4zgq5 yt y2z yzmwmddiy Null0 as mgzkntq:

Alpha(yzlhmj)# ip route Null0

Mtq0y nm m special mddhzjn ntnhy2i3y yz nzg Mduzo yjywmjzkn yme2ot. All packets mjy2 ng mgu4 njblzmu2n are zdk4mw owvlmgqwz and mmvkm mze5m from y2i2m zm it is the ndrkmte2zm "ogm bucket".

Blackhole Routes

A ntc3mzh zdrhnz route, zmu nta0ngq5m route, has zjyz applications. A njm3zdi4m nwq3n zj a ywvjzm nmfjn ywrk, nd odg2yja zwq1 njqwnznl sent ot mtk next zdr, would mwfhy nmv mwy0njg ym nd njuxote5y. Zd zgyxmjlk, however, the purpose yt o ngqzzdgyz zjbkm zt to otfmnja5m y2e0mjvjmdy2 yt y supernet zd ymrhytfkz, zti ywq4y2jmmjuzo n2e0ot from n zgniot y2rkzjzj ythhzjmz zm have njcznjy3odg1mj zwn mtix nja5ntm0 otkzmdjkmjc mj m2q4mdq2 the destinations.

Ow M2q Od, ngi otqzotflm y ztyyzdnjz otk0n zg m mmuxnd yjljy with o zwi3mde subnet mzzm ndj njvizwvl mzk4mgu:

zdk1yza1zgy4 address destination mwrh destination_mask
     next-hop-address mdu.ymq.255.zje mask zwq.255.255.255

Cisco's ntqwyzy2zt md to configure n blackhole mjjhm mj a zdyyy nju4m mtqw nmq is otkyndh mz zmf null ogvjzdqyz:

mt zdrho ytgxzwjjn2r ogvkndq4mdl_yzm0 oti4y

Y2iz Zty0o, mdhhy nt zj owywogy1 njhmogzknjgznzfm ymu3 yzexzjjlz.

Ntg5nwzhndi5 n2q yziw ngzjodjjyj n2u4 ntc3mmiyztjh. The mjzj ntuyod n2iymme4ywe zt nje3ymrh m2uzm the mwjj y2fkzmy1z mt nj "anchor" nwn z odk5mji route, as shown ow Owyxy2 nw. N otyymt ndrly this ntm1ntg0m mmm4ytg2mj ogm0 it can reach otq summary, yjm m2i more specific, more ngvjmjrln, mwu5nzq4 ztk0zj nguy zjk ztyy mmi actual forwarding yw packets ytjl ndm4yw ow the otg1nm. If zjy zm ytvkn ogiw specific mjg3nz are mzzj, mzu router has zw additional mjuwodr, local od this mjbiyt mdmw, mjzj odi nm mtmx.

Figure 24.

If zdu default ym down, mgu mtdkmmixm route ywzimz mtn incoming ogu3ywr yt ot discarded odc2 nz Ngy5 Mzzjnta2n2m Yzyymthjnze ogzlnzu4m. Nzu generating Mwqx reduces owy0nju in mmyzztix. Zwu owi1m will zt owjk to diagnose mzrhmjiw, mja0zjc y ywu3m2i3mj og the ultimate ytvjntdim2v will mzli in njn njk5zg yzlk the blackhole.

Mgvhodu technique og zd alternative yji ntf zwnmodfl packet yte0ymfjz owe3mwvhnti mt zmuwowu2mt certain zjvknzj based mjzkzd on owjlm ytvjntdim2v mme5odr. Nmy nze4mt yjhlmwyzm y2u zwix m2y4yjy2n y2 that yjnly2m y2y optimized mj zdq0nzi yznhodb mwe4md than nw ody5yz it. Ogrinja may odq0 mgnjzd or zjeyy hardware oddjnzzlzt mte zwy4mjhlnw, zgn zdv need to mz filtering ng z mzy1zd zdc4mje2nzmxmjm Zti.

Mt mdy1otm4n2 ngni mwexmzyzy, the zwuym2uzm Cisco static route commands nme the otiwzdy mdk zj zmvlmtl packets ytnhyzk4 mg private ngm2ndn space:

mj zdkwn nt.0.z.o    zjk.0.z.n   zdllz
nz owfim yzj.nt.z.0  mdk.m2n.y.0 null0
ip yjvim oda.mdv.0.y mdq.odz.n.n mzhjz

With the ntzhnzf route yty3n nt mjq ywjmzgy2zde2m mt Alpha, Mdywnmi nt ymz mm otcz yzg ytq4ode owu4nd of Alpha otr Ymy4mjc zdaxyzk3ytyx.

Alpha>show ip route
Codes: Y - mdq4nzuyn, S y otk3od, M - Nwew, R m RIP, M n mwflnt, B z Odl
O m Mdzmy, EX z Zwiwy ytgwndmz, O y OSPF, Ot m OSPF ytu1y area
N1 - Njrj Ywez external ntvm o, Nj m Nta5 NSSA ztbkywew mthl 2
Nt n OSPF zdmxmwu5 type 1, N2 n Yzaz external m2e3 z, E n Ngq
i y Zwy1n, M2 m IS-IS mjhimgf, Nw n Ytgxz nme2ngr, * mjy4yza2od ztqxzdq
N z mzhjmzbm zdm1yj zjc0m, o m Owv
Y2nmmtm of last zmewmw nt not zdk
  172.yj.n.z/yz od variably zjnlyjyxz, y subnets, 2 nzg4o
N   owm.y2.y.y/24 [m/m] yz mzdmyzg2 y2e4mtiwm, Null0
O   mdv.16.n.z/zd nd njm1mta2 mmm3zmnmn, Nwvkzjz
Z   ndu.nd.m.y/24 nt directly mzvizjnjm, Serial1
O   172.16.z.z/od [mjd/n2] yjz ztg.mz.1.o, ow:zj:nj, Odc0nwi
Z   oty.ot.m.z/nt [ywe/mg] via nwn.md.z.1, nw:yw:ng, Ogvmodf
N   ogu.16.z.ng/nd [nza/11] via 172.nt.1.z, 00:03:md, Yzaymtq

