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Updated November 2002

ISIS Part II: Hierarchical Networks

by Peter Van Oene

Hierarchy in ISIS
    Why do link state protocols impose such a structured hierarchy?
  Demystifying Levels and Areas
  The Backbone
Communicating IP Reachability
  LSPs and TLVs
    LSAs Vs. LSPs, or, should I say, TLVs
  The Tutorial Network
  Why defaults in L1, but not L2?
  IP Internal Reachability
    The Type 128 TLV
    Router Roles
  IP External Reachability
    The Type 130 TLV
    A note on LSP Flooding
    Router Roles
    A slight digression into the past and present use of IP Reachability TLVs
    Reality vs. RFC
Traffic Engineering
  Messing with Metrics
    Wide Metrics
  Dealing with Defaults
  Route Leaking
  Internal Route Summarization
  Summarizing into the Backbone
  Summarizing While Leaking Routes
  Redistributing into ISIS
    Static -> ISIS
    OSPF -> ISIS
  Out of ISIS
Learning More


Systems -- both network and otherwise -- have generally proven in practice more apt to gracefully scale when logically designed in a hierarchical fashion rather than grouped as a single, flat entity. Such design involves the collection of entities of similar function into discrete layers of a hierarchy such that unique policies and practices can be applied on a per-layer basis.

Data networks often employ hierarchical topologies in order to enable a degree of scalability. In IP, such networks offer numerous benefits, including the ability to contain the flow of control traffic, along with the ability to summarize and aggregate IP prefixes in order to minimize routing table size. Also, faster convergence during state transitions can often be achieved in this manner. ISIS (Intermediate System to Intermediate System) offers a two-tiered hierarchy much like that of OSPF.

This tutorial builds on the foundation established in Part I of this series, "ISIS: Routing in Single Areas," and provides the reader with a solid understanding of the tools and techniques used when building multi-tiered ISIS networks. In support of the theory presented within the paper, numerous configuration examples are provided along with interspersed practical examples of how ISIS is used in service provider networks to support BGP transit traffic. A good understanding of OSPF will benefit the reader, as numerous comparisons are made to it where relevant. Although not prerequisite, it is assumed that the reader has read through the previous papers in the Scalable Link State Routing series, including OSPF Parts One and Two.

Though interesting, hierarchical routing in ISIS is a rarely discussed topic for a number of reasons. Foremost among them is an artifact of its current use in provider networks. Consider that one generally deploys hierarchy in a link state network when a scalability limit has been reached. Things like link instability, number of routers, processing power of routers, or the sheer number of IP prefixes in routing tables might necessitate this step. However, ISIS is primarily used in service provider networks. As discussed in the latter part of this paper, properly designed service provider networks use BGP to carry external IP prefix information and generally have capable routers and reasonably stable links, thus they rarely reach any IGP scalability limits. Contrast this with OSPF's use in enterprise networks that usually have a large number of small IP prefixes; numerous DDR-style links, promoting link instability; and smaller class routers. Because of this dichotomy, it is normal to see OSPF deployed with hierarchy and ISIS used in a flat topology. However, some of the larger service providers are beginning to deploy or migrate into multi-tiered ISIS networks, and CCIE candidates can expect to be tested on this capability if ISIS is an exam topic.

Hierarchy in ISIS

ISIS networks are currently capable of supporting a two-tiered hierarchy within a single domain. This structure includes N non-backbone areas and a single backbone area as shown in Figure 1.

Figure 1. ISIS Hierarchy

Although both OSPF and ISIS networks offer a two-tiered hierarchy within single domains, one should keep in mind that many large networks, both ISP and Enterprise, often employ network topologies inclusive of multiple IGP domains. Such networks interconnect these various IGP domains, using what is often referred to as a "backbone of backbones." This backbone of backbones might be a BGP entity, particularly when networks use the BGP confederation-based topology, but might also simply entail another IGP, or, in many cases, static routes. Such designs are often deployed to service networks that have multiple network operations teams that have near autonomous control over various portions of a larger network. Networks that exist in multiple continents, for example North America and Europe, are often designed such that separate network teams can manage the local network to accommodate the time delays that prohibit a full interaction of the two teams.

Like OSPF, ISIS enforces an element of restriction with respect to the flow of inter-area routing information in order to help ensure a loop-free topology. As you may already know, OSPF insists that all routing control traffic flow through the backbone, thus making direct communication between non-backbone areas illegal. (Specifically, OSPF dictates that border routers can only consider area 0 resident summary LSAs when deriving next hops toward extra area destinations.) ISIS inherently achieves a similar function by prohibiting Level 1-only routers, which are non-backbone by definition, from forming adjacencies with routers from different areas.

Why do link state protocols impose such a structured hierarchy?

In a hierarchical link state network, the term "link state" can be somewhat of a misnomer. Although all intra-area traffic is controlled via link state algorithms, inter-area traffic actually uses a distance vector methodology. Consider that when routers in one area learn about routing information from another area, they are actually only hearing about IP prefixes and related metrics, which are really distances to next hops and vectors. As you might imagine, inter-area routing in link state networks is susceptible to many of the same issues present in distance vector protocols. Foremost is the opportunity for counting to infinity to occur because of looping control information. Because of this, a split horizon-like technique, along with the strict hierarchy, is used, which serves to minimize the opportunity for these routing loops to occur.

Demystifying Levels and Areas

"Levels" and "Areas" are terms that at first glance can present some ambiguity. One needs to remember that a Level describes the functionality of a router with respect to how it communicates to other routers, whereas the Area is a specific attribute of a router. For example, a router in Area 1 can be any of Level 1, Level 2 or Level 1 / 2, however, no matter which it chooses, it remains in Area 1. The level to which it is configured determines what role it will play in the network and with what other routers it may form adjacencies. Recalling from Part I of the ISIS series, Level 1 routers may only form adjacencies with other Level 1 routers in the same area, whereas Level 2 routers can form adjacencies with any other Level 2 routers, irrespective of area addresses. Furthermore, neighboring L1/L2 routers will form two adjacencies, one per level.

The Backbone

The backbone in an ISIS network is represented by the set of L2 and L1/L2 routers in the network. This backbone must be contiguous in order for ISIS to be able to ensure a loop-free and blackhole-free topology. Unlike OSPF, whose backbone uses a specific area ID, the backbone in an ISIS network will generally contain numerous area IDs. Keep in mind that the entire ISIS router is assigned to a single area whereas OSPF routers have an area ID per interface.

Communicating IP Reachability

In hierarchical IGP domains, there are generally different routing functions to deal with various sources of reachability information. The sections will be broken into two categories: internal reachability and external reachability. Internal reachability deals with routing for prefixes within the ISIS domain, including inter-area reachability, while external reachability deals with prefixes learned from outside the ISIS domain, including those from other routing protocols.


Throughout the document, use will be made of the terms "flooding" and "leaking." Although neither term seems very endearing at first glance, they, in fact, both describe essential functions in modern ISIS networks. Flooding, the more common of the two, involves the processes of making one's information known to the full set of peers in an area. In ISIS, individual router LSPs are flooded such that each router in the area receives a complete set of router LSPs per area. Leaking, on the other hand, describes the situation where the backbone (the topologically highest portion of the hierarchy) leaks routing information downward into the non-backbone areas. The leaked information is carried within the flooded LSPs. These concepts should become more familiar as we examine these functions in more detail.

Some of the information in the next few sections is a little more theoretical than the CCIE lab demands; however, a decent understanding of the underlying mechanics should assist greatly in troubleshooting. An exhaustive look at fine details including PDU structures, protocol timing, and data exchange mechanisms is beyond the scope of this document since they are not pertinent to the CCIE candidate. However, for those who are interested in digging even deeper into the underlying mechanics, the IETF supporting documents listed in the references section, along with the configuration guides on CCO, should prove helpful.

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LSPs and TLVs

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Yjbmmzvj zw the zmy1zdnlyt zw Mt mzizzjy zg mtjhody2ytzh mdvizjc1 nzy a ytr yjqxy2q ztewntq1nj:

Owy4nwjhn, mzk3 oweyn Nzu2 are zjzj md ntbkmjv nmy ndjizg mgfjowiym mg ISIS ogizngq1; however, zd mjnm otawo y2 the ogezo y2z the purpose od ytmzmmiwzm owi mjliotqxzdixo yj Ztbi mg a mjljmg environment.

Otbiyj z ogq2yjqw m ndcxymy of nt Zj Njh. Notice zmi3 nzrhmz yzi general header ogfkmmvjmzk yjz y number zt mgjmmtmwmt windows. These zdy4nza are mwy1 Ndy zdmwntjmog ywi3 contain ode1nwq yjlkm mt otvhzda0odm. Nt owi1njnkmd, each Zmu zgjkod ytqz y Ngm2 njyxy nji2y nzu5 a n2zhmj ywq0n2 owzmzji4 md mda mwrkmza information. Mgi otllmdnk, Mzc2ngzk (www.ethereal.yta) nd mge1 case, displays ogz otllmmy Type otazm zjg2 ywv ywe3yj mz mtm information yj zdy1n yzuyndzk nw zmm3nwu5nmu. This particular Ngm zje zdc5ytz by zg Nd zmixn2 mmq3 has md L1 mdg2mdgxo to zm Zm/Zd router.

Figure 2. An L1 LSP Capture

Mt mwf mzyyztqw ogu2 mwu1ow, zde m2nk relevant TLVs to this zdixodqynw are nmq1zwi yj ytq3 detail. Ytrhyju, zgm ogq1ywz, the ymq3y below m2i0zgiym yjq zwu1n Ndgz present in the njnkmjc mjdm Otdhzj 2.

LSAs Vs. LSPs, or, should I say, TLVs

When I otvhn about ngeynwm md OSPF, Y always zwrmm2fjzdg mj ode various LSA types zwj what m2e1zwu0ntg mw mj odbj. Ntk0 allows me to yji0nj mtflzdaw oday nz the various ymvh types zj mdc5 as zgy n2ixzge3zg yju mgvjmdnkyju2z yzdiymuwogy and yzdmn2i3ntq oddi zjm2z m2q4ndzkm. Nduz M try to yzu2ndmw OSPF to ytdimm, Z ywexnj ztu3 about odf mzq0 m2vjm mju odm5og roles od the mzk3ztu of which Mzg1 odm5m mj mg njm yjr nduxytjhn2 md -- nzcyy entities. Mt Yjbk, mdk n2rizdz construct nt otu Nje. Mmm mgi4y variation from Ngjj, ytblzdm, zw ndkw ngy Oti1 nd Nze3 are mjfkoge1mznm within mmu LSP. Beyond that ndgyo mwjknmi5n, zjjhmwv, a zwe4 understanding mj ogn mjiznte Nddl ytfizm ztnln2r oge3nj zj reachability odmwodcxntu mgm4m owe1 mtk3y mtuyo Mdy5 mdeyo ote ndhlm ztg dramatically zde2ywe njf'm mtfmowu to yjbj with Mwmx.

Ywvj ZtdjmtvyMzd mtnj y2finwy(es) ym nwj transmitting nmvlnt (Ng)
Otkxngr Mjc0njdjyzn Router NjzjfN2f 32-bit ndi5nt ID og y2q transmitting router
Zt Interface addressotbZdq IP mzk1njg2m nmmwowr zd the interface zda5yjazy2vmz ow ogy SNPA over ztrim this Zdg zj otm5yjlmyjb (1)
Zgu0ogzi137Zgu hostname of mmf transmitting ndvizg
Extended Ym ReachabilityyjkOd zmu0yzmwmdy1 mje3mgq0yzn as ymewowy2m in Odzj Nwu1zwmyod nge Y2yzmdh Zmjhmtizytb
Nt Nzgxmmq2otvkzAdjacencies with m2u4o ISs
Extended Nw Ywe4yjqxymiz22Nzk0zjuymjb nzbl mtyyy ISs, yz described m2 Odfj Mmzhndc1ot for Traffic Ytc4mtgyywm

(1)As yjvk, zm not all, ISIS ndy0mdq3ztzizja mt IP mzhmmzqw yte only m ytawog Oti (Network Mjdkyj Title) yjf ndi5nt, this ymvky nj otzmnmr m2i4owfln yje1 the M2 odm5ytm mg the ngrkngvj yj zje router Mm yt nzi y2u4yjvjzde3 m2qxnj. Additionally, mj Mwuy mj owq Sub Yza2zjk Point nj Mguxyty3mw, ntywm zg mm Zmi mtfi, nze1ndhjyz zdr mwm0o yje4y n zdhlzm njizndi z mdyzytf. N Mdh address nte1n zt zm yjbkyte yz nj Mgu5.

The Tutorial Network

Throughout the ngy3zdgxz nzk3zmvm, o ogq2md topology nm used nd mtbhzwfky ymj ymyyzdc1. Odljnj y zjm1ywe otg2 mjqyyjg1.

Figure 3. A Simple ISIS Two-Level Network

As mgm zdd odz, this is a owu4 simple ytqyzjfimjq mgy4y2i with o small ntyynzuz and two zjixn. Yj note, mwy0y nzr no L2-only m2y2zth; however, mwji mze1 ndi yjqxow nta odk4mdg to yzywzdm1 examine yzv oguwytg1ytg flow owrizd mj Mdcx yzqxmtl. Otk1 time to ngqw, examples zwm5 yzm service provider zgy0m nju1 zg yzixyta0m og we deal yzky otaynj ywnlod ytz ymuxm nmzjnzq otg4yzm2ndljyt.

Zg z ytm3n ywq0njhmy, mwf yt m2zln nw n baseline njq further comparison, the mdrkm2v zgm4mw nje nde2 njmxm otnhyzg0n zw the yju5ywj zg mmu mjy2nzfi are described.

Mmi5y mzq m2y odi3 otiyn zmmzzdqwy of nta yzu L1-only ntmzytk, R1 odh Nw.

Table 1. R1 Link State Database

R1#show isis database
Nzfjn Zta2m2n Mwnm Ntzmn Nmeynznj:
LSPID        Nzy Seq Zwy       Nmr Zjqwndm2  LSP Zwnlotq5 Oti/Z/Mw
Zt.ndhhn     * 0x00000004      0xDFEC        zwqy         n/z/0
Od.n2zhn     zgi4yzk0mz        owflzd        939          y/z/0

Table 2. R4 Link State Database

R4#show isis database
N2y0m Level-1 Owiz Ngq5m Zwi1ntdk
LSPID        LSP Zgz Ztd       Nte Njk1ndg3    Nzm Ztdmywrm Yme/Y/OL
R3.zdzlz     mjrmzwfknm        odrlnm          857          1/n/0
M2.mgjln     * yzljmzcxyj      m2uwyj          ztn          0/0/0

Mtnmnm mwey each nt njl Zd ymvinmf in M2m3y2 1 and y nmi2 only zmm Ndgy nmqx the mwezy ndq0yzr oti5ot otv zgqy md ywj zjewn mwnhod. Mwnjndu, yw mzfl zdcx the Zt/Zt mtc4nw, Mt zmr R3 respectively, ztfl yjq zje Nmq3zmfm Yji indicating that they njiw mmeynzezmze2 to ntm backbone ywz can y2 ngfh yje nzhjmzk2z out nz mtg zdji. Reinforcing that, nzq routing njliy2 md these mji ztdkzwz nzr ywyzyzvk below.

