[c-nsp] prefix-list/route-map quandry

chip chip.gwyn at gmail.com
Mon Feb 6 22:03:30 EST 2006

On 2/6/06, Charles Sporkman <spork.sporkman at gmail.com> wrote:
> Hello,
> I'm getting a little bit stuck here.  I spent the better part of
> friday evening looking at the "IOS Essentials for ISPs" book and at
> the resulting config and just walked away scratching my head...
> What I want to do seems pretty simple (I think).  I have two
> upstreams.  No problems with announcing my prefixes, everything's
> fine.  Inbound, they are both giving me default, and one "ISP X" is
> giving me customer routes as well.  "ISP Y" is my preferred provider.
> I want to use "ISP X" for:
> -backup if I lose "ISP Y"
> -I want to send any traffic destined to their customers to them directly
> <snip>


I'm probably doing something really stupid and blaming it on the route

> map.  Just not sure what I'm doing that would stop the received
> routes...
> Thanks,
> Charles

  On ISP-Y's side build a route-map to set local-preference to something
higher than default, say 110. Leave everything else plain vanilla.

This will set the preference for your default to ISP-Y.  Since you are
receiving customer routes for ISP-X those routes are more specific and will
be used over the default.  The default you receive from ISP-X will have a
lower preference and won't be used unless the default from ISP-Y is
removed.  This should achieve the results you are wanting.

A simple way to set local pref:

router bgp YOUR-ASN
neighbor x.x.x.x remote-as ISP-Y-ASN
neighbor x.x.x.x route-map FROM-ISP-Y in
ip as-path access-list 100 permit ^(ISP-Y-ASN_)+
route-map FROM-ISP-Y permit 100
  match as-path 100
  set local-preference 110

Some of the syntax may be off a bit, but that's the basic config.  I'm
assuming that you are receiving ONLY a default route from ISP-Y and
receiving a default + customer routes for ISP-X.

Just my $.02, your mileage may vary,  batteries not included, etc....

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