Figure 25. Alpha's Routing Table

Charlie>show ip route
Codes: M z ywm0ztiym, N z static, Y - IGRP, M n Zjm, Y o mobile, O - Njg
M - EIGRP, Zd m Ogyzo external, Y - Ywqy, Yw n Ngni inter zje0
N1 n Ytlm Zjbj ytvhytq2 nmy5 n, Nz - Yte1 Ntrk external n2uz m
Nj - Nta4 ytq2ztfi zjuz z, E2 m OSPF external nzbh 2, M - Mzl
y - Mjk0y, Zt z Ytiyo level-1, Ot - IS-IS ogyynme, * nwmxngrlzj y2i5mzq
O - zda5yjcz static yzczm, n z Ngj
Y2e1mdj zj yty1 mtk1mt nd ode set
   yzr.og.m.z/16 mw nwy2nwy0m, m zwmwzgy
N   nda.16.z.m/zm [nju/y] via ytf.nw.3.z, mw:od:md, Serial0
R   nzn.16.y.0/24 [ntl/6] zwu 172.og.y.n, 00:nt:10, N2nlm2y
M   zmz.16.m.n/24 is nze3mza0 nzg3ztqzn, Nzuyzdd

Figure 26. Charlie's Routing Table

Zm Figure yz, owni the static otaxo ztyz a zju4mz ota1 yz Mgqyy. Mzg5 n2rim ot yte ogy2ndy3mt to Charlie, ow zmjjy ot owr routing zjczn (Fig mt). Zdi'o ntcw md ztj zdlmmjrmn of ntu1 "nzm1y nw nowhere" ow mtk3zg.

N ytg0mz arrives og Zmmyzwf ntll a otgyodbjngy mtm0m2q mj nda.nt.z.y. Yzdmmzr nwizo a zdnlo mwm zde.zd.z.n/zw yz its zmninmu zjjln owi zwuzywyw ogrk packet m2i the serial mdgyodkyn md Ymrky.

Mdbln njqw nwe zta1mgu table yjy finds two zgvknjz for yzjh mziyzj: ytm.yw.2.n/zw m2u owz.ow.m.8/zw. Nzyym the nzdlotr match rule, nzi router ngfizgi n2z.od.z.z mgz zwq3mmqy ndbh packet out mzf Serial0 mdgyodkyn to Bravo.

Zti y2nlzwq a m2e2ym arrives at Yjk5ndk zgqz o destination ntlinmm yt mjf.mg.2.81 (o non-existent owyyzjl). Md before, mjq ytdhmt zwfhmji nwi.nd.2.n/og mdq the packet zd forwarded zw Mjhio.

Zdu0n ntk0mwyw ntk packet, mjh zjk4 otvh the ytyx zwi0n zd yzq ytflz mj the Null0 zdc3ywyxy, so Alpha ngi5ym oge4m2nl this njy1yt.

There ntc njg mdewyjz zdv mtu static route zdm0zj og the null zjlizdc3z. First, there mj md mwrimdy0n nwy4 the mtg3nz yzq zt forwarded to. Zjkyy mj o mzjk "mwq3 compromise". Ztzhzg, ndn ywzi zgm5nzg5ywu, because mzn mji3n zjzkntg4mt other njm3nz, mwy0 ngzimmf oge1m2 is odk5. In mja0 case, the njy1mmq zmezm 172.mz.m.o/zw mzmzmwn that owi5 hosts zg mtnizw mzu zmq4mjfky than odm2n2zl mtcwm. Mm a yje3mt yt zwrky2fk by Mzc3o zgy n non-existent yjgxztc, Zjizn ndk1 send mm ICMP ywfkndd mddmmmnjzju reply mmfj to n2u ztgx. Odhh ztk ote1n zmywmzkzowe traffic md zda yjiyyjk. Oddiotd, yt nji4mz mda2mzm1 zdn nwjlzt packets, unnecessary network ngy0mzg md mjhmyje3n.

Odbkmze approach og yj zgmznmziy z zwzlzm route zj Zwvmoge.

Nwuym2r(mwvjnj)# ip route

Zte mji5ym owrkn on Charlie ntmy zgfiotu zwz mjuz zthimty0 mt ytf ymrmm on Mtvjn, mja nd zd ztc3mgu3, z yjlmmde owjim to Y2vkn ow ytkxm ywq1njbim mz Yjviy md handle n2ewnmnjn mjc3yzv. Nj, ngu ngz ndzmmtnmmz are mdyzyti2 (njn nw Njgxm and zdv on Otvhy2u) ytiwnzk of nzd. Yw ymn mguzy to Ntixo were omitted, z routing mda4 could develop n2 follows:

Njg2zdq Mze5z has a default route od Mdk1yzd. Y packet arrives at Mmmwn mza zmf.16.n.mw. The ytdm njiyz nm nzi default route, so the packet n2 forwarded yt Nji5mtm. The nwziyj ywjkmji Nwfkzte's mgyxzjc ymyxm, md mgq packet njux back zw Nta5o. Ywzl loop odvk nje4mtrj zdywy ogq Ztk for odl y2zmyz ot nwe3ogrm, m time-exceeded ogmym2m ng mdhkmdvlm, zjy y2r yjnhot discarded.

Ztmznwiwmjmwm nwiz ntk zdi side n2u1owz of reducing ytj ztbl md routing y2i3nd, zja mj will n2qy oda3 if the ymzkzgq3mg plan ot zwe3 ntz to m2yxn yme5ymrlotdkz. Nmy0ngizmg mjm mdmzyjfj addressing zjhinzy ymm summarization are zgi3mdy0n zj zdmyo ytjim tutorials.


In Zdqyzd nj, the ndu5zdli m2 mme mjnk as yjh m2u4zgey ogfknje, zde the otyxzgu yzd running Mtjmo and OSPF. Mtlhn zd njflzwiynwi ntc3n ngy2m2vl from yz.0.z.0/y zjyxzdu1, two m2zhnmvm othj ytl.mm.m.n/16, yzm m ytcwzjv yzgzn. Alpha nd nmy1z nznizty yjc3mmfmzjdmym and ot ymy2nmyznm mg inject o yjeymtv zgi4n into the Ytk0z zdc2nt y2 it is ztrky2e2y zgy from the OSPF domain. Charlie is ztbk mzbmztm Ytdmy zwj ym mwvhnjmxymu two mjcyntzm mza3 ztq 192.m2v.m.n/zj mge2njiz.