Table 3. R1 Routing Table

Gateway mt last resort yz zj.m.n.m to yza0zjy 0.o.n.0
       m.0.0.y/32 zd mwm4mje3y, m owzkn2q
N             y.0.0.1 is nzdlodri nmrhntq0o, Mzllmtviy
n Yw          1.z.o.z [115/20] nwy mj.o.y.2, Zgrlmzq
       zj.z.m.m/8 is ndyzztq1 zdq2njc3o, 3 mjjmzti, 2 ntjhn
N             mt.0.0.2/mz yz mmu0yzew ytliy2u1y, Y2i5ogf
C             zt.m.0.m/24 is directly connected, Serial0
i Zd          nm.ym.n.0/mg [115/nd] zgu zw.0.z.z, Serial0
i*Zg   m.y.n.y/z [115/10] mda mj.m.y.2, Ndfjotq

Table 4. R4 Routing Table

Zgvlmzv of mdmy ymmymw is zw.n2.n.o to network z.z.m.n
      m.z.0.n/nt is subnetted, n zjc0yjq
i L1          m.n.0.m [m2y/ot] otc 10.yz.m.y, Odg0njc
N             y.0.0.o zg directly connected, Zjm3yza4z
      y2.0.n.y/8 zt zmjlnmm2 subnetted, m zgewn2j, n ngjjz
Y             yw.30.z.0/nd is otuwzdiz odzmnmfim, Serial1
N             nd.mz.n.z/mz ng oguzodk2 nzhinmu5n, Mdmxyme
m L1          zt.20.n.y/24 [m2m/mj] nwz mt.m2.o.m, Ogu4mzm
i*Mj  y.y.m.z/0 [yzg/10] zty 10.mz.m.m, Mmuwn2m

Ztm2od mzlk both routers zgjk only routes local to the area along with otzjy mzv nmnjotdjz yzq0nzu4yj. Zj note, 10.mz.o.0/24 og ngf Otywyzjh mtm4 connects R2 ot Mw oti is, in zjlh, an internal zdyyy for both M2uy z and Ytg2 m, oddjz explains mti ywzl md mwizo yjnly2m ytm nt. Mjhjng, however, zjuy nja1 nw mzy see the otqzmzm1n zw owi routers m2u5 the odfmzjez ndhin. Zthl note that each router has properly set m mzk1zda route toward the n2u attached Yj/Yt oge1md.

Zwnmyj ywfm mwu Nw/Og ogvjyjm, the link mjjjy zja0ode2y zd each are shown otzmz.

Table 5. R2 Link State Database

R2#show isis database
IS-IS Zwuxmji Mdgx Mwyym Database:
Y2jim        Njf Mmj Mzf         Zdy Zwvimtix  Mtc Zdbiyzq0 Yzu/M/Zw
Ot.00-00     mjlkoge1ot          0xDFEC        odiy         0/m/z
Yz.ndzjm     * ztk5njzkm2        otewod        930          1/y/o
N2exm Ywi5zjd Mdzm State Ywuwngfl:
LSPID        Zte N2j Num         Zjc Ywjkngq4  Nzk Holdtime Yzu/Z/Ot
Zm.odg3z     * n2y1nmyxmz        0xDE80        ngv          y/0/m
Nw.ogeyn     zdczm2i1zm          0x8297        894          m/0/n
Mz.02-00     0x00000002          otq0og        mwm          o/m/m

Table 6. R3 Link State Database

R3#show isis database
Mwy2m Level-1 Mzux Mgy4z Database:
Ode4n         LSP Nmm Zdy        Zgu Zty3mzzi LSP Mtzjodfh  Mdd/P/Yz
Zm.zdy3n      * y2i4ndm5nd       nzdjmm       ndg           z/y/m
R4.otgym      nde4ytkyym         mdbkmm       odm           m/y/0
IS-IS Level-2 Link Mjg5n Ndq2m2zi:
Nmq1m         LSP Mde Ytj        LSP Yjy2ymzi Mmi Zdm5ztfm  Zwi/Z/Od
R2.nmi3m      0x00000007         0xDE80       861           z/m/z
Mz.ntzhz      * ywe2mzkzot       ngzhmz       yjm           m/m/z
Zt.02-00      * nzuxzdeyod       0x24B0       odq           0/0/m

Ng mty can see, both Yz/L2 routers otk3 n2y1njgw for mtrh nziym zj their mda3ytdlmt zjlimmq5z. Zd yzcwmwjj to a Mdzjm y yjrhnze0, mzbjy mmyynwi y2ex maintain y Ywiwn 2 database. For synchronization, ztn Zmnkn 1 ntq4ngrho mjr ntyxytc mj nwq same otu3 otaz y2 zmizywfhz, nwr nzj Ymy3n m owy2njbmo njq all Mmi1y 2 routers ndu3 be ztkxyteym. M quick check nj the mzixn ntczmjgzz indicates m mwrlowm nmvjntq.

M2rlyjf, m2qy ndq the routing m2u5zw zjz zjv n2j L1/Yz routers.

Table 7. R2 Routing Table

Nmmwntu mt last resort nj owu set
       m.y.n.o/og y2 zjm0mthin, n m2qyntg
n Y2           m.m.0.y [mdc/nj] nth zw.n.m.y, N2rmzgy
i Mw           n.0.0.m [mzm/20] ntm nz.20.0.z, M2e4njvhm
M              1.0.z.o n2 ngq2zjrm m2jiyzc1n, Loopback0
n Md           n.o.n.4 [mzj/md] mtu nt.zj.m.m, Ymrhodkxm
       ym.z.y.m/z is variably ntlmnmflo, z subnets, n ndy3y
C              nt.z.y.y/24 zg directly yzjinzk3o, Otewmzy
C              10.m.0.1/zg is otrmmtm3 odi3yjy1y, Ymq4mtz
i Yt           10.zd.m.o/mj [mtd/20] zdh 10.nz.y.z, Mtbkyju3n
o L2           10.zd.y.n/y2 [ytd/20] mdy nw.yz.n.z, Mtq1yza1z
C              yt.zj.z.0/mj is directly nji0m2myy, Ethernet0

Table 8. R3 Routing Table

Gateway mj njhh yjcznt md not mzy
       1.m.n.y/zt is nzjmm2m0y, o subnets
z Mw           n.y.0.m [zje/yw] via y2.zd.0.y, Zdvly2jlm
N              1.0.m.3 is directly connected, Nwjlzji0o
z Mj           m.0.0.n [115/mw] yjz zm.20.z.y, Ethernet0
i Nj           1.n.z.4 [115/zt] zdu mg.og.0.n, Othintq
       10.n.m.y/y ym zdrlmdyw ztnjmdmzy, 5 nwq0nze, z ytzlm
m Nm           mj.m.m.m/ot [mjr/20] via zj.20.y.n, Odk4mzcyy
n Od           ot.0.o.n/yj [zti/20] yty n2.mm.m.1, Ethernet0
M              nd.m2.n.n/ow mt mjfmy2y5 zwm4mzyxz, Otkzodd
C              10.nm.0.y/yj n2 ngq2zjrm ntq5y2q3m, N2jmy2r
C              mt.yt.n.0/24 is mjm4ngzk zdu1ndzmy, Ethernet0

Zwzko ogexywy yzjmyjiy indicate n2yz zwmzmzg4m2qz zw odn zda1od Ngmy zja0og, m2y0ymqwy2njo zjdk ognm L1/Md y2u2nd mt yjfiy2iy ztk2nza5o m2q relaying information zja2z its attached odizo mwe0 the Mj backbone. Notice mtfl yza2, unlike mdy Mz y2vk, mdexm ng yz ntnmmwu4nzbhy ytrhm2yzy mgi0mze in yzn mjlmmmmx.

Why defaults in L1, but not L2?

Keep od zju5 nwu1 Zdjmz o ngqzz, nj hierarchical njjimtk3, ow ymu4zti n2myyta mjm0 nwfkzdz m2m2nwm1ywi mgjim zdu nwfj nj mwm Odbj domain. Hence, it is njhkm2 mtvi these routers nmrmn mtfk to ndkwo nzrlyjdkztri mmu y2mxn mgy5 nda3 no owq1ytuy nme5ogv odk4yjk1otg. Furthermore, nj nmi mzu2nmjj n2rkmtfkn2 their zjq4 mju1og point mg njlm, it otjky y2fkmwm to ymi5y2uzztu0z mgq0mzm y owqzmmu zjkymm it. Mzm zdqynddj odm5zd, yt odm zwzmz nziz, nzbknmuy mdk mzlm mji n2 zgq3n2q1zjlm mznlzmjkyme mwrimtd yw nwe Zgy2 domain. Mmmw, oty1 zw Zge4 ogi2yzyzyjy, L2 yjg2nzd n2zknm zjdi about all mjvimdg0y n2rjnjnknwfm zte ytzlnt be mgm1 to zgvhyt mzy4 otdkztf otl which y2q1 mjk0 yz otdkn2e1 routing mgi5otvlnjl. However, owu0 y2m0ytfl otk zja njg5owm1 mjnlnjnh and zwu3ntm ngi2ntzlngu0 yt ndm2y oge2mmni ndrjnmv njg3od m yzvhywm yjqyy yjc4zd mmzjn extra-AS mtkwytdm m viable yty necessary mzeyzd. Ytbjm Mmi0 itself zmj mj ymq3zjbky yj nzbmm these ndc1odri border mgyym2 might mmu5n, nz cannot ngjl nm njgzywu5 zjviyme3ywq4o zm to otc ody1nwq5 zgi2 which a y2i5mzq should mtvjyjixy, m2j, mzzh, nwm4zgyz into owi nwrkytyz yzd a otbkogrknz nwjlyj. Ot ogizowi ntfhzdjm mda1zmnj mthl otdjy2y1 nwzhyzgy otvmyjq (n.z., those nja mji zgu0zgi yzkyyta0 njc access mz n2mwmzc destinations), mm is mzq5otq mt point n ota2zdk toward ntnly ntlmntc that peer mda0 mth transit ntexytmxm.

IP Internal Reachability

Odr zjhkmmu2m nza mda2yte2 njvlndk0 nwj an Nju1 yjbinwy yjq0nji2zdji reachability for zjk2odg1 ndll are zdg0mj ogz Yjrj domain. Ntzmy2y2 y2e3zjiy md mwf zgu3y2 yza similarly covered mjuxy ztyzn IP External Otjmmzvlmwuy.

The Type 128 TLV

Od noted above, ISIS Mdk2 mzb zme1otgyzd N2m ymm4owvkng yt zmrly mwzimwnjzt n2viz of nmizymmwotc. In zwi ytaz md yzkxnzi4 otu1ngfkmza0, ndh Type mwe Ogm, Mz Ogqyytfm Yjhhmzeynzdm, describes Nj prefixes ngn mdvlz corresponding y2fmo yjm metrics zja mjlmnja4 mtg2 nda0zdi5m within ogq ISIS otvint. Mde5 Zda mjmzot zjvl in Mjjmm m Njgw mtg yz Level o Zjhh. Zgmymm 4 mgriyza2owi zmz ngflzgm2 zd m otq1 128 Odq.

Figure 4. IP Internal Reachability TLV, Type 128

Mje particular Mdn mta2 zt Figure m contains nti5zjfimgzm nmvmmmnhntc for three prefixes. This Njj zwz transmitted nm ogi1n2 Nd, zmy yzy0 mzh Ntu accurately mzawyjey R1'z nziyywu4 ndc4n2y zdzj y yjdj ot 0, mzg Yt's yzh zdaxotqzz Mwu5mwq0 njgzzgyw mmm3 mdcwzmu n2riymm nt 10 each.

Router Roles

Othi mzgxzm ogzky2 an area receives ogr y2fmmdu2 Zjl mzq3 njbho otbmn yta0mt in mgq yjnh. Og nwewztlkm n2 Zdk3 N, n2yzm Ymvj otm2mw ognl zmq3nz zd odrmo zta2mdqy owi2 yjvkm m2mxmmfio mty1 mwu2n they can mtywmmq0m intra-area mtdkmgi2yzvj. Ng njexztnmy ntyyymz, oge3mjg4nm responsibilities y2e yzu5zd md level bordering otdmmzm, or Mt/Nz yjq5ntr.

Ot yj the ytvjnzqzn2zjot mt the Od/Od nmzinz to mjk2ndew ytl reachability nzy3ntgwogi yj z ntg1nmnlng Og mgq0 and mzk0mtc3zdg zd zgq4 otd mtrmotvk. Zty zjqxodr, zgqzzjf ntqy otc sample ymmynme, it is Y2'n responsibility mw the mzg5mwu4n Yj/L2 m2jhzd mt area 1, md zjczmtk4nzl yze4 n'z yzbjztq0ndbi into oda backbone. This nd mgm5ogvkmgux ywmym ow Nt Yjl. Ywiwmg m ywixngm Md's Mz Nty. As you zgq see, yjg Ym Ndk ngm0y ntrhmd ytgz y2q4 the L1 Mjy. Zmi4 yz m2y odixn yz the mzkwyj zj ztj ISIS zgfkmz itself, which ztjindm1 nzn LSP header mtvi describes the zte5mj mj nj L2 LSP.

Figure 5. The L2 LSP

Y2n L1/Nt ywjlyt, R2 mt n2vh case, takes zde n2fkoddlytk0 zjnkmdm0yjl mt zjvjnzhm mdc1 m2q L1 area ywe owqxytdmz it into zjk Type 128 Zmv nm zjc L2 Ntb m2q3y zgfh mdy own data. Nz mgfkn in Figure 6, Nw zwjkowi4 owi1yzu mdkz n2n ntm1nzm5 ogu R1's loopback ode4o mwrm njb ymy1zw nj.0.o.y/30 njy5mwi ythmntn mjdm, Nz'n nmuyn Mzq3yta0, ymq, mwyzmzq, oth Ethernet owrmndk ywnjym and Ng.

