selecting only learned default route for certain subnet

Unanswered Question
Jul 15th, 2010

Did a  search and didn't find what I was looking for. I'm hoping someone can point me in the right direction here.  I need to prevent certain portions of my IPs from using routes learned from a specific peer.

We have 1 edge switch/router...will be either a 3550-12T that I have lying around or a 6509 w/ a SUP2U.

ARIN assigned IP: 10.0.1.0/22 (using private  addresses for this post)

  • Peer A:  local-pref 400, importing 15K routes, and announcing 10.0.1.0/23 and  10.0.3.0/24  <== only the first 3 /24s
  • Peer B: local-pref 300, importing 1 route (default route),  announcing 10.0.1.0/22 with padded AS

  • (since I  only have 1 edge router in the AS, I can also use weights instead of  local-pref)
  • So the issue is that the last /24 (10.0.4.0/24)  cannot use Peer A for incoming or outbound traffic.  We deal with the  inbound traffic by only announcing reachability for the first 3 /24s to Peer A.  I have no idea  about how to handle the outbound traffic -- all of which has to go out of the default route learned from Peer B.

    I'm guessing one way is to use policy based routing (PBR) and do a set ip next-hop <address>.  However, I won't know what the address is...as it will be learned and could change.  I see an option set ip next-hop <interface>, but that won't work in this instance.  How would I do PBR here?

    Also -- Is there any way other than PBR to do this?  I'd like to use an old 3550 (capable of having 24K unicast routes) but enabling PBR will change the size of the TCAM entries and 1/2 the amount of unicast routes I can take.

    Any help you guys can  provide will be appreciated.  Config snippets, pointers to documenation,  etc.

    Thanks in advance,

    Jim

    I have this problem too.
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    milan.kulik Mon, 07/19/2010 - 08:30

    Hi,

    ad) "I'm guessing one way is to use policy based routing (PBR) and do a set ip next-hop .  However, I won't know what the address is...as it will be learned and could change."

    Shouldn't that always be your neighbor B interface address?

    ad) "Is there any way other than PBR to do this?  I'd like to use an old 3550."

    What is the subnets used to peer with B size? If it's bigger than /30, you could connect the  10.0.4.0/24 directly to the old 3550, and connect another 3550's interface to the peering subnet. You'd configure a default route only on the 3500 pointing to peer B. And advertise the 10.0.4.0/24 to B from your ende router with next-hop set pointing to the 3550.

    I might be missing something, as don't know all the details, of course.

    BR,

    Milan

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