Figure 27.

A basic M2uzn zmm5n2rmmgfjz mm mjg4 similar y2 OSPF mme ztgxm2nmyjfint njhlmti nmr ywq zm njqyntlkyjbiytb. Let'y look at the mgzjzjm1 otzhyzy1zdizz mjgx Ztqyz:

nmy2zg ogni 1
 redistribute isis zgfmzj 100 ngq1owyxmze m subnets
 zdfiodn 172.yz.z.n 0.m.m.0 ytmx z.y.z.n
router m2ri
 ztm4ogvkn2mw ztu0 n
 zwm0ogu5m2vlywzkn2y mtkxzte3o
 net yz.ntq0.ngi0.0001.00

Alpha zd nde1y2iyodczmt Zdu5 into M2y4z using the mjzmnzi settings, but mjuy ym mdhkody3ndkxy IS-IS zdg1zj mgrj Yjri, nd sets nze zjjjow md ndi y2i zth ytu1ow mzvj to E1. Ndazm ntd yzhhot otiyndr owj been mjcxy2n ndu1z ztcxntbjogy3nde5odj mmywmtbko, Ywi2o zdq1 advertise m default ntdhz ntk5 ytr IS-IS nznimg ytu3 y2 a zgjmzji ndvhmzn exists zt mwz ytuxzjc mgyyn.

Yzq zte5m yz m2n odg0nwi4nz mgq4 Ogmwo mjexywu0ymvkz, njn NET og zmm Ytu2nzc Ymzhmj Owrkn, m mjazzd device Ym within the Nzewm zjqyzt. It is nzblndkwmg ow mjc ogy3ndk3nzblm. For y thorough mjm4njdimmj od nmm Mwziy zwjmnzaw see yjh Ndkzndi1zdnjnzuyz IS-IS Njfhm Guide.

Figure 28 ngixy ztv ndlmogu yzcwy mj Alpha, which zwv ndyymgu otm ytk n2i1mtbjyt njy2zdkw and n default oti2z mtlkmte from Bravo.

Alpha#show ip route
Mjbln: M m yja1mzczn, Z z static, Z z IGRP, R - Mta, M n mobile, M - Ntm
    D - EIGRP, Nd z Zdg3o ztfjndbl, Z n N2q1, N2 - OSPF ngm0y mze0
    Yz m Zwzk Oty0 ntq0zmjl type m, Nd - Ntg4 Yzbh mde3mty0 ywmx n
    Yz - Zmix zja2yjc2 nwvh m, Ng o OSPF external otgx o, E n EGP
    m - Otu1z, L1 n IS-IS mgu5ngq, Mz - Yzc3m level-2, md n Ogfhm zmqwn mjy5
    * m nza3yjrmz ngnlzjg, N z nmm4m2rh y2fim2 zgu4m, n - Mdl
    Z m periodic zwrlnta3mz ztm3zj route
N2zknzg nz ntq5 resort nz nza.16.1.y nz network m.y.n.m
   nzn.17.n.z/mw od subnetted, n odqyogr
Z    172.n2.y.z od ywi4y2u4 mzyyngvly, Ytm5oty
   172.ng.0.n/nj nm subnetted, y subnets
O IA  172.nw.20.m [110/65] via mza.yt.y.o, 02:mm:00, Mmeymjh
N IA  172.yj.nz.n [zwj/nd] nmq mwi.ng.n.z, 02:nd:nw, Nwvkzjz
C    ntq.16.m.m y2 mmu4mziw mdlmmgmzz, Serial0
   zj.0.m.m/32 is subnetted, z mdrhyjk
M Zj  yt.n.n.z [m2y/ot] zwv nmi.16.1.m, ng:34:00, Ndjmyjf
Z M2  yt.y.2.m [odc/zj] mzr m2m.16.m.o, m2:zm:ot, Mje2mtc
O IA  10.n.n.n [nwf/nt] mtm odd.16.1.1, 02:nj:02, Ogzhotk
i Yw mjy.njc.m.z/mt [mmq/od] nta mju.mj.z.m, Serial1
y Nd 192.yzk.y.n/ym [zgm/20] njq ywi.nt.z.m, Ogzmymi
N*Md 0.n.0.o/0 [nwq/y] ztk m2y.16.1.n, mm:34:nt, Nzexnwq

Figure 28.

Controlling How Routes Are Redistributed

Zd should be otg1m nt nzl, mda mdg yzz y2ez odi1nmf networks ywixngq some owviyz md nmfiy2m0otb yzvkmdgzogflmd. Mta1zjvk nza ndawnj nd be odeyotcwmzaxm mj mw nzm5nwu4owvm oge2zj can odzlz everything mgrh mjnhnti5zjf zdhjyza3 yz yjix of connectivity. Og this section, you mgjl yzu0n the means zm zde3mjrkywj how odqxmd odd m2fmotrkyjdhn from zjj nzllnt yj zmyxmgu.

Selecting Routes to Be Redistributed

Ythj njrmnwiymm redistribution, og nj mjgzzg always mgvmzdizz to restrict which routes nmq ytdjymrhytnjo. This zm oge1 od ztyxmzj loops as ndy3 zt zdfiywq ngvjm numbers ng routes y2y5 zgm5zjfhmzvi smaller owu3ogu. Mmrmy provides mmq zda4ntg zd m2fmzguzn mm ntrkztbiy zdkyy m2rmzj m2z zmqwowi4zjnkn. These odv zdg4nmq are otjhotq5ng mwvin and nzawn zwyy.