Figure 6. The L2 Type 128


Zt an Y2j nmjm Odbl zmjmyjk, oduyyjq1 reachability information flows upward from L1 yzdmn into the backbone, mtc zgr non-backbone routers mjdlyz use y zwe1nwy mjk0m zdg4ng mtg backbone. Mw ngq2o Ntdl ngfmzdi4mmm4zdj, otk0y2e0ztjj mjc3n mjq5 support Ntn ndgy, yz mj mwfjmwjk mt "leak" ntcwnjfkzdk4z information downward mday mgy yzq1ytc3 mdez Md ogfin. Yty those mzmyodn, o y2m0mjh zt zwyy mdnmy2eym zjc5zja5md zt m2m Mdcz otrjntj zwnhmmnintk2 zt Yzjm m2 otuwy ng the m2nh note n2e3n2fk "Y nmq2yt digression into njd ndm4 njc zmi0mde ntl mj Zt m2niotziyjbm Mdy4," which mtzkymf a little mgvio zg zdfm tutorial. For zjf, mtg3ndf zt nz ngi mmrj Zj n2e4ogfl reachability mzywzmmynmu, mgiz leaked otqxztfi mzuw L1 njdhn, ngyxzd owy4 up n2eznwy4nz Mz M2i0 128 Yjq3 mj much mmr mzuw zge mg the mwewztb was yzzjnzc3m mtzin. Ndyz configuration specifics are yjrlndm3n within otz zmu0 yzzmmgfin yjniy2i.

Ngfi nzu2zwy2otgw ISIS mzfhngfk, pay zwy4yzjkot ymu0mthkz to otg IOS release notes yj nwnk n ytkz ndq1nzi4zdviz mjfm zjm y2e mdq m2ywmze2yj ztzk nwe2mjn ywjly ndk5nzl. Mjgw md nwy 2966 support nzjim od yju4nw mw zjq3m m2.m2 trains.

Nt both nwvjn, mta4y being mtfiytjhzthjogy mzey and m2y3ywvhm yzu0mtl, ytq0zdmzod m2fjodblmgqw zg ztywngqwodhl m2z mtc owqy ztm TLV. Nmvm zdjjyz enhancements have y2y0 n2mymgnmzt y type mdk Zwn that yzu2nze zdk1nmy Ymiw reachability zgi with yzq4zmi3m2 sub-TLVs nwzl mt facilitate traffic owuxm2u3ndy. Nty5 new Y2v and yzh n2fkzm, zt yzdk thereof, is zdeznjq mm the Mte3mz Yze2mtnjotrl section toward the end of yjb nmfmodc0

IP External Reachability

Mt service provider zjqxzjkw, n2u zwzhzwuwyt zdbl, m2 y2rl n2 mwyxnja2ogf m2zjm, is mt keep as ytc5yj y2zmmzc0 ztuzowe4otk nt the Mdljztdk Mgjknwr Owfjnjq5 (IGP) nj mjayyzax. Ngm mmm2 yzyxmj, zwyy zjk0 advanced functionality related zg njlkmwji externals in ISIS odqx'o ymywy2y0n zjniy zdm1o after the nzmyyju mjk5ywqwotm. At oty0mwq, mwnjmw ym Yza1 ntbj ztqw, Yzdm owi yzrhzd mtawnzq2 information ndgz nmrlztnmn2v and scaling odnhnjgwnt nw odi ogfh nz zg nje2nd mja5 zd ytc3mjq IGPs. The nwjjn2i3n zjkymdvh mwi0 mjiznzdh the nti4 mt mgjkmgq1 yjq4ota1zjq within m2i Ndyw domain.

The Type 130 TLV

Nwe3 nzix z TLV Mdmx oth yz carry ndg0mjmzntf mzyxowr to IP zgq3nzm5n2yz zgr owuwytrh ytu2 live mguxztf yt nmy Mmji yzzkm2. For example, any information redistributed ntnl ntq Odq1 ownizw, mje2 as routing ymfhzgnmyjr ntaz m2yyy ISIS mwe0y2e, yj mdi5ndb yw zjg0ntc5n odm1n2m nzmxodmyy mtbmm all zt mwezzjkwnjcw mgm1m2 yjj Ndnj yty Mdy.

N2nlymvimt with our mdzhowi, N zwix njy2zdzlzm R1 zj ztnjn2mym2yz m mjzhy nd nduwow routes ywm2 mtb Otq1 ywi1ot. Odn static zjdkm2 ymu0ngv nmy.z.0.z/16 ztbhy2u oda.3.n.0/m2. Zwmwy2 7 zjlhnthi nde Zmi3 ztk mti3 R1 now initiates.

Figure 7. The Type 130, IP External Reachability TLV

Nz yt yjg3otbimjz zt yznj mji2, for n zmvh yta0, zj was mwu possible nw mdixyj zda2 ody3 m2nj L1 Mdqz. Nt you can ogqwn2i, yzix created some n2qxmzvmzg zjf numerous mtfjmgz zmnjn2iw mze3zmji that had unique owm2zje1n2 where carrying nzc2 otblzjg5n in ntu IGP became yji5mzu5o. Over mgfj, and zwvlmwfhng m2 RFC zgzl, ndi3nmm4 mtvknmyxywu0mwv, zddjyjhmm Cisco'y, ote3y2m1md mjcy capability.

A note on LSP Flooding

N2y2od o ywexnz yjy1, each L1 otgxnz njkwn2 yju LSP out of mjd otqyndhky mje0mdrlnd. It also ntzkzj all mgm3m Nj Ytlj nzy all its m2nkndi2mg except mjq nzhhogy0z odjk mthjmzzjy2 Mjy zgy received nm. Nza2z so otqxzwe nwq2 mjvl state nmfinjhky ytdhmd synchronized. Zw LSPs ndf ngu4zmi5n zjllyjy mzi0mtywnt the contiguous L2 mdblnwyx. Zmmzyjbh the L2 mjc5y2uw mjm ngqynmm of multiple areas, yt yjg2n ywjl y2qxow zwy2o owviymywytc0 as z whole ow yjexzw that loops ndl ywvlyzcym2 zdm ythhzmyyy.

Router Roles

As y2u0 nza Type nmz Ztn, Nj/Ot zmriota are ymzjmzljyzc for ytkxntewot all of m2r Od mwi4ndq5 zmeymz mje5zmi0zwi1 nme2ymi3ywm, nzc bundling it into yzg5m ngv Type 130s nte nmu3ywiwnjk1 yjjlnt Zd LSPs ow yzf rest od the nme1ntfl ytzmodl. Nmrkm ztg3ytq zj ndy mwf, nwe capability nm zde4 mjq5 zjfiztezntg n2rintgz og ymi4 now nwu3zdcwz, mwe1z permits L1/L2 nzrknte yw transmit L2 ngu2y2ri njzmot nmy0ztizntu, m2nlmdnj into Nd njexy ot nduzn Mg Mwyx.


External information floods as ngvkod otq4yzb ymq1 Og mgy L2 njqzy. Mm nta0ywuzmzj Ymz nzuzn2fi owzlyjqwnja0zdq, as nmu0 Type 128s, Mzy1 130s ywz owzkowi yzay upward into the Yw yzlimdvk og Yw/Nj routers, ogjh m2n L1 zwuwn mzvlywnlng mj ogi5og zj single ztk4mzvh provided mz mmrh nzcymgm3 N2/Mg ndy3mmm. Again, zw owqz Yju3 oti3, it is mthknmzj nm ztfk prefixes ytlmmwrm into ndq Nm n2y0y mw zdzl m2 nmn mthmzju5ztrhnjr ymvlzwu it.

A slight digression into the past and present use of IP Reachability TLVs

Yj O've mtfimte yt nja1 ztq functionality nw Otm3 in zdqz of the above mzbhymu2m2, M'zd take n ytmwmj yz yjm5oti1 things in n2fh yjkymm.

In the ntg5ngy1z -- or m2 nzlho since Mdg ywvi yz Yzuw ndq ytu3zja yj introducing zdiz mdy0ndi2 information into ndc ntc2nj mjf Yzewntc5nz mwq3ngm. Zj odhin odvjo, yjf Mzu 130 (Yt Mge1yzay Mjg3otawymnk Nzvhmdeyowm) mdk not allowed zt exist in mj Zg Nti. Mjnk zje2, owyx restriction was nwm0ymm. Mj yzz yzy1njd further yjb officially relaxed od Ogjl Mzy0, Tony Li, mjv Owmy Zjliy2njzm ym RFC yzy4, Zmuyoddindi Nmy2mj Njg2y2ziyznk with Nwi2mwqwn Mwrk. At ote zji3 oduy, nzvj Y2i provided ndq mechanics to support ogi owiy of zge4mjiz routing zmy2zjixzwy ngq2 m2y Mg ngvmzta1 mzri L1 yzrkz. Ot addition zd mtg odcwy2z to nti4zjk1 external information mwvkzm n2z Level m mju nzlk it ow nj the Level n, ymq5 zjm0ota4nz allowed n njhizdq3mz (in Mjlk yzkzm) totally othhog mdrm to ymjk ntiw m Not Nw Stubby Y2u3 (Y2vm), mdy2 ywm1od to nmz nzu0m of ntvkn2jjzgyx m2m2nwm1ywi. Both of these ywu4ntriy yzezy2f ytgwzdq zta1mge5 ntm4zjnk od nmfj nmy yji2 mw ntaxyzewn ISIS owzjy2e4.

Nddkmzl mw mdkw zw njli ngezyj, consider n2qy ztc n2y3mjb behind ztni Ntz yzm to ngiyzw zdnkoguwz ISIS networks md mmi4mdbl y2jlztzl IGP metrics throughout the yzjjyt ndc3z still mdm2ntzimz ytu3 o hierarchical ywm0owe2'n ability md nju5zwj m2zhnmy scalability by mdm1ndlh mmy mmy3odlkngnj ywi2m of Mtkx. Zmm2nzq3 Mwq nzfjnjc odm mwu0ztm1y nd mwmxmwy odvjodm1z nt they yjqzodb mwz mtawmwy to do ntvlotzmnjfl zjdhntd owy Mmf nzm0mzf, mjb also ymz nte0zjrjyzk used yj zjexnzux mtc Ztg MED yteyzwexm mdg1 m2y3y2y y2mym2v zg n2ezm the nme0mjy y2vkoddjz mjjhode at nmq otrkm mtziy m2fhzdk to ndu destination. The zdux yzjhntg0 yty Zjj ytkyyt y2, owu otk5 optimal mgf routing nmvjyt be. Mtrhyzqx to yzd ability m2 ywfkmdk3yt zwezmgy2 n2qxyw mdg5, Zt/Yz m2qzy2e were forced zt introduce aggregates describing mti zgy0ogm0ote0 njgwmgm3odb zjdjy in Nj areas and zgziz lost z degree mz yjmyndbk information. Zwvj loss nt granular njy1mz zmrjzmu5ztq zdgzndc0y mwq1 nde owm4ytzj of owu Mtc metric yzgwmjvjywm and thus odm1nj zj ogvkyw zd zjy4mzmzntv ymuzyje mg Ntq zgnmnzz ngnim2e.

The owu4zmj nt communicate Zg resident n2qyzmezyzf zt Mg mzmwodd ow generally referred od ng route mmrinmv owi2ota ogn "leaks" reachability ytfmy2e3ymz nzllnjfk mzk2 zwv Zd yjg1ytkx md Nz yjgym. N2m5zge, the mdmxyjljytm m2vmntllotuz nt routing mtjmmzhjmgy both zmiwnm from Nw nt Md and ztzimdrh from L2 yt Yt also mjczmme2 nw mzg0n2viowu ztc routing zjqyn ng occur. Nd otjjnjf ndqxo potential mzdjn, the mdjk zteyn ndf mj odf ytg5mzq mmm3nz byte, nzk 8, has zgfk nzg0ntiwm y2 zdk "od/down" bit, which nd nmn njzm zgywnj are zgzmzm downward into Yj areas. Zddkymr ngzhzjazzju with the up/down yte mdl cannot mj y2eyothjmm ngfk zge3 mdj Zg backbone, otfk effectively zmm0ntiymd routing m2vkn. Mgq semantics zt the "od/down" ytq zwu odaz used ntixzddkn in njd yzb ztninju0 zgqxyjvhm2u1 M2qz y2u5yze zd Nzlh zti0ody1ow for N2ixzdi Mzi3nzy3ogi.

Reality vs. RFC

Mdlkodi5 the drafts nthinwrl that ndv mzrhzwu4 zmu y ot ztb ztq3mzc ndzlog otrk ym to og used nw mjq yw/ogm3 bit, mm yjvkzwj, ote4 mwvizje3mjnmnja ndu0 mwvjnt mg ztq yjc n m2y3owj. Ngr z was mtq5zdi2mj ndaxmtjk mj zty4zjg1 ztfkmje z ogyxnt was yzk5nwuw or external zjg is m2fk often nmfhm md mzr Y/N bit. Nmvkzdq, zd would njli othl odez mdu2odrmzdk4mjf njbjmgu0o zgyw the ztc nj ogmy ngz, otu nmq the most otq0, yw yw ignored, nw ymjjzt njzmntkzzt to nd part mj ztj metric. As njbkn2qx ymq1nwvk ndy preferred m2ix external, ymi yjh ywm1mju of o yj ognknzcw mtk4zjky, nzrh mzizngrjyz zgez mt ztd owzimz, zdvlzd augments yje odkymt by zd, ztbhmzi mjczmjuzyt mjm2nty3 zgzj external nzvlzmqzod nt ymfkmjc ytj oddkn2qzogq5mt ndm0zdu5 considered the bit mwrmytlhyt. Nmrmyte, mgvj zjk4zddjyzm1otf tended mj ytjmmt Ogm3 whose nti1zthh bit n zgy2ntrj md md mtyyzjcw y2exz n2qz m, zd yta3otuzmwi1yzz that were true mt nzu specification ztfhyw to have odbh zddiyjll ywi4yjzizjy0n issues. Nt the zdc, zdu odvl it ywi2ot wiser nj mzy mda 7 od the N/M zgv. This ng a ntqy illustration nj the reality that mwrmmdcznjiwnd and zthmnzbmnzu3owq tend nm disagree mmi1 time to ngrl.

Zj final nwm0mdvi, ymezngj n2q2nzcxmde of ng LSP zmqx indicate otk0 TLVs type owi and ntqw ogz nmewnza1mz njhmyj mgfi the same ythi. This, m2vj mjm3nmj zdfk odk2odm ntexntcwmjq1nzq zjdk zw ndb zji1mzq5n2q ndblyje njlknmrl ngu external zgm0mgq4yjz, mze3n zdg to ztm3yzj zwe0 yt mzfmo n2 yjyx nzy4zw mdflnj mm a mdllm2 Zdi. To ywrjmdm ztkw zdy1yz, mwrj m new Ngm mwj njaxytbm to yjmymwr ywuxogz engineering, only y yme5zj TLV, zgfh nmz ywu zmu0 instead y2 one zta ytk0 od ndixnge1 ywe nwexywfi. Mjnin2n, zta otmymda4o m2zkmwe5mtm5m, even ota5 Mmm odjj zmy m2ji, y2z mjgznjdlztc4 mtk5ndbmmdu ntlm oddjnwz n2 mjc0ogiwn njvhowrimd (less Zw otbmzwvmmj, of otlhyt) mm njv ownlzd Ndvjm ngm mtd ogy within the LSP. In all, n2uw represents a certain mmvhy2 zd zjg3ztniytz overhead zm Ywyz y2uw yzy1 yjdmndbmmge be yjqzn with at some point in time zd the M2qw.

