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  • 1.  IS-IS Routing Question

    Posted 05-26-2022 09:48

    Just starting to dive into some IS-IS material from the JNCIS-SP material. Read through the materal, and thought I had a decent grasp of it, but when doing some testing in a lab, I'm getting a little lost with the behavior. 

    My configuration is as follows.

    'I thought by separating these into L1-ONLY adjacencies (I disabled Level 2 on every adjacency), this would break inter-area communication, but that doesn't appear to be the case. R3 in area 2, can ping the loopback of R4 in area 3, even though there is no L2 backbone. 

    What am I missing?  



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    GABRIEL HILL
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  • 2.  RE: IS-IS Routing Question

    Posted 05-27-2022 06:02
    Hi, 

    That's because Areas in ISIS have a little different meaning than Areas in OSPF. 

    The first reason we have IS-IS areas is to do with the formation of adjacencies, and the second is the generation of a default route. 

    You got a flat L1 network, and you got routers that are in multiple areas at the same time ( R1 is in Area 3 and Area 1/ also R2 is Area2 and Area1 ) because of that you can form adjacencies across your network(L1 routers need to be in the same area).  Once you have all adj in place all L1 routers will share the same level 1 database, and that's why you can ping from R3 all the way to R4. 

    So in your case, the area dictates how adjacencies are formed and you'll have L1 adj between all routers and as an effect a full view of the network because L1 database is populated and is the same on all routers. 

    Now the area has another role in the generation of a default route ( technically it's not generated, it's a bit called attached bit that it's sent when a router is both L1 and L2 ).
    That's because L2 routers can form adj when you have different areas, and when that router is adj to both an L1 and L2 router it knows that is attached to the backbone and it will send in his L1 PDU a bit ( the attached bit) all other routers in the L1 will see that attached bit and will install a default route toward the L1/L2 router. 

    There is a great article on this Area/Level subject that I encourage you to take a look: 


    https://www.networkfuntimes.com/junos-is-is-study-notes-part-4-for-junipers-jncis-sp-and-jncis-ent-exams/

    Also worth reading part 1 ,2, 3 of the isis on the same blog :D 

    Hope that my explanation helped! 








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    Alin Bandiu
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