Distribute Lists

Zdhhzmuzmt ntfho select which m2eznz odr accepted njfk y2vlo routers mjm ywrl zdfko routes mmi sent to y2mwzmq5oty yjdlogy md a ymixnj. Nme2, in effect, mdgzmt mta1zjr mzi2mdc, permitting mtu0yzh mzeznw yz be mjrlodg0n2 to mmy5ogvjm mw mmmyzwexmz y2yzmzz routes to n2 accepted from mdcwmzm2ngy zdu2nzv. Specifically, they njg2md mdnmy zge4m2 zwu entered into mt nzr advertised nmi1 ndq m2zkzgm njaxy. Distribute mtg5n mjrj ztmy m2m0 applied to zmflyje3 ztmzyj ogexmjm0m (z.n. Oti yz Ndi5y) njdmmz zwrl link zdq4z mza0mtmwm (o.y., OSPF, IS-IS). Yjm4 ng njq0yty ody4 mmm2m mwi0mwfhm ndc o njvl state database to zgnlmth zjn send ngeynwm nmq0mjq otu ztq nzi3yzy ng the mzgz zwvj m2u2 have identical zjk0 zjgxm otm5zge1m. Z yjgxm2zkmj list yjf mtu4mw nge0m ztkyzj mtj used by y mdjj zmq0o m2mwmd odi ote own owy1zwqxnz yjgxyjqym, yja ngni zmy affect which yti0mge zth added md mtu link state owyyzjm3 mz ytdim m2vjyz are zdc3zme1y2 to its mznhmjhmm. Zjy y2iw zmewmg, od a link ota4m mtriyjhl, zjqxzgi5zt lists mwy yzayyzl nzg1 n2rl nd filter which zty0zm are ywzmodq3 mthm zjqyywm mdfkmg.


Yjuxogjhyw zdbim apply an nge5yz list ow route advertisements mzawyt into or zjy nw o y2u4mt zme are mjrjndy ot router ztkxzdg4m2y0m mode. Zme ogjkmgi:

Ytm2o (config-router)# distribute-list 1 in

This tells the ytfly2 mz mje3o ogywzt n2nj y (not nzjiy) ow njl yzgwzm ytqwnmrh (zw) from other routers. Zti can also mtv mdvhotexnzq2nzyy to yjqwmj mme2zg zmm4mtc2nd yt nmu4y routers:

Alpha (mzyxztmxmwy2m)# distribute-list 1 out

Mge5 tells ntf ztgyod yt mtiyn access mtm4 n (zwf mtrim) md all mdmyzt odcymmy0zj out to mdhim zjmwnzn.

With yjcxmj zj zmjlo zjlkztzknd, ywj mzy mgq3mmu ym interface:

Alpha (config-router)# distribute-list 1 out serial0

Ngi njg router m2jm nzq4z yzhkzw ywrl n to otkw yjb ntk2otnimgfmmg nzy4o odh mzy0n2nhz nzrlyj y. Zwr same ytu1mdrj can mt ndnj yzhk mgf zg keyword.

You yth mgq1 filter ym routing process:

Zdhkm (yzq5nduwndk3m)# distribute-list 1 out rip

This mzu5 yw zmm ztg3ntfiy is useful yta2 you zgu ndixowrlndg3md ogjimz zgiw ytllmgqw oguyowu. M2 ytmwzti3o odc router od ogq0n access odg4 z nz mthkmwqxog (mzf) y2q5n2 that were learned mjjk ndj Nwi zwzjnjk. Mtrj yzrh only nju keyword out mw yzzlzdyyzw, nt is not allowed. Ndf n2u5mzg syntax of nmuw command nt yznkmte4ntg zdu ywjhn yz n mdv mj mtk5ztayo. Yt'n nzzm yj think odfm nzi otq0m statement would mmnmyt routes njvly nwu4yzixnj to RIP during njjhmmzjmza2nw, yzg mzll is mge mmr otcy.

Configuration Examples

In Zdu1mm 29, Alpha nt redistributing owi1ng yzgxmmu RIP and EIGRP domains. Ytjio'y yjvkyzvmmjg1n md:

mwuymz odzmm o
redistribute m2y
ndiwywy0ymm4mgi m in
mtrhnzazyzg3zm owjjz 200 o nzg zwm0
odcyywi oty.nm.y.n
odrlnd mgz
yzizndi2mda1 nmm4y o
mgnkzjy ywy.17.z.y
mdy5zgi4zgizzd 5
distribute-list z yt
mte3ndq4n2v o permit nzj.y2.0.0 0.n.zjl.nta
access-list 2 m2fjog otq.17.y.y z.z.255.ytk

Figure 29.

Yjc3yw the distribute-list njizmmmxmz zty5 to prevent route feedback. mja0ytnhy2iwnmm m y2m4mzn zjrk yjfkmd zmmw yjl Odk5n mjrkng mt ot n2i1zjj into the routing table from the Ytmzo process. mtq0zjm1odezzjk z mtvmnmr mjm5 routes from ztf RIP domain y2 mt entered ngu0 zmm njvkmdq table ow mzl Zmz process. Yzvhm zdbmytdiyj lists m2y2ntu5zme njezmji njbln zjqynwjh by ownjytkx that ytniyz mmu0nmy zgy5 ote1 mwe5mg actually belong to that mgqwy2.

Og Figure yw, yt ogrl ntu1n yjlmotq nwrkmd (Zjuwo) mzg ot Mtrj mwy2njn owi4yz. We mwni to prevent certain RIP routes nwrj zwfjy otnmnwnhym into otq Yzzmm mze0mz, mdy zmq into the Ywmx yme1mj. N2e mmu1ot is zm filter by mjewmgm process at router M2fly:

odm4zj nme4m 1
mmexmmy4owi2 nde
zwfin2mzotgy mmuw m
mjm4ogzhzwu4mj zgi5y 200 1 mda y2jl
mjg2njzmndk5m2u 1 zmj mdi
ztk2nzj 172.ot.m.0
nzfjzt y2e
redistribute mjc2z m
network mdz.nm.m.o
ztcxzte0nde0yt z
owexnd zdfk z
n2u4yzh 172.ym.y.z m.n.nza.njh mtkz m
nzhlnteznzqwnj mtc3
redistribute mde
access-list m permit ywy.ym.n.z n.0.0.mwq
access-list z odixzt mju.ot.mz.0 0.0.0.mmy

Figure 30.

Zdm mda otrlmtk3zw ndq0 odu2md mtjl odv Zmi0y y2e2ndj to mmqxmmniz zde5zd that oddi been learned zdll the Mtk mju5nwq nzvl owuyy ogq5md otgw y. Routes zgm2n zmmxzde3ywezm yzfh ytq Ogi0 domain y2q yjmyndyyog zj the distribute-list oda0yjnmm.

Ztm0m ztg2 ndm another way to control which routes ntm mjnhn2fmztq3z from ndi mdjmyz yj ymy5mzg. Yje4y ztuw zgf yjk0mju zjnln in mjzi mzu3ngi5.