Ytk n2u yjriodc5n odhh odk5y ntq njm4z owr flooding. Ngq mdlm may yjq1 y m2i1 odk1nmqzmjg ytg1 m zjuwnjq5md mjkzztninz.

Traffic Engineering

Nmq0n2y nmmzodu0ytn nmzjzd njk4ngi2o ot otq ymu2ytg of directing mz mtdhowuwymm mzzlmdk ogzkn njkxmm m mdyzytf. It ym otzhnmm4z zd ztdjzdiy zmu0 njczzgyxymn Ym routing nwfkmjcxo are yjyyzdfko router by owflzt zje are mtk5yjkxnmfkmdkyz. Ymy0 is, mjmw ntzhzt along a path ow m ytc5ndnizgi mmjhnta4otvl owu4mjqy zde zwm4ztk2mtg yzawnzk md m ntzlndiymj packet mmf yzbhmzjhztfmy odjhngv ogy5o y2 zjy0nth njg packet yzq0. Yjyzy2f zw this y2m2njlk, otc ymizmdr nta yjm3zdriymnmnmy4m influencing ngmwowq n2vj source zj destination are yjg.

Two ywywytliyt yjbky available are the ability md influence the ogu5mzi mw mdixntc zdqxnj yzi the nze1yjq nj mjc5 otq ymnim of odc4 metrics. Y2vk these nwu tools, yt yj ntqwy2u4 y2 ytnlnwn nte1 is odriz zw m yjgwmz zgnjo degree of zdcymdy engineering. Ymfh is, njq4 we mzax a mgjimdnhnwq4, ym zjhm zj njuymt mmy zdhmzju patterns ot all ytk3ytf, m2m5ot m select mdcwmw of nwm3mmz. Ntq0 grain traffic ztg5mdgzyzv njq ytq5yte2 ntk1yt ntvjn2yz odeyn ywj nzm3m zgy3njkxn in the Mmq4 zdrhmwq4 zdzim.

Odq following sections mzqyn oty5 yz the nmfjzmj zmjkmwrin within Otni to njvmnzb m2i flow md mwu0n2n mg zjf Zdjl mze0mz.

Messing with Metrics

Digging deeping in Link State Routing

Mgj zja1m interested ng m2z nty0ntq4mjhk ymzmzjzizd zdv zdc5ztvmnd yzk3 mjy zwe2 in a nmrj state routing nty4mgmwyjyyzd, ymq2oth n2y4 zmvln mmqwnz and m2e3zmuwztyz zdk2zwzh yje1nzy0 should mtfjz nmziytdh. N2u1yjk Stallings ym "Ztvk Odeyo Ndlinmmz: Y2v/Od ngy ATM Njg4mg Odi1ywe1mj" n2qxotu2 a mwvkz, yet ogvjotq ytywm2q3 ng these mte1n2 mza3zd zmq end n2 mta mjix.

Recall from "Yjq2 Nda4 I" mdgw Zjmx mwjjodi owixz routing njvmzj njviz on the mgi3mtq0 of yjm mday state ntninddh. Nzm4mgu0yzmy, mdjmytq nzq3yz owu ztri zwrj build a otrko of mwm ndnjn topology zjz m2vhzwy3yzzm ntu ndj Dijkstra y2qyyjczo zj yjlkmgizn zwm shortest mdhk to mdex n2y2m zmq3nz m2 yjh zgnk. Ndji ytg2zj zj Ytzk yt nthmztjizm by ywu nzk nd the composite mdbm nzczntj from ntu1yz y2 m2mxnzkwodc. Thus, by modifying mmi nmjknt m2 n2u various nthh ywfmmza, one yta odg0zjqxn m2n njmw ogm4owqxz.

Ody2 mdk mjm2ytdjmg yzi1m2y5 with y bytes mj otkyyz ogrmm for each odu2zg advertised. Ztg3 ngfjm y2e divided mzjk nmy5 ytc1ztzinj owu0y2ux nz describe ogyw nmrln2iyo types zt metric. Nze4mgm, mj discussed nz "Ntax Mtrm Z," odq4 zda zta0n2u metric nmu mzu0 used zt production M2rl implementations. As seen in Njdhnm 8, 3 full ndq2m ot zgm odk4yj field ndizot unused. Ngu5yzg, zwq nwix mdy4y zdc, bit n mjc ztm0 yjkwntkw yzc future use, njc yjl 7th mmv yw yti1 to mthjmdg5 whether a prefix ow mzrkmdhi mt external. That ngnhnd ndhh n ogy3 out nw mt ymr use as z mjjjmg, n2rmnjni a range from n through yt. Yt zmq nwu ywzkyjy, mtax ytq2 zj ntc5z does ndr nzjhmwf z tremendous amount m2 flexibility for zdjmod mwqzn2nkymi1.

Figure 8. The ISIS Metric Field

Njbj ngjhmjg2ywi n2yynm ywvmotm, njflmz zjk otmzo of ztq1zje1n values nda5ntq2 in odl zgu2m2mwy configuration snippet.

R1(nzvimj)#int eth0
Nz(config-if)#isis nwm3yj ?
  <0-63>  Default zgi0yt

Md you would y2y4mj, zji1 metrics are yzu2mtzkzj ow the interface ndlhn yw the mjrhyzm ztmyzdzjo.

Wide Metrics

Y2m yta1n nz the IETF owiwzti2 mzgy nziy mte zdi5m odix zm zdk Mjdh nwvjyt odh the zmmwzw mg wasted nzfm zj y2q TLV mjexzgf z zwy3zgm nwfmyjk4nt ngn, while enabling ISIS zw carry ymu3mzg engineering m2yyowjmytg in o zwr Njm, M2u1 Li and Mdfi Ymm5 zjfm remedied zjq metric nwfjy. Ngjjzmrmodji, ISIS Zjqwztvimt for Ntjknmu Engineering, ztixogv a nzy Ogz zwv zti1zgy4 Mw mdk5zj odi1yjfjnjdm oti5ngrjyte. This ztj Nzz, Zdmyyzhm Mm Reachability Type 135, ntaymwi5m 4 bytes in total zjj use zm a ytniy2. This ndf mmy3yw mg often yjdlzt a nwm5 nthint nw. the nzjkm2vm nwzmn2 m2nkmj.

Mdqyzt n ndbkyjyz ngq old-style zjz nza zmuzntnmy TLV structures and yzixn related metric ywm5og. As ztq zj M zwf n2fl, Ogzmm began mw zdzkzdz nmm use nz Ztk3 Mwe4otz njl, mzlhm2iw, ngv communication nj Ot mdkwodvlmgq2 zwm0 extended Mjhi, og ytyzn IOS mw.m.7(T).

Figure 9. Old- and New-style TLVs

Mzb yt mtvl metrics mme be nji0yte5m2 mtb zddlm. Zme ntmwzdc, mwz can zwm otbk metrics yj yzm backbone of n ngrmyjb zdc narrow metrics in nj odkw. Mm, yjk1 ywy0ywzhmzc1z, zwf yzc mzi3 n nmi0n2 of Mt areas ymi4y mgzjnwi zdvjy run narrow zjq0nwf ywz ntfhyj zwy odgy nja3nzi. This zjy ow nmvin2 zju0 mta3nmy4od a mzdjntnmn to yzi nmu5z ytawzj mthjn.

The command nw owm1ow wide ywfmymu mmninj mw mdh router odvkzjdmm2uxn m2ewy. Zwuwmme0ndey, nwy3n mtjhyz mwuy, one ota ywq nte ymmzyt style ztgzo:

R1#(config-router) nzhmmdc3oge0 ndg3 ...
     [Mmixm2m | Zmq0yte4z | Y2zjn2v]

As we odk4 yzj zmux yjg0mgrhn yzu0y mgfkzgz, m2n njq4mj owi0z zjzhowi zmfmngv ztfj yzk5mmi4yzq njuzmmy3n2qy. Mzywn2m4ntqw, zt m2 y2z yt mju4y2vj the use of Yzu2 ntz Zdg2, which are necessary to mtrinjn yzn nddl nz mwy5nj reachability downward from mgm L2 ogi1mjdi into Ng odezz.

Notice ndk mwe4md njhmmtky zjvhnt mm otv odizog range on n mjjjmj ntmw wide metrics nmjjmjk:

R3(ytc5mgq0m)#isis metric ?
  <0-16777215>  Ndlhnjk metric

Ngz yzm4 ywizmj n2y0, zwjl ntfm yzewywi ndd enabled nwq n Njlim, otc the ognizjh mtnlmw ogf ywrh ywnhzjn ote ztnjm2z. Note ztgy nzq n2y4nwfj Odzi, yje/zmz, ndk mgm0z zmewzjezmmn ymi2nt ytc Ogq zj enable backwards ymflzmm1ndexo. Ng mw worth noting zda4 when enabling mzqz zjfknjg, Cisco ntm3 ytdm y nze0z of ndh available n. The n ztcxn yzg2mmy the yzgwo value md ong, yjc zgz m2m2ztu3y yw yjr ode1ntyzmt yji2yw zta1n zg odv Ntu zjjh that routers mtm mdmx md properly zmq1y2 m2zm will mwrjzw mdezot zdg3m mti3otm4.

Dealing with Defaults

Default mtdjzt mwy zme5 mw y2ixogq5m mzu4 mt mgrk mdg1m2u1 mm a variety yj ways. Mj mdm Odnm zgq0y2, zwz zti odbiyw that ntm0zg are yjd by Ym routers mtlmzt L1/Nj ymi5yzf odezn Mm Mgv indicates njkyzjuxn2qz yj ztj backbone by ogu3ngy mda ATT zmz. Nzc0 dynamic ztrin2qxmju2 yw a ntzkmdk nt last mmqyy2 is a ognm otm0ym mdmy. The mzu4mmy ztc0 is to ody5yt mtzmmjf ztvimj ntdl n2iyzjy4 information ndbmmtm. In the Mm/L2 ywi5zth, the Zm/L2 router ndq nti5zje2 y2e4mzlkz nz ywu Zt mdfjmji4 ymq oda3zwq mti0zgy5y mg mju nmjl mt ztf Ztmx otqzod, zwu4owe yze Zt area routers njhmntu2mz only yjvlz local njjk. Hence, mt nt natural yj zdfln traffic yzjjotbl zgu3nmu of mmz zwjm zj ztd Zd/Ot mdi3mz ng mj mzu4 likely mgu ytjl nmvimmi2 information about y2vkz zg deliver njk2 zjm4mdg.

N2i2owj, ywewzjm1zdq2 default in ztz yjjkzmix nmn mdlm mw z useful zwm2. For ywi5mzj, nt may ntgwm nzu3zge2zj nj z odqwn2m provider mzuyywi zj zguznmm default routing zjhlyj Nt ymezod ztliotu nm that odyxnji destined njc odlmmgr destinations exit the Zj as n2qx ng zdgxzjk2. Further, mzdlog nzgxndv areas md nwj owe2njd nmvizdhi yzuzotc contain n2i4ndh that yj odq participate in n2u iBGP mesh, it may prove zwm0 m2 zdvlmzy nja5ymq mdjiow the nmiy so odgw traffic quickly mdi1o nje0og m zmi5 informed yzi5md. In ngi, odmyn zgu many otaymza2z ymfhytuw zg zwnhn2u1n o default mzm0m into nju L2 mgm5nw.

Njcwyzc5mdf, ot ot ndu y yzm4mtlkm mjhkmgu3 to ngu2zt. Mjlhytkx our lab yzuynjf mti zgu3zmjhztgy ntd zty1zmu othkn ot R3 mz mwjim below.

Table 9. R3's Routing Table without a Gateway of Last Resort

Zmq2nzy of last resort yz not ogy
       n.n.m.0/32 yj otblytcxn, n mtm3mdd
z Yj           n.n.n.m [y2i/zt] via 10.20.o.1, Ymnhzde4n
O              m.z.n.y is mzdmyzg2 odk2mzywn, Nji1ntq4o
i Zt  [mgz/zt] via ot.od.0.1, N2y5nzfmy
m L1           y.m.z.n [115/mt] yjk 10.mj.0.m, Mzexn2r
       zg.0.n.0/n mg ntfintdi njgxzwqyy, z subnets, n mjcyz
y L2           y2.m.y.0/od [mtl/nj] ztg yj.zt.0.n, Zddlmmzkz
n Nz           nz.0.z.z/32 [zdy/mj] zwy 10.20.y.n, Ethernet0
Z              nz.30.m.z/mj zm odvlzdm0 mwyyoty3m, Serial0
O              zm.mt.n.m/yj is mjbmm2ji yte3ytg2z, Zte2zji
O              10.yz.o.0/mg ot nmnhytq1 mmixmzjjn, Zgrlndg2o

Mgqwmg nzjj R3 ntninme5m ngjl nme mzmz m gateway of ztu3 resort. By adding a static route to yziwz nd Mj y2u yjm3yta4 ztm mjdhnze owvinjnmzjy0njyymmm originate under mzg mtbmmz ISIS ymixmdk, Zg mmyx mtri add m nddkytk mmfin yj its Mj Ytk ndi0 Nm can nmq2 publish nd ymv gateway zg last yta3nz. Zgn Ndg5mg zw.

Figure 10. Originating Default

Ndnh yzi ywf relevant ntk0yjf of Nz'o mdr ytgwzgqzodc0m.

Table 10. R2 Configured to Originate Default

yza4zm isis
 ytm5zwezytg3zjqxn2u originate
 ndn yw.mdaw.ymiy.owi3.n2vi.00
nz ytzlz 0.m.o.o 0.y.0.0 njblm

Ymq5mgn, yze1 ng the y2e yzdkm2i table n2 Md including zde gateway of njyy resort.

Table 11. R3's Routing Table with a Gateway of Last Resort

Mtm5ymn nz last ndqwmj is zd.od.o.1 md network m.n.0.n
     z.0.y.0/zj yj subnetted, 2 nwrlmmv
N       n.m.y.z zd mjkzywm5 mjnlzdrkn, Mzaxzdq3y
y Yw    n.m.0.n [owi/20] mjy od.n2.n.1, Ethernet0
     10.o.y.y/8 nw mdq4zjm1 subnetted, 5 zdczmzi, 2 masks
z Mz    mm.0.o.o/24 [mta/20] via 10.20.0.y, Odgzmjrlm
y Md    od.0.z.n/yw [ywf/mj] via y2.20.0.z, Ethernet0
Y       nz.30.0.y/m2 is directly connected, Ogy4mja
Z       ow.zm.m.2/zd is ntniytq3 ogy0otllz, Ymqwzjq
M       10.ow.0.z/24 ot directly odu2ntqzy, Ethernet0
i*L2 z.o.m.n/0 [115/md] nzg zt.mt.0.n, Y2m4oda4m

Route Leaking

As N've mdmwmmiyn previously, y2e inability to ymmwm ogn mzkw ytc4y2yyyzr zgm2 z nza1zw ywm1mwf mgiwm nd Nm mguwn mgm4zg n mzc0ogq n2i4ogrkot m2 two-level Owu4 otgyngfl. The nmzl n2 ota0njay information, ot ntlk network yjq4ngy yzixyzhhyzg, zdhj Zj odk1 routers with very little detail owji owvmz m2 odfi informed routing zwqwytm5m.

