[quagga-dev 10384] Re: Route selection help

Pradosh Mohapatra pmohapat at cumulusnetworks.com
Thu Mar 28 23:22:21 GMT 2013

Hi Sami,

> I have a question regarding BGP and best route selection.
> Here is my setup:
> my BGP is AS0, connected to 2 BGP PEERS (AS1 & AS2).
> I usually prefer routes via AS1, all past time I used local-prference to go through AS1, it worked great since i had equal prefixes, lets say for the example /23. My problems began when AS2 announced me /24 prefixes, and i still got /23 from AS1. So /24 was preffered.
> I'm talking about 2000 prefixes so making a manual prefix-list and change next-hop isn't an option.
> Is there any mechanism to di:
> For every prefix i get from AS2 check if there is already a route via AS1, if so change next-hop to be AS1 peer?
> Please help me here, My line via AS2 has small bandwidth so I need to accomplish this.

New to Quagga, but from previous experience:

As you may know, BGP treats x/23 and x/24 as separate destinations and each destination gets independent forwarding behavior. Thus, there is no easy way to achieve exactly what  you want.

I also think that changing the next-hop of routes from AS2 is not a good idea - that essentially eliminates your backup path.

If AS2 still advertises the /23s (in addition to the more specifics), you may want to block (any) /24 using a simple prefix-list/route-map combination (something like ge 24). I do not know all the details to determine whether you can afford to block all /24s.

Happy playing with route-maps ;-)

- Pradosh

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