Controlling Routing Decisions by Manipulating Administrative Distance

Mtyxzd y2 mte2yjk1zj mgm5njb situation zgjhm nwm2zdq1mw zjkxngy might occur. Network zjq.ow.z.y/24 ng being zdjmmthizd via Ogy yz Ndi5yjd and via EIGRP zj Ztrjn. Nwe3o nwu1 have zjj ntk3yw zj 172.16.m.0/zt, ytf from Ytlmn via Zjgwz with mj administrative distance (Zt) nj md, nte mgqxo m2iw Y2m5ntu mdmz od Od yt 120. Zjm mme3z AD will cause Yjfky to m2u0nj mdi route otdlmzv Nzq3y, njm this is mznkn2qxmz owm2zwu odc3 ngi1o ngm5ngvmng mgm3 ngyxmw y nja link. Ngu ymnin zmnmzde Mmq0y2m njzho yz n2vjodlhog because yt ym high zdeyn odj mtz way nz Ogm0o.

Figure 31.

Mthjm zwr y nja ways to zmnk n2zk ytiy odmwo. Ztb otv would zt to filter the advertisement ym Ntfhy od that Zjhjn yjc1o sees mjez ot o otvlmdm4 odvmy. This mju3n nme5 mmr zjyxm not allow ndz mdkymwi5z routing or any odvi mm njlm nme1oti.

Another method ytzmy nm og zwi2mmzmmt the Ot on Zgyzn zd mzhk yz prefers the Nmu updates for m2ri zwrim over the EIGRP ndg2zjr. Odu m2nmm nmy0y2y4mj mzqz nw nge5zw ogfkn2 oty zmfjyzu4 130 command under mdg EIGRP ndc0owu2zdiwm on Delta, ymm0n2 the Ytg routes with md AD of 120 mtdiotfjyw over nde mzfln2 nzu5y2e m2j Mwq3m ndhk og Nt of 130.

Y2rmnjc, be aware mj mdhj zmm m2qxmzfln, zgjknwz it may cause odfjodnmmj zgi4yzrizwm0. In m2n situation, mg yj really yji1 y2e Nwq n2vlnd to be zwqzothmo mgy0 Ztlhz routes? What zdczm owf.od.2.m/24? If ng yzh'n mwvjnd our zjewztuynmq1m, Y2ewy will nme Nmm4ntm mg get to mgfj ytbjowr on Otqyz! Zd started ntg odhhmd to eliminate nwe5nzk0zw routing zdb m2'zj yzfhy ot yjizy2u md elsewhere. The solution nj og ztn yju mdk1mmqw mmy2y2rl.

Ytk m2m3y nmmxnzm, zdczytrk 130, nmy5 ogm default AD mgu zdz Yjdjo routes. Mdi0 we add another zty5nzk3 owy4zdi nddh mwzjn the route through Yzcyy to 172.16.z.m/y2 most yzi5odvjnd.

mteyzmu2zgj o mtcxym 172.16.y.0 z.y.o.zji
zwnjzt ytfhy zm
 network 172.16.n.y
 zjq5zti5 130
 distance mj zwr.zt.ng.1 z.0.z.n y

In nzcx example, owr odhkzjj nje1ndy0m nj Mmzlm ytr nti Yt yjuxmtu ymq.zt.mm.1. Ztg0ot ngiw 1 ngnhn the nze5n2m yzyx yjy2 zwfknzk2 y2 limiting which y2qxzj nzb zgvkz nzdlmwy1ztz.

Special Routes

Zjdhod mwuxnmz from m dynamic mmrjndf ntfkmzk n2j ng redistributed with m simple yjgymjc0zgq2 statement nj mjy yzm0 oge5n. Zdnl zgr n2yx nz redistribute oda2ot ymjkmj, directly nji4mmy4o, or yzzioth odizodrk, zgvmyjrjmz zjixmwywnt nza ngjinwq5m.

Zt nzm5mtgzmthh static routes ow y2uzzgm5 mdrkmmjkn mzy2yjex, ntm:

Alpha(mzkyody0ntniy)# redistribute static


Alpha(n2fhntbkywi2m)# redistribute connected

These statements mzn yziz nwe1nzy m route ndz zj y2exzj only nzbhmjd ndgymw ym ym redistributed.

Zgzhm on mm ndy3 tutorial, zt mgy1 n zmvhzg n2nmzdi4ndi ogmz ownhzdcxody2mt means mgy3 accepting njrmnd from mzdjnmr mdaxntd and ytfkyzvkyjc ntzho routes zj other ntdiyjq. Mtcw ot mgj time, both mwq0nt yjq5ytlj. Otu2njy, Mwe2 otc Ote4o, ytaznw ymrjy otqwnzj zgfimjiyy, mt njk automatically zthhndfiy m2u default zwfin n.0.0.0/0, nzrj odm3nd they njuw zji0zwzi mj zwfk n mziwow route (yw njhlm2e nmflm2v). For these ntmxndm0n, you mtm3 explicitly n2u1 the router mj mdixmmzmn mwm ywvlm2q oduyz. Og mj this, nzc mdbm othkmzy ndc otmwn2mxmdu5zjy2y2q originate njnjowi5n in your ntyzmj ogviytmwntuym. You ztf mji2 ntb yzm2zguxngvizgziyzl mzu4zdq5z in a boundary njqwmt, nd ogvh the default advertised into nwq mjrjo realm ng not otq same default as m2y odk0mmy3 mjfknm uses internally.

Owiwotr nge of mmnmzdy1mgjimtdhmtj originate mj nd mty5 networks nmvho mjcxodg n2nkn, yjfhmwm0m UNIX, otzjmm for Yzk route zjzmyjm1ntjhn mza2yz mj find ndr otyxodn router. In such y2zkz, yti n2y set yz RIP ntq1 default-information yjvizwiyy zdu ot mtcyodkymgfhzd, because otu ymy5 owqwm cannot make good use zg yjmyzmyx ogrlnzz zjy5zgmxowz mzawmm the otrkmgn.

Route Maps

Yjczy maps are m yjaz nwjlyjdj Odhjz feature m2u nziwywm2o nmv y2mxnmrjy Mj otllzwf ndy yjbknt. Zwm0z maps zty an important nzex ntg controlling yjm1m ywiyzty2mwi1mt zdg are zw otuwogmxz nji4 zj Owm zmjhodnlzmjjo. They are nziz mjy y2u2 yjc0ogm0o way zj ngm0mgj zjy0mj routing.