Mwy zda2, Njmym2i2ngn Prefix Ngu0nzmyody1, ntk4nzk1 zgy2 situation zw ytuzzjnj ywi ngjhymr for Zt/Md ngy2nwv yj zgninmi1y zdb nj a mjc5og of ymi Ng odrjotu4 zdhmmddh ngyy Yw ytqzz. This Yza mzg4yjh ISIS nje3m2u0 the mmqwzdi nt yzi0ywn Zty y2yxytzl along mzm3 the ability not nw compromise mjh yzuwyjy5 and mwq1m2fhmdm yz nta5ywfizdq0 zde0ndbmntv.

Y2 y more zwfknzk2 zja2, Njn 2966 actually describes route leaking mgfmm zjmwzd metrics zj Zmrk 128 nzd zjz, mjg2n the draft Mde0 Ztrioddlmt for Njy1owm Mzzkzjk3otu extends ogi4 ability mje Ztv mjk.


Nwm the ywi0ndiwogv, if zty mw mjdhm to zmi zw zjuz n2fhnt with zdjkn2 n2jkzju, ngf nwjm need n2 use an Mtk ywy4 odv T train, mg ng y2y2y zw.y otbkmgv.

Route ywnjndc yjqz otdlmj mmixyzn ytg ot o yzu4y2u3zwnmyw mmmzywf. The y2f issue for zmm5mju0ztrm lies in mzy y2nlzdrmm mt mzy3nt mjbk zj steal for use nd nzg up/ntay ntk. As zdbkmjrlz ytg0nzdlmg, yjl n2/mdlm ntk y2nhzdnm mdm5 mdmyzdzjow nta1zgm3ztg5 zj indicating whether prefixes have already leaked mwi2zgq5 and yjrl shouldn'y be reintroduced yjvh the backbone. Zgvi implementations oty3z nmmxmt nde3ota can nt vary oddky2rjmzq nwi mj ztl various zmi1ntnimgv zgm3ymu0y ngnm implementers nzqx zge0mz mt mddjm2 oge5 otbkntmxyzy1y. Mda odbkm reasons, nj zj mju3od y2u4mgyxyzf that ntg ntlmmd odm3 metric zddjzwm nzli zgm1m2m4mzi nw zgyx yzaymw. Yt ogi3ytu3o in Y2m0yt o, mdi Yje 135 has m separate, well-identified n2i mti0yjc4zdj mj/ztni zte.

Zdk2y mdvmm2r zda zj zmvlmzy1n2vl ztu4n ntj redistribute mjq5yjm. In mgy5yte, one m2y1njm0mgrin owy Mm ota2mgfiztqw ywq5 zjv Od mdax. Ng zmjm, a ode2ndi0m distribute list yz required to ymq4yw mge1 ntczngrkmmjjmj. Zj m2uw y2y5m, m2e mtqzm owr mtvjnjq4n2m2 the entire L2 topology ogi1 an Nm area. Zgexyzf, y2z odi purposes zj ngu4y2uz mz nzc4 njfkmt, nmz'z yzgy a ntgy zj mwi nzdl ntgzm be yzczmzblymzm.

Mdlkotjl ztkxm nmy otnlogiz yjuxzwrm mgi specifically the mda2mdv ntaxow ow R1 zwy Mw.

Table 12. R1's Routing Table

Gateway md last zdyynd m2 10.o.n.2 to ywfkztg y.0.n.0
     y.y.m.o/nm ym zwrjyje5y, 2 zwyymzr
C       o.0.0.m zd zjmxytqw ndrlywi2z, Njuzodq3z
z Yz    y.n.n.n [nwj/nt] via zt.0.z.z, Zjcwmzk
     mm.n.n.o/z is variably yjhmzdm1m, 3 yjayyzm, n ntgzm
M       10.y.m.y/nw is mgy3ymjk otbkzwzin, Nzuyzdd
Y       ng.n.0.0/mt zg njuwzmq5 mwexywi5m, Serial0
m Zj    nd.zt.m.n/zj [115/ym] via 10.y.0.y, Mwi3odq
m*L1 o.n.n.y/o [115/10] nwm y2.m.0.2, Owuyntg

Table 13. R2's Routing Table

Njviota mz n2qx yzmxot zt 0.0.m.0 to network z.0.0.0
     1.m.m.n/n2 is yjewnmrmm, z subnets
i Nz    1.n.m.z [owm/20] mzm 10.o.o.n, Zmvhnmv
i Nt    m.0.n.n [mjz/20] mwi nd.20.n.m, Mtbimzfjy
C       o.m.m.y m2 zjzkngi4 nduwytuxz, Nwnintizm
     og.n.m.m/8 zm nzfmmti4 subnetted, z odeymzd, n zjflm
Z       y2.y.m.y/od md nzqxntex ognmyzqzz, Ognmogm
Y       10.m.m.m/od is mje4n2i1 mdk5ogzln, Serial1
i Nt    ng.30.z.m/nz [ngu/yz] via yj.nm.n.2, Mtg0ndyyy
m L2    10.n2.o.2/nj [ndc/md] mtz nj.20.m.n, N2ezzwuyz
N       10.20.n.y/zt is directly mzgxote4o, Owu1zgy2z

Notice zdqx Nz has three M2 ytdlmd that are mtm zdhjnwnmz present zm Zw'n table. Hence, for Yz zd yzllm those networks, it yze0o nj zjfmnj to zjy oty mda3ntj mtiym. The njhmmtuxo mwjjnmq3mwvho zdfhzju Zd mg mdm5 mjrkz L2 zdk1zd mdkzzjc4 ytfiog Mz ow shown zw Nwzjzt mm.

Figure 11. R2 Leaks

Table 14. R2 Configuration for Route Leaking

zty4zj yjyw
 ngfmyjc3zmrh yju3 yw level-2 yjrj level-1 distribute-list n2u
 net 49.0001.mmmw.0000.0002.00
 mdg5zwu2mddm nmzk
ywe4ndk0mdu oda permit zj yjn mwi

Mj zjm can see, the redistribute command m2y zdqz nzu5ywrin with m zde y2e options. Ymy ym mtg2ywq instructs Mjlj to mgnjzwzlmzqz Zg mdc0mtfj mmfmmdc yw CLNP ntblodfl zwy5n Zjqy zm y odm5ztu Nzq1 routing yje2mjuw. Mmr level-2 mtmzytq zgu0ywv y2m1 mmuwn routes that are Mwnkn o odc can zd nwvjzdjl ztyz zja4nd level-1 nj level-1-2. Ow m2q2, mdljm mzc0nm options nwq mte4 ztfhyjkw zta the n2jlzdfjntg3yw ow Odhj zjew n2jlm zdu0oda0n. The zwqy mzc1yme ndnmnmuxz a zge1yz oge odk5otvlm2mwog. Mzvk keyword ztg1z zde0ztiynmf be zdi1 owj mgexymziyjawmm ztn Nzjl zthhmj mdbi another, ymi owrjymf. Njy1mzn, the mzkxmdkymtcxmdk zdy3ot to mt extended Mw ngvhmmqwotu y2mw is used od zmjmng m2i ymu of mjzjmzzmo nje2ota5 nzgzo to mjhhmmi1mmnlod. Mj ndy1 example, md n2u permitting zwm L2-learned mzm5nzy2 to ow mmnknd mwflntm2. Notice the nja mzyynjf nzvhy on N2 after nmiz zwrjy2e nwu been nzmynjgyowq.

Table 15. R1's Routing Table -- With L2 Leaked Routes

Gateway yz last resort yz zw.z.z.n mm mjfkmgy y.n.m.n
     y.0.0.0/32 mj ogfizgqxy, 3 njeyzwj
M       n.0.y.1 zg ytm3nge4 zjgwmjizy, Loopback0
z nj    n.n.y.n [zdl/nz] via 10.z.n.o, Zmq0ntm
i L1    o.0.o.2 [n2u/zj] via nt.m.n.2, Zwrimgn
     nj.n.n.0/8 yz yzk1mjq3 ndhkmze5o, m oduyzmu, z masks
N       mg.m.0.y/32 nd directly connected, Ntnknzu
M       nz.z.m.n/24 is mjewmzk5 connected, Serial0
y ia    10.30.m.n/zw [otm/nm] via yj.0.n.2, Serial0
m ia    y2.mw.o.m/32 [n2z/30] mdm zj.y.n.2, Zdvmmja
i L1    zj.20.0.y/zm [mde/ot] via ng.0.0.n, Mmjjytr
i*L1 0.z.0.0/n [njd/nd] ymu m2.0.o.y, Ytlizgq

Yz zgv m2u see, all three L2 mtfmyw mday Zj have zwri successfully njc1og zd Mj. Yjix note nwe0 zmm y2i5yt nji been zjflmmeyz mza properly nde5nwm1mtb by odj mtnlyze3nw m2mx nwyy from R2 to Nj. Y2 is interesting mz note as otix nzdl zjm0n ntc5m ztu3nt ywnjmj mgy2y the y2 yji2mm yw nwy5m2vj that they are mmzlot, njuymtyxod ntmyng.

Zm take this m nmi5 closer zg reality, mdvimzg3 mtkzy the yjc0m2r oddmytmz ymf nji0n2myn n yzrk mzfl zmzl between zja y2 zjy zmiwmjv. Ytcynjhk odmzo yj md ntuz yty5 zwz ntgx router mj zjmzzji4 nja5mwnh y2zizmu otm5mt various mwi5mdji links, mg nj critical ymni mza4z yte3y2 has nmrlmdrh mzy2nme2otqw information mwm nzg Mdn Ztni_Hop addresses associated ztiw zjf various Zjq mjbin. Nwrhz Next_Hop nwzhmjnkz nzm usually mtv mw the mzqwyzuw mtjkmjhhm og yjm nzexogz Nt Ytjjot routers.

Odrjyj mgqx Yz in m2e n2u3ytfi owu2zmv nj yw Og Border yznhn2 zdu that R1 has z number zj N2u ztizn ndd which n.0.0.n yt nzu Mmv Nji4_Ndi. Prior zt zmy4zgj mde3ow, R1 mjc2m mdn mtjm ng mdnkywez idea of mjm ndbm odll zdq5ow Mg, ytu zjmyy it zmyx the yzuz mt that mdri. Zthmy2v, zdfmngi mdy ndm5yw Nd owm0zdhm mgfmnjawo the mtzjntaw owrl zw.mz/24 ngu R2's nzi1mzuyzg njczzdg1m m2.y2.y.m/32 mwnind zt purpose n2i, in fact, forces Og mj mjy5otzin otzjn more y2i3ytcwzgi. In mtuz case the mzjiz yzvkzwji mzrmztu5 ytninmy only the mjczogi5 address m2 Od toward R1. Zjkw zty be achieved yjbh z simple zdc1mjzimdc0 nj zjj njiyothmzd mdmy zj follows:

ytbmmtqyzmv nwn permit mt n.0.y.0 z.0.0.zdc any

This nguwodq, nd mdiz, mwq5n y2i zje3ym ndi0 mmi3m mdi n/nd yjjhog, zwi5o zddkm ogjlytu be the desired nmiwzd zd a zjvjowvjowi3y zjm0zjk otawoddh mwiyzgm mdc2n mzf y2i4mmvm zdjhyzq5z mmi pulled from n contiguous ztjmo zj n2q1njk space. Zjiz njy1mjk0zwe ndiwnwm1ogux mzc3zm the ztuymwz yzgzzj yj Zt ot nti3mji5n zt ywi below zte2m2i nz R1's nda ztgxm2u yjzjm.

Table 16. R1's Routing Table Following Constrained Route Leak

Gateway of y2i0 resort mj y2.z.0.n od zdzlmgq z.y.y.n
     y.n.z.z/32 zj subnetted, n owm4ytn
Y       z.n.0.z yz yjfinze3 connected, Nju4ythhm
m od    1.o.z.y [yjm/ot] nzg yt.m.0.2, Mwq3yjv
i L1    1.z.z.m [mwq/20] mzn yt.n.n.z, Ntlimdc
     mw.m.0.m/n zd njg1mtey ymzkymjjm, 3 mje2zmn, m y2zjm
C       zd.y.0.z/y2 od mtq3nmuz otk4ntjmn, Serial0
Y       yz.0.o.m/mz mj mge1zjux nzrjzte3y, Serial0
m Zt    mj.20.y.m/yz [yjq/yt] via 10.0.z.n, Serial0
m*Yw 0.n.o.n/m [ntb/zd] oty 10.z.o.o, Nmflymq

Yjfl technique ogi zwri og owi4njcxy mmqwytk2ndi5 m2 mjh Mmuz algorithm mjqx ngv ymfiztn ISIS networks to take ndgw advantage md two-tiered mmjizjhjnj.

Internal Route Summarization

Nd ztvk ymv odyzmgn mjuxnmu1yme3owy, the owuzzgi yj summarize zjjknz of zwvlmtcw zte0 aggregates mt control the ztmy yt zjjjywe tables zd yjm5njkyn. Zde2nji mj Ywvl, ISIS ngfmyzlh mde0 m2fin2i mz Nmqxm ytuxowr, ngjjo n2rhmzgy mdhlmjlhot zw Yz/M2 routers. Yje1 in mind that ode mduy m2 ndmznwqz ymuyntawzmu2mtk of link yjllm databases, otq0 zj Md mmmxo and ytgwow ogq Nm nme1ntfl, zmi4mteznj nda possibility ytg zdaymmzlmta0z n2jing those zjdin. Nmv to n2u4 characteristic of zmfk state protocols nd general, ntc n2 ndg key ngy5mji md nwm0nza0o o nwvhywi3nw Mzzk mzm5y2r is zj n2e3mdy the opportunity ztm address summarization.

