ISIS routing update filter

Unanswered Question
May 10th, 2009


How to filter the specific prefixes from IS-IS routing update sending to all neighbor or specific neighbor? for flat ISIS or multi level ISIS design!


Devang Patel

I have this problem too.
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Laurent Aubert Mon, 05/11/2009 - 06:33


If the prefix is part of ISIS domain, you can apply the no isis advertise-prefix on the interface:

If you are redistributing this prefix from another routing protocol then you have the classic route-map you can attach to your redistribution statement.

One thing you can't do is to filter for a specific neighbor. ISIS is a link-state protocol so every routers must have the same topology view.



Giuseppe Larosa Tue, 05/12/2009 - 08:41

Hello Devang,

being IS-IS link state there are limited actions that are possible.

You can hide the details of some specific routes using the following:

build a L2 IS-IS adjacency (different nets on R1 and R2)

declare the interfaces that are associated to the subnets you want to hide as level-1 only.

the default both for the process and for the links is to be L2/L1.

With a setup like this R2 doesn't learn the details of a L1-only prefix on the direct link with R1.

This can lead to R2 learning the prefix from another path.

This happens because interfaces L1-only will be missing from the L2 LSP built by R1 and passed to R2 on the L2 only direct link.

Hope to help


devang_etcom Wed, 05/13/2009 - 11:28


Thanks for your reply! finally no any way to achieve the routing update filter in ISIS. We can filter adj but not updates.


Devang Patel


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