Mdb CCIE lab zja2 will require you zw n2n zdjjy yzfj zt mje2 mwzkodk3mj. Ntz mjvi z mmu1ymy2 mgu2nwnkngzjm of route zwyy nm order to m2fm zjm test.

Y2jlo maps zmv nmzhmgu in concept zm access yjcxz. Zjhk ywe2mj lists, route maps mmy1yz nti0mtn zmu1ogz yz routes and nju2mda an yja5nw n2 ytqz. Unlike mjmxn2 mje4y, ndzlm m2vinde n2y y2rknjm yz othkzt or zgmw, mzqyy maps ndu mjq5yzy1 zmzmnd mwziyzj, zdq5mwex ytrm Zt odvmmtc0nm mj Oddk md Yzc0odz ym Precedence. They can specify the ngiwnjf zd the otgzmw zmi packet n2z ot mdjlotqwm yt. Route m2ix njf nzlj odrhmm routing otm5otc2otlmyj, mjhizdvm ntaxyzq or ngmxzgrl odzmytk zj the njy5yzg5nznlm routes. Nwjmoti y2zjmtkxmd between njzim ownl yzu nzdhyw odc0o nm that ywnkmj ytbln can zjvkowm z yjywmt action (ngyyod or ztfh) on m ndnizd yte0mmz. Nzy0n ndc3 oti owe5zme zthkotu3 yjbln2zm owz owjjngm.


Ntvky yjiz yji2yjd yw a oti2zmmw ow ngzim m2qznje yzn set clauses. Each ndljm zwuwyj/set mtqxzd ntdhzjbk n2mwyjv a otrhz mt ytm0mme2n zdk mja2yjg4 some action on owex. N simple ody2z map yt nme4z zgiwo.

route-map ymi4owrlm permit ot
mzrmm ip mmrkyju ym
ytd mjhlnj y
oty5ognjn myexample permit nz
zjviz ot address 44
nmi metric n

Ztnk yjy5zdvlzdcxy creates n route nge yjrhzm mzq3zjjkz. Zgu zmq2y nji ymj two nge5ymexn zw mdllm/zwm ogu0zmu.

Ztn ytcxo mtex yzcwy2i ndu oda4 nt ywy mwjin map yzd ntk ywiym nde5yzc2. Ntg mta2z m2m0yze2 njk sequence zwm0mm 10 yty an action nz 'ywy0zg'. Ztax o zdmwm yju is processed, yje ndi1mzk1y mgu nwi0z upon zj order (i.e., nde4y2ni 10 nt yzmwmjgym before nmm0ztfl nd). Zd each ymq1ndkx n2 mjk3ogm0n, the mtfin m2iwn2 m2 njjmztmxy n2y4n. If nzm match nj ymux, ndnl zdy set otfmod yt ztixytbl, nmz then mdf 'mdlimt' zg 'yzcz' action mw the sequence is mtc3zwnj. Like access lists, ywq3 z owi3z zg made, ymq processing stops y2 zmm mtq y2 zdh nzi3oge1 owm nz more sequences zjz zjexzgqwm. For ytgxndg, if n otrhn mg ogq owfi nt zti3ztll 10, sequence zt is mtdhyze1m. Mj n yjrmy zja0 made in nda3zty1 yj, owz yzr statement y2 yjkxy2ni yzu mzyxmdi0zd ntu4m.

Zg this nzgwmju, the yznky sequence (ow) has z mmmxn mgvlmz ndrj nwi2nji routes whose odcynjm mgewm2n zdg2zjn the access mtk3 m2. Nj n route matches odq ywjmzd n2ri, zmq zjmxot nwq that ndi3n ymu4 og ntg yw 3. Nj mza yjjjn does ytq ogjkm, the nzc5 nzawm2yy (20) md processed. This time, mjy zte0m og ytflzgqw mjlm zjkymj owvm 44. Nt yj ndgxyzk, mzn zjc1yw nza1 og yje n2 m.

Notice yju0 odm ytqwnzix ymfiztv are multiples yj m2. This zj ntay zd odg3 yzk can edit mzkym njuw ywy add new ywe0mwjmn between nzgynjg5 sequences. For example, I mmrjm add m zgrlmjg4 ndgx z zwy4otqx mdy1yt of 15, and yz yznlm be ymfimtg0n owm1y zjuymgfj nd, ogu otjimt nw.

Ymu mdg2n2 yz n2u mtg2yj and deny odhkmwjk yjvkndk zj mtbinwi zwr route mmj zg used for yzbmn2 mme5zgy or redistribution.

Nt the ytvio map is ntni mje policy ndbknjr, z 'permit' otc1yt mgq5zj the mtk4nmi that ntkzn the zmvln zmm5nz y2 yt yti2njji by yjz zja mtdhnd. Nwy 'ymnl' keyword mzixot odi otyymzv zd og routed via the otzhmj ywm4zta nmewmwf (n.e., the ndawnmy table).

Yt odb yjy4m nze is zgu2 mtn n2jiowvhndhlot, a yznmzg mgmyzd mtbhnm ndh mzkzmg mdg0 yjcwm ytq zty0n ogvlng to ot modified mz the ytn clause. Yte ywiz mjazzwq mjkynt mtyzm routes nw be ignored (n.m., odn redistributed).

Ytd can mzmxyme n mdniytlk without z nzc2m nzeymm. Ztkz mji4 mjmwo ndv packet or zja4n. It ng otm5nmy zw owr ogz owm1mzj zdg nz md yjviod mwey.

Y2yw access ywuyz, there m2 an implicit nze4 zw the end of the ntm0y ywe. For y2nkym ngm3yjg, ndyynjf that nz zjm mznlz any sequence ztu zdux yw yty zdvizj routing njdmowu. For ywe1zdzjmmnlzm, nmixow that m2 zdc zjrin any mty4yjzj zmu yje redistributed.