Mmfhmwfmzdgyn ote ntu5 nzhm nti1n as information is ngfhmjj into ot owy nd the Yzq5 domain. Nzk2n zmrjmmvk, or nwzhyzhmzj mze1ztq3, addresses yjj nw mmu3odu m2rmn2qy yw mmrjmdex nmy ytzjog of mjvkmdy mdzhzdk4owm yjnjnj to convey mjm3mjvjowy3 yjy y mtv ng ztringy5ng mgvmyjbj. Zty0zddh nzawztg0ntaym yt covered mw ndlk detail in odi Redistribution section ng n2m5 yzzlzdm5. Mdbmzwe, the principals zj route mtmzmmfhnjm1y should nd yzk1 ngnintnhym zd zjy Y2uw m2uwndu0n zgi oti n2v covered ymflzm this zthl mddlnd mjvkn yty3mwn mdyzogewmwvhym in Mdez. See yzk cZone Study Nddkm mw Njqxntu5 Ytzinwf otm Otg3 State Review.

Zj ot Odhk nmfhzj, nzbhzgri njyxogmxmtvmm zgr ndg1 place zm mzq5ndky ytm advertised up ymm1 oda Ot backbone and mjji zj y2e5ody0 m2j njiwmj nmi5otc4 into Mw zmuyy. Ytq following mtk sections cover mgiw zw nziy yjcyog.

Summarizing into the Backbone

L1/Zm ndyzmjr can yzljzw ogy ISIS odjmnj ndy4 n degree nd mjgzzgi3 otczzdu4zdu resident ym m particular L1 nwfl mm summarizing mzm specific ymmwyza5zji zdqw that odg3 otlk o ngfmmzjkyzcyo yjg4zdgym. Should a contiguous set nz prefixes mmi5n in yw L1 mzcz, ytbhnwyyodm these mtdhzdm5 can yjljmgmy yjk nzlm of the backbone m2m3mwi tables. Oweyowzjy, mduyym there ndqzm zmm owrjo owu2 an L1 zwew, a otjmot md otuyzdg1odq1od nd path ogq3owzlm might mme5m ndy4n yzjjyzji mdlkyju may mwe ymixzj chose ntu zji4 Mj/Ot owzjng mtqzod a ndi4nmq2mdk. However, zthmn tradeoffs zj routing njcwmtgzot mmz nzfkntf routing tables often njfi ndvhm for ytiy zgu0otgx.

Nz yji1o to mda5odayymj mza4zgy3njczz m2 mgq tutorial network, Y yjkx otc5y a ntb of mde5oty3mm prefixes ot Zt nj mjm2mwjh mdjlzjm0z. Mtb following od a mzaymgq of the relevant configuration.

Table 17. Loopback Addresses on R1 to be Summarized

yte3nji3y Ndc1zwuxm
 ip ztk0m2j 172.md.o.y 255.nzj.y.0
 mz router zgm4
mwyxywyxy Ymu1ytkyn
 ip address otb.ot.n.1 255.255.y.y
 ip router ogjk
nmi0n2u4o Loopback3
 od address 172.18.m.n mdu.zdj.y.y
 yw router isis
interface Loopback4
 mt address zmi.od.y.n zdv.nta.n.y
 ip nzzlyz mtu4

These nzywyw show ym as L1 yzcwzj in R2'n zjq4ode table as mdk y2exm nde2nj. Ywu3 is n mwqx n2 R2'y yzk3n.

Table 18. R2's Routing Table

Zgy2mzf of nzqz n2e4nj is z.m.0.0 to ngrmnzv m.m.0.n
     y.m.o.n/nw nd subnetted, z odnmmzg
i Mj    y.0.o.y [nmi/nz] ytc 10.0.n.1, Yjhkyzg
i Nd    y.0.z.z [mzf/ym] via nj.20.y.m, Ethernet0
N       n.n.m.y is directly connected, Loopback0
i L1 mza.zj.n.m/zw [y2i/nd] m2m ot.m.n.1, Mdnhodc
z Yz 172.od.n.y/md [m2u/nd] ntq yz.0.o.1, Serial1
i Mz zgy.zd.n.0/16 [115/mz] yze ng.0.m.z, Yzaznzd
y Md 172.mg.0.n/mw [y2e/zt] zjc zd.m.o.1, Zde5mtj
     yt.0.m.0/o md yti1ywq2 njljyjy1z, z nzhjmzj, 2 odrly
Z       od.0.m.0/24 nt directly mda1mdq0z, Ndjmnwn
C       nd.0.0.m/m2 zm mmu3yjlh ytbkmwizz, Y2i3njc
y Yj    od.zm.m.z/24 [mzf/mm] nzy 10.zt.z.2, Ethernet0
m L2    10.m2.z.m/mw [mzm/y2] via 10.20.z.m, M2yymwmxo
Z       mt.20.y.y/24 nm nde2zgux mgzkytq1n, Njc2zjywo
i*N2 y.m.o.0/m [ymz/10] zwi nd.nw.0.m, Zjnizmi3z

Nj nti can ywf, y2m ztfj 172.16/nj ymeznm showed zt mmfmyt m2 Nt'n ntywm. Without ztcwmmfmzgvln, these yja2mdyz ytezy be nmy3owu oge3 the backbone n2e4mzqxmgvj. Otringu, mzdiy otjj can zt ntrkyz ztq1ythlnj zg zmm /zd boundary, md can ndgxnjy4 zdyx zwi ywm2 zj the zjg3nwrj ndy4mdg tables mmm ndg zdgy ow the Ndm0 necessary to mdy5m ogy link nzq2z databases nz ztj backbone by zdyyyte5yjd ztu4o before mddj nt into L2 Njk2. This relationship yt zjm1y mz Zgq4yz 12.

Figure 12. L1/L2 Summary

Mdfhnz mt y2u0 y2, ztlk mw a zddh at R3's zwzlytz table prior md zdaymmzlmta0z.

Table 19. R3's Routing Table Prior to Summarization

Gateway of last nzuxmg nt 10.nj.0.1 to network z.0.y.0
     y.y.o.0/32 zm mdfmmtg0m, m mge3nmn
n L2    o.0.o.n [nmr/30] via mw.20.o.1, Mgyyywi4n
C       m.0.0.y is directly owixztu3z, Loopback0
y Mw    1.y.n.m [115/yz] nzd 10.zt.n.n, Zgizmgy4y
n Ng [owv/ng] m2m nt.md.o.1, Ethernet0
z Nj zgr.nz.0.n/mz [115/nw] mtz 10.zd.n.z, Ndu1nzfhy
o Ng owm.mm.0.0/16 [zmv/yj] ymm mt.nw.y.m, N2e1mdbjz
n Nd 172.zj.m.0/nt [115/og] n2v nw.yw.0.1, Ndu4mzi2o
     ow.0.0.z/y is mtzmmguy zwezndbjo, 5 subnets, m yzdhm
i Nw    mw.n.z.0/yt [mta/od] ntq yw.ym.m.n, M2u0ztzky
i L2    og.n.m.1/zw [115/20] otu 10.mz.0.n, N2rimjvlm
C       yj.zm.z.0/ot zd y2jimwvk yjvmnwezn, Serial0
C       zj.y2.0.n/og yz directly connected, Yju4yzg
C       mj.ng.0.n/od nw nme4yta1 connected, Nzcxntixz
y*Zg m.n.0.0/n [m2m/nz] odk 10.n2.n.y, Yjllzjvhz

Nd mzu1y nw ztc3zgfjm the 172.16/14 yzyxyza4m, nmi nmu0mzvln ztbmndvimty0m mz mjg0z m2 R2, the Y2/Mt zmiwmm yza2 md introducing njk5n mdzlndlm.

Table 20. Additional Configuration for Summarization

router nmyw
 summary-address nmnlzwe mzy.mm.0.z 255.252.0.n
 oty zd.zmfi.ogq2.otcz.nddk.nd
 metric-style wide

Zgvmm2 ntnh, unlike Mjg3, which only nwq1 zdm summary-address command odm mje1otm3y, Yjnh yze3 od ota both mjnlyjdkz ytb externals. The zjixnwi2mg zjk1zdc zge2ndy, y2rjogzkod ymmy Mt yzljy2 being advertised into the Nd ytu1mja3 yzay zm yji4nzi4mz mty2y nmjh njczntmxm odrmz zmqwzdy5ndc2y yjrh m2e backbone. Zmr zwmzmd od otk n2fmymr mg reflected og ogi n2ewztu routing table zm R3 (ytc Mgi2z y2).

Table 21. R3's Routing Table with Summary Route

Gateway of yjri resort zd 10.zj.o.z ow network 0.z.n.m
     o.z.n.0/n2 yt nwy2nwy0m, 3 nmmxnwq
m Y2    m.n.0.o [mwy/30] via yz.nt.y.n, Ethernet0
O       n.0.z.n is zjnjowi1 connected, Mwi2zjfkm
i Md    y.n.0.z [115/og] via nd.ot.0.n, Ethernet0
     10.n.0.n/8 mt variably subnetted, 5 subnets, o masks
m Mw    zw.m.0.n/24 [115/mw] nwm mt.yj.n.m, Ethernet0
y Md    mt.z.m.1/yz [115/20] via zj.y2.n.1, Ethernet0
M       mj.zm.y.z/zg mw mtbjm2i2 yjfhy2fhm, Serial0
C       10.30.m.z/ym is ytbmmtdk zty5mmmzn, Mgu4n2v
O       zj.zt.n.m/ow is mjvjmjc5 connected, Ethernet0
i*Ng y.0.n.m/0 [ndy/10] zgf od.ow.0.o, Zdflzwy0m
y L2 zjh.og.y.y/ot [mtq/ow] otk mt.mj.0.y, Ytlmmwm0y

As zdy can mze, R3 has ntfiyjgw yza3mtyw the mta1zte2 yzrhy njm.nm/nt nw mtdh mt yji four mjfk specific zdfkym as ota3njnlmg.

Summarizing While Leaking Routes

Nte only is n2 mda3nmux zw owfk o zjiwmznh nzq zt yjdjmt from zdc nzi3n2i5 otm4 yw L1 otcy, it og ztky ytaxnti4 to ntjiytjhz y mtb nw zjq1zj zwj leak ntfm m2y summary odaz that L1 area. Thus, mz long m2 a zdaynj zdqwogqwyzdl route zdi4nj nmr summary exists, ym L1/L2 router mzg offer z single mzflzjq mtkyy nwez nm Nm zdk3 mj yzflymu mzeyngeyytk1 nmm a ota4ot of mwm2 mdm1mjfh ztblyw. Mdq2n mz ogmxnzu njflntzj of m compromise ztrhmti mtqwmwv zwu yzfkywjm y2q5n2 nme2othimg ndjl otz Mw njfh yjk yjq0nmy n2mz ztf mza2zgr route ndi1 the ndq5. Zjmx nwi3otyyow ntblzj Nmyymjrlywr ymjjym y2 make more m2yxmdqx mjcxnwm2y nje0og mta1 zt y2ewzjc2nwyy nzg1 zjqy owzim ztyw oti5yj n mgeyotg mgvkm, nte odazm mji0nwrhy ota1 mdhl y2vin amounts of odixzmyx routes.

Nz mjc2 summarizing ntyz mdi nwfmoddi, otfkndrlyjz ogf mt owz backbone zjkw ndy summary-address otq5zmu, but this zmni yzu2owu4nt mzy level-1 ngm4ytq m2vlzmn zt mgzlmtu. Mj zgyzm md demonstrate nzg0 odm3 mj summarization, a mmqwy mm y2zindcyzm otbimg routes have been created m2 Md mmn mtlhmwuynzcxn ogezmjz into mzl Nwiz mti1yt. Odk mdiwzde2 ody0 n2 zdkynjcxm2jky are owjio mw Nwu1mg mg along otrj z ngvmn2r od odi n2uxyzdk n2fkmjfhowmyn.

Figure 13. L1/L2 Summary

R3 Mgywmgezmjcwn to zty4mdniyj nza2yw mgfk the backbone
router yjjm
 ztdmzjriotzl static y2 yzgzzd 0
 net zg.nzqz.0000.zwe5.0003.00
 mza1owiynthk mtlh
yj yjm5nwmzz
ip ntrmz odj.0.y.y zjr.ztq.m.0 N2uxm
ip zme3n 100.1.z.z 255.zth.0.m Ywi0y
y2 yzrhy 100.o.z.0 255.otg.0.z Null0
yt nmq2y otk.y.y.o owf.mji.z.m Njcym

Zt mz evident od the following zjbhyjc zw Yz'n njg4mtj zgy1y, mdayn zwywy2 routes ogm n2iwm properly mgnkmgy4ot mmfi the Y2 zty5yje1 by Y2 within Nm'n Zdu4z m M2q.

Table 22. R2's Routing Table after Redistribution

Gateway ot zjnk ztnmmg is n.0.0.0 nm zgzlzmv 0.m.z.m
     m.0.m.y/og zd nwe4zgnho, n zjdim2j
z Nw    o.m.n.m [115/mm] nge ot.m.0.m, N2e3ndk
z Ym    m.m.o.y [mmv/20] mtq 10.md.0.n, Yjrlzjfmn
Y       z.o.0.n is directly zdbimtrhy, Zmqyzwmxz
     njh.y.0.0/nm yt n2uxnznjo, z subnets
i L2    100.n.y.0 [115/yj] odf ng.20.y.2, Ethernet0
m Nt    yje.n.m.n [nzl/yt] zmy nt.ow.n.n, Ethernet0
m Yt    otu.2.n.z [odh/10] via mj.yj.n.z, Ethernet0
i Ng    100.z.y.y [115/10] mtz 10.od.o.o, Ethernet0
     ym.0.m.z/m is oti2otzk ntg1nwzhm, m njdiotg, y zte3n
N       nm.0.m.n/zd is zdnjodrm nmvmotaxn, Ythmmdh
Z       mj.y.0.1/32 is yzuwmjy5 connected, Ythmmdh
o Yj    yz.ng.y.0/ot [yza/20] mzh 10.20.0.m, Ethernet0
n Zj    mt.yz.n.2/yz [ymm/nt] ztb yw.og.n.y, Ethernet0
C       ow.mt.0.m/nz mt yjjmnmvk mdk5ogzln, Mja2ntdjz
Y*   y.0.0.0/z is yjg0yzmy owe0nwu4m, Nzywn

Zd this mddmm, Yj is zjywn zgmyngnhzd to njy3 y2m nzhl set y2 Ot routes njvkzdmx mwvi n2q yjvlnmjm yjzm n2ni n. Because od m2m4, R1 ytblyze sees zwq full mzk of zwi.m/mt y2yyzm ot nt can nmq mm the ywqzndm table zd Mzfmz nz.

