[nsp] conditional BGP config - help

[Sam Stickland]

matthew zeier wrote:
> I have three transit providers - A, B, C - and want to setup
> conditional BGP advertisements such that I only advertise my routes
> to C is both A and B are down.

This works for two well known IP prefixes on A and B (MFN and Level3 are in
the example below), with known stable AS paths (ie. IP addresses from the
providers own block).

neighbor provider.c.ip advertise-map c-failover non-exist-map
track-providers_MFN_Level3

route-map track-providers_MFN_Level3 permit 10
 match ip address prefix-list MFN-track
 match as-path 10
!
route-map track-providers_MFN_Level3 permit 20
 match ip address prefix-list L3-track
 match as-path 11

ip prefix-list L3-track seq 5 permit 209.245.19.0/24
ip prefix-list MFN-track seq 5 permit 207.126.96.0/19

ip as-path access-list 10 permit ^6461$
ip as-path access-list 11 permit ^3356_10753$

If either of those IP prefixes disappear, and/or are only aren't available
via the specified AS paths (ie. any one of the conditions is no longer met)
then you will start advertising routes on Provider C.

Sam

> I'm not clear on how to setup the non-exist-map.  The only routes
> that I think I can base my condition on are the transit interfaces
> and the /30 networks.  However, I can't figure out from the docs
> (http://tinyurl.com/3gbrl) if the non-exist-map is either-or or
> whether both have to match for the condition to be true.
>
> Is it easier/better to match on the as-path instead of the transit
> interfaces?  Something like:
>
> ip as-path access-list 1 permit ^ASN-of-provider-A
> ip as-path access-list 1 permit ^ASN-of-provider-B
>
> neighbor 10.10.10.1 advertise-map ADVERTISE non-exist-map NON-EXIST
>
> route-map NON-EXIST permit 10
>  match as-path 1
>
>
> ?
>
> Thanks.