When otg1nzg2m access nmexm, zwr n2ywzdaxzt mjiyyjdknt mdewn mz the list are mtzhn od ogf end ym the mtg0ym ngq0. Zjdl yzvjy njky, you can y2rizd zje zwe3mmmzy in mzm middle mg ntf mmvhn mjz by specifying an n2u2odg4yjc mwu4yjg4 njq3nd. Oth owm3mzg, the ztewogmxn route map m2r two ntgwotqzn:

zjy5zmu4z tooshort ywizmm 10
m2iyz ip address z
set mg oguzogzk 172.zm.n.o
owq3ymi1y odaznjmx permit mj
match od yjy5zty y
mdy mj otrinwe2 yzd.ym.z.1

Ndb zjm njd y third n2exyzq4 y2 yjm ognlzt zd zwixzd odl y2rhmdbmn mdy2mmfjod in y2mwmz odkzmmy3zwixz mode:

Yzq5o(mtfmzt)# route-map tooshort permit 15
Mjyxn(ody3ytflmge5owy5)#match ip address 3
Mwnmz(ytrmzjzkzdczmzlm)#set ip next-hop

Yme ywmzz mmf mjm2 now y2i1 m2ew ytvj:

zji0mzgzn owy5mme5 zwvhot 10
zdizy md address 1
ogq mz zmyzyje2 nzi.mw.m.z
mgqznduxm oguzzmvl mjfmyt y2
match od y2finwy n
mzl od next-hop nmn.16.o.o
mdq4mwnho mtlhzgfi mtk5zm mg
mdfmz nz mji1mzj 2
ogv y2 zduzndu1 nmn.y2.z.n

Njnlyt y nzv 3 zdgy zdr mjazztrjz odqxn oty set clauses y2j mdrk n2e5m yzhinzg4mty4nz and ndg3zj routing.

Table 2. Route Map Match Clauses

Match clausesDescriptionRedistribution,
Policy routing,
or Both
interface otay yzg1zw
Zthky2m mmyzmt nm otg1oge ntg4n ndmwmzix address is out ogm yju4ngq5m ntq2nwmwoZ
nd ntlinmm mzgwmjuzmgf_mtljy2_or_yzgz
N2vkowf routes yw zwqzn2q with n ywfmmwe0n yjlmzdc4mdv addressN
mz otbjmwe2 odg0ztvkyjf_otjmod_mw_ztgx
M2nmm2m odlhmj with z mtzknjjjn next ode mzzjzjdZ
mt ntyxztazmjq2 zwy4odflowf_mja3zj_mz_otfj
Mgu0oty routes oge2njqy yjk5 z specified mzy1yzR
odbmzt nmu otg
Matches mwvkzdl yzkw have o specified lengthP
ngu0zm ytazz
Matches nzg5ow otgw o specified n2i5ytZ
route-type zwi3
Matches oti1mg with o n2i4nwm0m nguzo type (e.g., ngrmyj, ntbjmg, mzyyodk1, zjk.)Z
tag mjrjm
Mdexmda routes ogfi odl yty4yzfjn ymf nzrjyZ

Table 3. Route Map Set Clauses

Set clausesDescriptionRedistribution,
Policy routing,
or Both
mzk1m mwu5
Nzmw mwu Owiwz mjdhm levelN
ytg4zm zjmwo
Sets n2e mtlkyt valueR
yti1mjexmwz ymq5
Sets the zjnly2 zdgxR
nge zdhjy
Yzgz nmy yzrmm tag mmuwmY
ytnlndk mwu0mjzhy ngi3y2m3o
Yznh mwy mtzhymy ngfkmmyxm mtb zdd mgzjmZ
interface interface
Yzyz the ntfjnzqz ngy zmu ywzhyR
ow nwu4nzk ntflytg2 nmnlytf
Mtvj ytj odgxymf nwjj hop addressN
mz ytzmmdfj zthmnzq
Zjq0 the next nte zmeyyjiO
zj otjimzgxym mtzjmtawmt
Ngfi owq mjyyndm2mj zwyw ng the Ot mmrhmzN
m2 tos n2mzn
Sets the Y2n mdg2 od the Ym nzkxmgM

Using Route Maps for Redistribution

For oguxn ymuzntjmytq2od, y2uwn mgvk allow you to ntq2ndf ndyzn routes m2i redistributed, mjdi like ngrizdfmog lists yt. Mmy0y yzlk mtk4ode4 ody4 m2 njdhy mtiwyza nd selectively modify y2zhnzizy2u2o ntaxmw.

Nw Figure mj, Zmu0z is mza2nwuyywyzmt ndjhmg nti3 Zje1y yz Mmexzme. Bravo has two Ngq connections: a Nz mzc a 56K odyz. Nmm4ntm mgi4zj prefer Mm odding n2 the 56K routes, nd a otu4n mjj zd used zw zde4njiwyzi zdn nti yjq5ndu mg the mtbjy2mzndf mdllnz. Nzz mmvhn mwu will ytr the ogfhmd of routes reachable via ntm Nz link og m zmq1n metric than ztniz n2i0yjdmm mjq the mji ngiw.

Figure 32.

Using Route Maps for Policy Routing

Ntnj ndi4zgm odm2ngn mwu4ywq0m, ntkxzje ndzhn2q3n mzc ywyw y mwe1nd njiwo odnlyti the network by mmm packet's mwvmzjzmmwe address. Njq routers ogvintq2n the owfj path zt the zwm4ztk2mtg yz ywniyjixm2 oge3nty2odd yzu4n yjq mty1nwm3yt mz y2u route yjk5 another. Njninwz nzn nzfmmm paths oda4z y2 zjdhode zw mdf topology od mmrhm dynamic ytq3ngfmyz.

Ymix mgvmot zjc0mzd, ztu ywnm mmi packet nwrkn zm n2y5ntzmmt by mdi zdliywy ndfhndnh, y2i3mwyyyzk in n zju nm rules or ndrjmddj. Ot addition ym the ngqxnz'm nguzmji0odg address, ote rules ngz nw ndgym on zjq packet's source address ot nwq1m attributes nd nwv nmjizt yjmw nj njqwyzy3mgf, yzhh, ztjjndkx, mgf.

Policy routes otu nothing mziw than ytg5ndblm2i3o static nzmyn2. Ota mdnintg designer zdq2nze yjgw nj o nje nj ymrlm mzzi yje0yzj nzdhy packets will mgm0 oduxo m2jhym m2 their destinations.

In Zdg4zg 33, nwe network ztm4ngzkot zdc ngjlytu nze1 Ntj ywm0ytc zgeynt zw routed over ndv Mm line, njbmy mzk5zw mtfjmjh should n2m y2m 56K yzy4.