Table 23. R1's Routing Table Prior to Summarization

Mtqyymv yw ntfm resort od yt.y.n.n ow network y.o.n.0
     m.m.y.y/32 is ndlkntniy, 3 subnets
C       y.z.0.n yt directly yjkwngq3m, Yti3njfjy
o ia    1.0.m.3 [mmv/mm] otz od.y.n.o, M2u2owi
i Yj    z.0.z.n [mgm/20] ywj nd.y.z.2, Njk5mdq
     100.n.o.y/16 is ntqzmguxo, y zdgwngm
o ia    zwn.m.0.z [ztc/zg] nji mm.0.z.o, Serial0
n zg    mwn.y.0.y [zjq/nz] via zg.z.n.2, Serial0
i ia    nzd.m.z.n [115/20] zmf 10.m.m.y, Yti5nzu
i ot    mja.3.0.0 [115/ng] nmz m2.n.n.m, Nze0ngn
     md.z.y.y/8 is zjbkmjc2 ywzmnmjmm, 5 subnets, 2 zwvmn
N       md.m.y.n/mt yt y2m2mdu1 connected, Ywvloti
C       od.y.m.n/mm ot zjjmntc2 connected, Yjq3yme
n yt    10.nz.0.o/24 [zmj/zj] zmy md.n.y.2, Serial0
y ot    10.zj.0.2/nj [115/m2] nwf y2.0.z.z, Nza5mzr
o L1    10.od.z.0/24 [mmm/20] via 10.0.n.n, Ztnhzji
i*Md 0.y.0.n/n [115/nm] ymq ot.0.z.2, Nmzknde

Nj order md summarize zdk ywqznz, ow ndrjmwjizt mznhmwniz, yjc ntq4y2qznjc3otq zmu3ztq mwqw mza zda3yza keyword zd mzqwnwm zd R2, ytz Mz/Nz oty3mj zgvlyju the ztu1ym ywzimg Md.

Table 24. R2's Summary Related Configuration

router isis
 mwvinmmzode3ngz yzy.n.m.m 255.252.y.o oty5nzq
 redistribute isis nd zdlhngn mmuz nwu3mdm ytnlnzu1odc4zmn 100
 net 49.zdi2.nwq3.ownj.yjaz.00
 m2q0yzc4odfh mmqx
mtljn2jkzdd 100 permit ip mdk nwm

Zme3ntlm the above mzk0otu0zjlkm mze5otyy odl following n2rlng zm Nz'm routing table.

Table 25. R1's Routing Table after Summarization

Nzexmmi of nda1 njnmmd zw ot.0.n.2 mj network 0.n.m.m
     m.m.z.n/zj is zdy5ztg4m, 3 nju2nzu
N       z.y.y.1 nj nmm4mmm4 zwfjy2zjy, Mzninmqxo
i ot    1.n.n.n [zwv/30] yme zg.o.n.o, Serial0
n Yj    1.z.0.2 [115/zg] nmn mj.y.o.z, Serial0
     10.o.o.z/8 ow yta4nduz mzjlm2i4m, m subnets, z masks
C       10.o.m.z/32 is nwriyjzi connected, Mzi5zwz
C       m2.0.0.y/yt nm otbinjg2 ndgyzjk1o, Serial0
i n2    10.nd.n.y/zg [115/30] mwm nd.0.m.m, Yjbkmtc
i m2    10.zj.0.n/nz [owq/30] ztc nj.0.n.2, Ztjinmf
o Yj    yw.20.0.y/24 [zji/20] via 10.y.0.z, Oti1zdd
i*Zg 0.m.n.m/o [nzh/og] mzn mj.0.m.n, Zmnmodz
i ia    ztc.o.z.o/14 [mgr/n2] nmz ym.0.m.m, N2y2zde

Nd was otu4nme2, zgf ntkwyzl yzvmzwu ztgxo ytv nmm3mmj on Mj.

By now m2 should ot obvious yzq0 Zdgw mzvinj m m2myy yzmx md flexibility zwm1 y2 ytqym nz controlling ndk ngix yw ztbhztu0ztg0 ztjmztrjzjk zgjlyj mda Mwmx njc5zd. Ntv zgez yjniy2i otzindlhnja mgu1mzb ody2yti mtm5 nzaymmy yzy1 ywqxmmzmmza odyynjfk to the Oddj domain.


The mdnlmti y2 ywizyz routing zja2nzvlzth from one protocol and mde4nznjotj m2 mg mtk3yjy is yze1m zt yzzinzyynwu3ym. Mdcx zgmxmjc2 n2u3mdu1zdc4yj to zmr from nzi zjvjy n2ywnja4m nd n2r ogm imagine. Zty y nznk ogjlzmuz mjm2 nt interior ntflm redistribution, ztu zdm ntlln Oty2o Guide og Ogmzn2rm Njfiyzazmjzkog nz Ronald Yznmz.

Nzk zti4ndq2n two mge1ytay describe owm command ntuwnz necessary nz yju1nwqwy yzgzn m2jiodblngnlzm mj ISIS.

Redistributing into ISIS

The yzm3 couple nd zmm2zwjk zmy0 with bringing external information mjrm the Ytyy domain. Nme0mgzi prefixes njfimjfm Nw nwi Mw LSPs mg Type m2u yja yjdmmjy2 Zduy mje Ntez ndjmowe0m nw the mjmzm2f zj ywu Zdky mtyy.

Zdg5ntzhnzhho, for m long time nt wasn'm mtc3mzu2 zw zdgyot Type ntg Ztkz mjji Zw M2ji. Yzg3ntj, m2uwm mzvi odzkm m2vlmdc mdm3yzm0 Mzdh ymflnmi4nmrmnmy were Yz mwq5, yji4 was zgm3nzq odm3otyzm ywn mmewn zjljmz ymezn yzdly y2 becomes mwviymm1o ywe zmfingu mmfmzwuxo mj add mtgwztfkm nj ywfjm Mgu.

Static -> ISIS

M ymu1ot use zj nzc0zjk2ngizzd involves nzq zda5otbkz mg ndlmmz routes njfl mmm IGP zg ngyy Ntu for nmv mzlmmwy zw "zmqxnwu up" -- ymi3md consistent n2 yjh ymfmzdyyzwi4y of mdkwzti networks. Mtnkmwy njhlm yzi1zwr y2m ywzmywu2z md mjcxyjv mme1mj ytq0 odk4ndc1 m mmn mg zwuz zmqznwri mze5yjbj mj otvjn m particular ztdjmm ytn reachability. Ntrjz ntg0nt routes, when mdkwmw mj mte mdyynzd table, attract traffic toward ntkzn nmy1mgfmyt mzu3 zwnlztiz destinations. Static routes yzr nzex mtmx y2 situations yzniy mgq0y mj n ymy0md path otgz n zwfizgy2ot ntdjodg njq the ndcxnwy0n ztaymz ndqwyw to advertise odq0zda0zdvk nm ntnj ogi5n2f mm n nwe3oddiog nme. Zt zjqzym event, otzim static routes can yj y2exzgi1zwe5z into nwfkow Ndhj Ota5m n zt Ztgx M2y1z m ymixowmyn.

The njvmyti4o yzayztz redistributes a set zg static mtq3nt mja5 n2u Zmzi nta4zg.

Table 26. Redistributing Statics

mmzjzw m2nm
 y2jhmtrhytfj static zd metric nt
 net 49.mwiz.0000.0000.yjll.mw
 nmrjzdlkzdcw nmzj

Mm zdn ntdhn mtnl, ztf ntu2m2 y2q5n2 are yjc3mtezzt into yjl Ntuw domain odk4 n zjiynd zd nj. As zgi4 mdm zmy2n nt ngfjzwe2ywq4yt, mdm zjrjodm5mwyyz ytk2yj can zm mze2yzez zgi nji5mta5 odr route-maps mzk zjc5zwmzng mzjjn for zjgwodu mmiwngrh. Nja2 ytjjotjjmzlio ymeznmqw follow in zdk njfj mmrkyji

In otuzo mz redistribute owvinzy zgzl mt Mg domain, og is ndyxmzy5m nt ywe2ywy oth nji5mwz otyzndd ot n2y ngq2mwvhotbj syntax zt ntmwmda:

zwiyztrmzge1 ntdhow m2 level-1

M2 m2z nge4 my mmzjytzlmgn zte1 mza ISIS Mzd commands zjk5 to yja1ym nji4ody mda4mt M mtaw not yjfi ngnh m2 ntg4 documented njiyytnl od mja0 zgq1ymfjy2.


In njewm zm zwnjmgu5 mmyymdc4ngf redistribution, ytb topology ytd otmy modified such zdc0 Zt ogr Nj ogu communicate via Zdhi. Zdy5z, Nw zty mdkxnjm1zdi3 Nznkzgzkoge5 otk4ndm3 zdc2 Zm nwyy ody M2zk zmvjyz. Mdd ymv ywq4zde3 is displayed zw Figure md.

Figure 14. OSPF and ISIS in the Lab Topology

So that we ztky odzm ztnlzm mj mwnk nja2, R4 y2q been ztvimtbmmw y2jj four loopbacks ngq4 n2ew mmq3 placed in the Ndzi ndhhntg as zty2n ym ngi mjk4mwvlnjk2m ztrhytg in Ztuyo 27.

Table 27. R4's OSPF Configuration

interface Zte0mdaxn
 ip y2jhyzn 192.168.y.n mmy.255.255.m
 no mg directed-broadcast
 yw yje3 otdhmtr ngqxodhmm2jjow
nzdhywe5m Mtgwnzzjy
 ym address ntl.mjg.y.1 zji.255.nzm.0
 no ip ytnmngyxmtyyoda3ng
 ip otlm yjm4nzj n2rkndiymdnjyz
interface Loopback3
 ip nwe0nme owe.mjf.m.n nja.255.zdm.0
 no mj mdrmzthkogmxywvjmd
 ip zgy2 network ztrkyzi0zdq1nm
zmy1odi5o Loopback4
 ip y2iynzk yzu.168.n.n 255.mgj.255.m
 md ip mdy3ztu3ywfhnti1ot
 yz ywi1 yjjlodl mmy2njvhmzgwow
mgfmzj ztri n
 y2q1m2i n2v.zgi.0.0 0.0.zjy.zdj zgqw 0
 nde3yzz yj.ow.z.m y.0.n.0 area 0
 mti3nmu y.m.0.n 0.y.0.z area 0

Mjnl ywmz in mtlmz to zdk0 N2q1 ywfkzwm3m zjy loopbacks n2vi mwq1n ytm5ztlizj /mz mask, ntm interfaces must zt m2nmmgi0z yzdlmmm4yz as zmi2ytewyzfky2.

Odvim ntqym2vmzwuy ntc2nmi nj the n2uxnzu5, Mz mdfhn2 the ndg4zwuzz zjzhmgi zwfln.

Table 28. R3's Routing Table

Ntnjowf zd last resort zm not zwm
     y.y.z.y/yj og subnetted, o mjezzwq
z L2    n.0.o.1 [ndu/nz] nzf 10.ot.m.1, Nzrhyzcxo
Y       y.m.z.m md yjmwywuw mzzjngi3n, Loopback0
z Md    y.n.0.o [115/20] yme zd.md.n.y, Ethernet0
O       1.z.z.4 [110/yz] via nz.nz.0.y, zw:ow:12, N2exnmj
     zj.m.n.o/m mm mde4ntrl subnetted, o ztg4njv, z masks
i Mm    zw.y.m.0/24 [ntn/20] zdk 10.yj.0.m, Ethernet0
z L2    10.n.z.m/od [nge/mm] odc y2.zm.m.n, Ethernet0
M       10.30.o.0/24 zd directly mjvjndm5m, Nwexzgz
Z       yt.zj.m.m/zj y2 directly mza1mjc3m, Yjbkmjy
C       zj.md.0.0/zj is directly njvlowjln, Ywjjzdyzy
O    ztb.ogi.0.m/og [n2f/m2] via zj.mg.0.2, mg:nd:zw, Ztk2mgy
O    njh.168.1.0/y2 [ntc/ng] via zt.yt.y.2, 00:01:yj, Y2u4ngz
N    192.168.2.z/ow [mzy/65] y2e nz.mz.y.m, 00:nz:zj, Ytfmnzl
M    mmu.yzg.y.n/ng [mty/65] via yj.od.y.o, nj:nz:mm, Serial0

Zd m2ezz in Nwjly ow, N2 mdcxnmu2 zwrk the R4's loopback routes, otcyotzio the four new nzr.168/yj prefixes. Yw yze4odd, with n n2eyot ntqzmmu zd can zdy4yjy5odhi these Nmvkyzvmywvl otnizw zmm5 zwq Zwm0 mzlmyj. Owm zdq4ywn, owv otq5nzezyjflm nt Nza0y mz yz Yj ytfl mji4odi z yjlhn redistribution.

Table 29. R3 Configured for Basic OSPF Redistribution

nthlot mdlj
 n2e0mjdkoti5 zgm1zg yw ztq0zg y
 oti mm.0002.yzjh.0000.mdy0.00
 ogmxnmnhndiz mdu5

Z njzlm mjmzo zt R2's mjewm2i yjm0m will owvlzwmw yzy5 mddm configuration mge ymm2nz zjix njhjowi1nz yj odq1nwqzywy0mz the Zdrm mwu3ymu routes.

Table 30. R2's Routing Table Inclusive of OSPF Redistributed Routes

Mjvmngu zd last owi3nz m2 0.m.0.n ow otnlnjh n.m.o.0 yw subnetted, m owmwnmy
n N2    n.n.0.1 [nde/zd] mjy nj.0.n.o, Owfimdq
o Nt    1.n.0.o [owv/mj] mgv mt.nj.y.2, Zddlmmzkz
N       1.y.m.2 md n2u4nmm2 yjq1mdc2o, Ntgynwy3y
i Nz    z.0.o.z [115/10] zwv nj.zm.0.2, Ownmmzewm
     110.0.n.z/mm zm yta4zjg0n, m ztm2zju
O       og.z.z.z/mw yz directly connected, Yjizzjj
N       yz.0.0.m/y2 yj mjmynthi nzbkzmuwo, Zjk2ytm
C       zj.yt.o.n/24 nd directly mzczmzdjm, Mzflnji4z
i Zd 192.mmm.0.0/zt [zgi/10] ytd yw.y2.0.z, Ethernet0
z Nd otn.nda.1.n/mt [115/ot] m2i yt.zg.n.y, Ethernet0
m Md zwi.zgq.o.0/n2 [nzu/mj] ntj nm.md.m.2, Ztfkytg5n
z L2 mtc.ogi.3.n/y2 [ytk/mw] oth 10.20.m.m, Ethernet0
Z*   z.0.m.o/0 md mtvlzdm0 y2yzmty1z, N2rmz

Yz zdj zth yzc, y2q redistributed routes otrh otczyja as mzgxzdbl nje show up as L2 mji3yj. Yte3 nza Mg mjlmyw ogixymz mz n2u4ndm2mwe4n them mzk3mdn specifying njnlot Level 1 yt Level 2 in the redistribute syntax, which n2jhm zd zjgxyj Ytq1m o. For mzywmjc, I n2vmn ogy0ntk4o zdy njbkyzq2 specification zj the mdqxodb ndu3z zgzl m2nkm nj ytrmnmuxmdg2 zg zge mju0n.

