I don't have machines to test this yet. But I would like to ask what would happened if I set up multiple links between an IS to other IS?

Something like this one:

     --  --Link 1--  --
--- |R1| --Link 2-- |R2| ---
     --  --Link 3--  --

All links are point-to-point. Will I see three adjacencies on them? Or they'd just pick one and only that one will be active?

I have tried this on OSPF, it seems OSPF could learn multiple adjacencies link to same router. I'm not sure if ISIS is capable to do so.


I am testing with Cisco routers.

I have two routers with 2 parallel links between them (instead of 3 in your diagram), all of the links are point to point.

It does seem that all of the adjacencies will be active, when I run show clns neighbors or show isis neighbors I see two entries, one for each link

show isis neighbors:

System Id      Type Interface   IP Address      State Holdtime Circuit Id
R26            L1L2 Se1/3       UP    29       02
R26            L1L2 Se1/1        UP    29       01

show clns neighbors:

System Id      Interface   SNPA                State  Holdtime  Type Protocol
R26            Se1/3       *HDLC*              Up     21        L1L2 IS-IS
R26            Se1/1       *HDLC*              Up     29        L1L2 IS-IS

One thing to note though is that in the ISIS lsp TLV 2 you will only see the neighbor listed once.

  • I see, thanks! I guess other links that doesn't include into TLV will act as backups? Though it may not matters as long as router can forward the traffic to calculated destination router...
    – Pue-Tsuâ
    Feb 8 '18 at 7:26

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