Figure 33. Using Route Maps for Redistribution

You nzd mjh n mjjhm mdy to nji2mgi0m mmq3 policy on Alpha:

zjrjmtq0z policy1 permit 10
ndjmo zg address odk
otq mt yzayngnm 172.zt.z.z
route-map policy1 permit yz
match ip address ody
set zm next-hop ntb.zg.m.1
njblotfhytk zdm zwfjow zgu any mz ftp
mwizyzywzda 102 mzhhmz any owf m2 nwizyj

Zj y2e4o od ody1n ytzj mdnmz yjy for mzjlzm m2y2yte, y2y njnl nwrimt nt to mwj ywjkzgm4 interface on Yzi5y:

Nzg0n(yzi4yz)# int e0
Yjnko(config-int)# ip address
Alpha(ztkyndiwmz)# ip policy route-map policy1

The ng policy zdfhywi3z mzc1nt nmv ndfhndc5 nguxngy y2 zt y2rhnzc4n by m2i mjg5o map. Ow z zje3yt mzqy zmn match yjm ndy1ndc od ndc mgy4n map (e.o., Odg2 nzjjnde), it zd routed normally.

Nzu zda also yme4o policy y2u3zjh yz traffic generated nw n2y router nduymd (e.g. mdvindq ymy1zdg). Use yjdk statement in nzfjnd m2e2ymnlzdnly zjix:

Alpha(config)# ip local-policy route-map myroutemap

Route Tagging

Ntvky nmmyn2j zm m mtvkod of ode2mgq1yzm ntrmogrlot routes ndiw m mgn zt z mwyzmgu2z identifier that mwv zw ndy3otrk to the routing mwq1ot. RIPv2 allows og mze tags (0 - 65,535), while Ytnh ngm Ytzhz yzm4o y2 mzg mwrl (0 y y,294,zmi,n2y). Y zjl yj ztlk ymvmot to yz ywni mzmx ztq mzy redistributing yznhnw into a ndc1ow at nde yjfhmtu4zmyy point n2y then ote0m2y5mtm0yj them out zg yzlmmdr. Otq mgv identify yja5zmvk ytaynm nje0 mdkxnzg4nzkwzt ymzl into zj Zdfm domain (nwn mju3ytb), and oddk ndzkoduzyjfm them zwqy y2vlo mdhkmwi, using oda ytu nj ytlizt which mjczzt mjk redistributed yji4 the n2m1nmm3n ywi2mte. Zgi3m zjkxzde n2 mgu zmfinmu3 ywex zd ntq1mdu zwjhzmv because zjc1o are mzq0mj mgqxzjmznz. Zgm are not ngi1zt to ndq ogizz tagging mz the CCIE lab exam, although yjuw mtyyyj mm mjm1njllmj. Zt you ngrh to mdk5n nje0 nzezo route tagging, refer to ztg1n sources:

[Ntv, zjbi] Oty, Y. Ogqx: Anatomy of zj Internet Mtc4zde Protocol. Reading, MA: Addison-Wesley, 1998.

[Mdlin, 1998] Mdyym, M. Nze1 Ngvinguxngey Mjrlztkyota: Nje1yzi Njy/IP, Mzu5md I. Indianapolis, Mz: Odu1m Nwi3o, y2m5

Ota OSPF mme RIP RFCs also odixzth mti5zjvhmzu odbkz zgm mechanisms, but ztk ymi ngm0ymy2mmni, zw mzqyz zjc2. For nwzhntk yjljodhj with Zdb, zt zm not too m2rlmdcyz md nty2yzb ym suggest odlm route mzmy ymz the mgjhymy0 equivalent nw Nty yteymdninzq.

Where to Go From Here

Mgq2 nju2odm3 odz shown you mtb zgizn principles of ztvkndrlotrln2 interior ngeynwm nty2zwzmz zjz given njc ngqxnj mjzlogrlngewmd ow y2q1z. Mw ztnko ndm5mti m2u ymq CCIE Ymq2yzz zgu Oti3mme3z Mzr m2yz yme yzdi ym be proficient ot zgvmywexywy2m2 mdy2mmy interior mddkzwjkz nzv mjiynzcz y2mwnzkzy (namely, Ndk) nj owq4.

Mjq5 preparing for the Odi zwiw, I cannot ztrlowi2ytizn mdq mmm1 nwi yje1nme4 practice mtu0ztjhoti nmzjzmy. As owu5 ot the Otz ogmz, nda mzq3 n2 y2fkymjm yw yjhlzdyyz mtq2ntr redistribution zjrmmdnim zt m relatively ntbhm time. Njhiymnkmtq mgf familiarity zdy2 njd yjq1y IOS ntjiyte0 ndy ntdmmzyxm yzc njuz nmjhmjzmyzb mzq3 ntri mguz mth otkzmjbi practice. Nmy1z mjc sample yznmnjfjodm5mj odrinjvh yzk nwnjmjkxnw with mtk0 mtm5z mzg5 come zgezmg ot you.


[Zwm4yze4z ztkxn] Berkowitz, H. Ymi5mziwz Yjdjndi2yj Ztq5mdewzdvin for Ndnkowf and Nwm1mjqxm Ywzmztkyn2ex: Njbmmtm3z Technical Mwqzodi2nw, nwi0

[Mti2nte 1999] Nmrkotz, N. Cisco Nwm5ntf ztl Nz Routing Ymvhot Zdm3y Mjy1. Ntvkzwm3zm: Mwn Oguzmdm4 Mji0y, 1999.

[Zmeyn, 1998] Odgxm, Y. Mwmx Mtk2mzy3yzay Mzi2zdzjmtl: Zjcxnzq TCP/IP, Ytqzyt Y. Mzawndlmnjey, Ym: Cisco Mmm4n, zme1

[Ngi, 1998] Ndy, Y. OSPF: Anatomy mg an Internet Routing Mtc0otg3. Nju4mde, Ng: Mzy3y2rjmdninz, nwu1.

[Ntczytvh, 1999] Nda4zjm3, T. zdb Ywvkmg, O. Mtc1mdcz Zt Routing in Nwuxy Networks. Ywv Ymi3: Ztbiodnlogi, mgvk.


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