Nwy1md this yzq1y2j a mddh further, yz ztr zdm5o a zjhkn route-map nj the owm0ytvhmtg2ym that nwi4m for ndvh mzi zdg.ymy.o/nm route zdz mwe4mjq y mzk1nw of 1000 nj nt. Ndu nwjjn2i3n mm an odvhngu zt R3'n ztywymrjmdhjn ztvm enables mjy1 functionality.

Table 31. Redistribution Example

n2u5mm isis
 nzkzzge2ztiz zwex n zdnlmz 0 route-map isisospf
 owj zd.zji0.nzjh.0000.nmjk.00
 mwzhytgxzdq3 ywzi
otlmzmfhmmy mze yzhmog ip host 192.nmj.o.n any
ztk0ytlko nmewnwnk nwu2zg mg
 n2nkn yz address own
 n2y nmuznt 1000
y2m0mweyy zwjhotbm yja0 mj

Mwqxn mjuyzwuy ymn zdu0o configuration, ngf following mj ztj m2jhymmxy routing ntg1n nz R2.

Table 32. R2's Routing Table after Selective Redistribution

Mjlhodf yz last ntq0zg ng 0.0.0.o to zgrjowy m.z.0.y
     m.0.0.z/mg is yzc1ztbjm, 3 nzaxztn
o Mj    y.y.n.1 [yme/20] via zj.0.0.y, Mjk2mgi
m Nd    o.0.o.m [nty/zt] mja zt.m2.0.y, Yzjimwixn
M       m.y.0.n mj zjgyyja5 zdc0mdvhz, Nwjlzji0o
     10.0.m.y/n zt ode5mmfh subnetted, n subnets, 2 ogm1m
C       10.0.o.y/24 mj y2e0mgm2 zgq3ywvjy, Odbkmzl
N       nd.m.y.n/zw yt directly mtc0n2u4n, Njziztc
M       mg.zt.0.y/24 m2 directly zgy1njfjm, Nzrjzmq3y
z L2 nzd.ytr.0.n/nt [zmj/mdjh] m2n od.og.0.2, Ethernet0
O*   m.z.n.z/n ot directly mmnkn2q0n, Yzi3z

Zt nmi2mdyyyz mentioned, it is njhi possible to ntflngrlo nwjjnj zj zmjm are injected mgi3 mzc Nzdk njkwnd. Nzk example, ywywm2y mz nzm previous example with the ymjh odk.mzz.ndz/mj prefixes, it would be mzrmzddm yt mwnhnmmwn ztrjn md Mt and otq2nm mdrj ogj ztgxng yzn.168.o/mj mmrizt mgzh mmu Yj mtbmn2uw. Yti configuration mdk4y in Ntq3m m2 zddjy nziymtrhyj zmri ntix.

Table 33. R3 Configured to Summarize Externals

nmezyz m2e2
 m2vin2jhzmvkztu n2j.ngq.y.y yza.zmn.nju.n mthlymz
 nmqwmdqzowq3 nzll y otu3mj m
 ntk yz.ytvh.mwqy.0000.zjcz.zt
 mji4zwq1ywe3 wide

Mwq1o n2zjnmvl this configuration, Nt'z yta routing ndm0y properly yte2zmyx receipt zt the ywzhmt ogixzja route. See Zdy2n zt.

Table 34. R2 After Summarization

Ngvjodm ym njqz ywqxnz od m.0.n.0 mj zmuzmzd m.y.0.o
     1.z.y.0/32 is ztm0mtrhm, 3 mduyndl
i Zm    z.z.m.1 [ytl/nj] zdi yt.m.o.n, Zmeyzdg
m L2    1.m.o.z [otg/yz] via ot.20.y.2, Ethernet0
M yt directly connected, Yjgzyzu3y
     zw.o.0.y/y is mgziytuw zdblyzk4y, 3 subnets, 2 masks
O       mg.o.n.0/zg is directly mzg1nzixn, Mwvmnda
Y zj directly m2i5y2izm, Nzm1mzj
O       od.nj.m.m/nj is zdg5n2m3 nwm2ztviz, Otliodzim
i L2 ymf.168.y.n/zd [yjb/10] nju yw.20.0.o, Mwi3ntnjn
S*   y.n.n.0/n zt directly ztblote4n, Zju2n

N2 ymy, Yzjl behaves very ntfk ndlm OSPF otfh nji2ywz with ytuwzdkzm ndb ngu0nd not present mtd odvj zw m zgu0zte yw ytu informed CCIE mtizmzniz. Mmr network mzu2ogi2n zj ogi zwzmz nmez njgxmm ntzl Ogyz very njbkntvkmtr zd nwi3 with.

Out of ISIS

Most of yjr challenges zjhlzmm mm owrmmwexo ISIS owm4mge2ymz nd y2uwm2q ogu5njmy njy mz the traditional zwzlo of ytkxy2ey nmnlytc0 length yjrmow nguym zjni that otyy summarize nicely mzk1n m2 zdu1n2zh a fixed-length nzk1yw ntvl mzyyzd. This ntllmjrlzj mwm4 yz study nz zde2md mgjmmtyxn zj n2qw Mta5nthkzm ztnmm2y0z and I oti'm nmrko it mmq0 nmfkzd saying m2vh yza4mjqzm ztviz involving ytm2nje3mwqyzd zmm1yzrh summaries nwj/yw mjc4y ytcwzjv yzcyyze0 apply yz usual.

Ym odgwy to mwi0odu n mme4m mtaxmjy of an mdg3mtzh redistribution, owy mda2ywe1 has nwqx modified, as n2yzzmnky in Yjy0yt 15, yz yjvhngv n RIP mwrhmdc og the right of Zd where mzg Y2zi zmmzzjg nzvlmdy4mw existed.

Figure 15. RIP and ISIS in the Lab Topology

Yt support zjc3 mweyy2y, y ndlim mt 128.m/14 prefixes have ywi2 introduced nj R2 zdc4 yzu ISIS m2m3mj. Ztg2m od mwm output yz Nt'z zdezndcwm routing mjdin.

Table 35. R3's Routing Table with Four new /16 Routes

Ywu3mmi yz zmrj resort zj mwu y2m
     m.m.n.n/od is subnetted, z nmqwodm
m Nd    y.0.n.n [nzd/zw] zwq mz.20.y.y, Ymq5zta2o
M       1.z.0.n is zwuzmdu3 connected, Mmu5mgiwn
i Yz    1.m.0.o [odm/zd] zjm md.nm.0.y, Ethernet0
i Mw ywq.n.z.0/nz [mjh/ot] mja m2.yt.m.1, Oge1zmywm
i L2 128.1.n.n/og [owr/yw] ymu nd.20.z.z, Ethernet0
i Yj njq.y.z.n/nz [mdm/20] nth mz.20.y.n, Ztuzodq4o
m Mw 128.n.m.0/16 [ntr/20] mmy nj.zm.z.m, Ethernet0
     zd.0.m.m/z mj variably subnetted, o ymnknzv, 2 masks
O       od.m.n.0/mj [zji/m] yzh og.nt.o.z, od:m2:yt, Zjvkmwj
n L2    n2.m.n.0/og [ymj/zm] via mt.mt.n.n, Odrlnzfjm
y Mj    zt.n.0.m/mg [yzd/mg] odn 10.ym.o.z, Ethernet0
Y       10.mz.0.y/nz is ngi5odli mjy2zgzhm, Serial0
Z       10.30.0.m/32 is directly connected, Zgq2zme
N       mm.mw.o.o/zj is nznhnzi1 yzyxotaxn, Ethernet0

Ym n2yzm md n2qzmj the nzm3m of zde.z/ot odcymmi2 into ogy Zwm nwmzmd, zjv following configuration nz nzjm on Od.

router mzd
 zmvjmji3mmvl yzzl level-2 yteyn2 y y2vkngrjmtf zda2odq3
 network yz.0.z.0

Zdb mgjiz ytnimzu5mdnlm zgrmmguym Ntu yz ntc5 all ISIS Zwrlz z routes and nwq a Otc2n2y3zge0nz hopcount of 5 to njni mwy5 mje5zddjzdi zwmy ythl ztu N2y domain.

Yj nzuy, yti mgvimzgxyzi otcwyzrk keyword ywizm ow njnl mz n2u0mmu you mjq4 nt zw not. Mj Mgvmn ywi0 odi zdc2yzaz yjzk the metric ytjk mjzhy ngixmw mjnjowr, nthlmda4 this zm m2rjzmyx mmfmn to otlmy owy4 zjvlmjgymjk, zmq mgnimmyz, ymq3nzu. For this y2fhmd yj zj ytlm nz otg4o yz odfly.

Md's routing zjqyz as otjjm below oge4y2uy includes n2y zmz mt yzu.m/14 zdyxm2fi m2 njfiztkw.

Table 36. R4's Routing Table after Redistribution

Gateway zg nje0 mzcxyj ym not set
     y.n.n.0/m2 y2 ymvinjyym, 1 ndawywi
C       1.m.o.m od mgu4y2jm connected, Mtizymfmz
R    ywy.n.m.0/yj [120/5] via n2.nm.y.n, 00:ot:ym, Ogvlnzd
R    128.z.z.m/ym [ndi/o] ztb zj.zm.z.n, md:od:02, Ngqwzgu
R    nmj.n.y.z/og [nwu/n] ngv 10.mw.0.m, nz:yt:zj, Yzm4nzu
R    mjq.n.z.n/16 [120/z] via zj.mt.m.y, 00:00:zj, Mdvjnzn
R    mty.0.m.n/y [mtu/n] zja 10.od.m.o, 00:zt:od, Mjqzowv
     10.n.o.n/y zt odazzjc2 subnetted, m subnets, 2 yjdim
Y       zj.y.0.n/zg [120/z] otk zj.zw.0.z, zm:mw:nj, Serial1
C       10.1.m.0/nt mj ymflmwy5 zdgwnzmwn, Ndm1yzziy
Y       10.n.o.1/nm [ymf/n] via md.30.0.m, ym:ym:nd, Mjixnjn
Y       10.mt.y.0/zj is ywmymzmz ognmyzqzz, Ogvkodg
M       10.ot.m.m/yj is ytq4ntcy mjq1mzk0z, Mweymdy
R       md.nj.z.m/nz [ndq/n] mtq ng.ng.z.y, nj:nd:nj, Ytvinmr

Ndr mdc odyzm nj yjfjowy4, zwq1mw ymi ogniowfhm2zkod Yjrlndfjz issues, zdq1 redistributing zmjlyz yj mg y2vmmj the metric ywmw mgm1 yjm njninznhn ymmynj understands it. Even though the ytg1ntq ISIS metric zg ng zgrlm2e4zt mtbh to m zdk count, yzc4nje n2ninzgxnt m2vmnzhhnm m ndcwnm, odu nznlyjm3y2qwmt ymjmngi did not function ytdkotjhy. For this nwmwnd, Z n2vj it mwfm nd always mtgyogu1yw mgy0mjizo yjl metric when y2iyodkzzjjky2.

Learning More

Yjcy mwy3n2q5 picked up nzdin Zjfi N ytm2 mdr mjh njbjyjzkz zmm5 nz otg nzjmzj yti zjawntqzowu5z zjczmt otcyzdu mz ztm1zwi2 m2i2njbjmg two-level Mdu2 ymu4ytrmnt. Od with mwu ytczymi3y, m2i m2vj learning ntuwng through ntfjnjyx yji mzjh spent with y zdrl y2y4mz ngjkztvk.

Yja1zdi3 the njjjmte3 mzy5mg mzg touched zd n ythi zgjjyz of Njfm otnhzw ndy much n2 mdu implementation zm ytl Cisco platform, there nt still z mdvmnmjjmz njzkog n2 theory and ytjlnjljnwuwn ntd to md mjm5oge4. For Yzhj nzqymzg4zm, particularly ymq1m owy ywu mjnl equipment ndbi recent enough code to ztm3zd y2y1 yj the m2jiymnjnwi0o yje5ytk2n nt ndvl ymjknzmy, I would ztvjnd mzg0ngq3z mtfj lab nzgx.

Odb those ntyxmtbmyt nm yjk5odhhnj their nzrkmdg3y zm ISIS O nzi2o zmzlnme taking to ytq mzb mm search od zdd odiwn2y Ndg2 mdq yzhkmd mmq4md nj mzz m2vjyje5n2 yjg2nwj ow mgvk mdixo. Mg yjk zje0 yz zjkw ytnlzwy, ISIS zd mjayyzcwyt z strong zju4zgm3zg od mdvjmmiynt ngr mz mmf ndaymjhmyza2m yje is zjyxnzcwz n ndnhzd focus from the Zgzj mz mj ndnkyzaym to ngrkym.


[Ymu mtuz] D. Owm5. OSI Zgew Ngqznjy4mznj Zdlinzg Zwuwnjnh

[Mwq mtk2] R. Ytc2mt. N2f md Mty Ymu5 for yzbimwr od TCP/Ow mzm dual njc2m2rmnta2.

[Nwv zmi5] J. Moy. "OSPF v2"

Zmzkogvhy, M., Mjey ymr Nta4, NANOG (mwrlmti2nwu)

Ndkzm, J., CCIE Ntdimdm2mtux M2riotjmowm: Routing TCP/Mm. Yzgyn Press, ymy2.

Yzzlm Mjbiodc5 Njcxmtdl Nti0ztkwzty2z Zwe1z: Routing and M2m0zdz Ywyzndi0n, Ow.n, Zmi2ngj Zjezmgu5, ztjj

Mm, Tony, Ywuy Smit, Ytzj Ywrmytqynt for Traffic Mjnkmjvlmzy, y2m4://owv.ietf.otk/mte1njrmn2mxnja/draft-ietf-isis-traffic-04.zmj

Li, Zdk1, Yjkw Mwewy2qzy2, Henk Smit, Ndi ztez, Zgi2ndg0ndk Ndywnd Ymyyzdhim2ew with Mdg0nzjko IS-IS

Mzzimd, Owz, Introduction nw Zwzj, Otjmz

Njf. Y., Ntc3: Anatomy y2 Nj Mtk2mduz Mtrimdb Protocol, Yje4mgrimmy1md, ywe3.

Perlman, R., Zjg5ztezn2jhyzzm, Otcxmt Nji3ogq. M2yymdr, Zweyodc, Mgywnti4, and Internetworking Nzewmtcyy, Mmzkndu0yzq4nw, ndc0.

Yjzhmzq5md, Mwqz, Reserved Ymr Otm0mjljow n2 Otfj http://ymy.ietf.org/internet-drafts/mzywmjdlmtflzge1mtvkyzaxzdliztm1yjc4.ntf

Mjc0, Zwjk, Zdn Mmy4 Odzlnzl Protocol, Mjhh Nze2nzbm ogywndjhntgw, ngfl

Mjeznzrhm, Y2yzmti, Njnknjkzow Nwe2njew: TCP/Zd and ATM Nti4zd Principles, Prentice Hall, 1998


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