[ixpmanager] BGP communities in Route Server

[Barry O'Donovan]

Salvador Bertenbreiter wrote on 27/08/2020 19:41:
> Cool, great Barry.
> BTW, does the order of the communities affect the behavior like in BGP 
> filters? I mean, if I use "64500:64496" "64500:64503" "0:64500"  instead 
> of "0:64500" 64500:64496" "64500:64503", would it have a different effect?

The relevant bit of config is:

     # Implement widely used community filtering schema.
     if (0, peerasn) ~ bgp_community then
             return false;
     if (routeserverasn, peerasn) ~ bgp_community then
             return true;
     if (0, routeserverasn) ~ bgp_community then
             return false;

which is contained in the function ixp_community_filter(peerasn) which 
is executed when determining if a route should be send to 'peerasn'.

Remembering that the default is to advertise the route then you can see 
from the above that the order doesn't actually matter.

In the above sequence:

1. if the route has the community 0:peerasn (do not advertise to 
peerasn) in any location, then the route is NOT advertised and finish 
processing the route

2. if the route has the community rs-asm:peerasn (do advertise to 
peerasn) in any location, then the route is advertised and finish 
processing the route.

3. if the route has 0:rs-asn (do not advertise to all peers) then it is 
NOT advertised and finish processing the route.

4. not shown above but the default decision: advertise.

Hope that makes sense,

  - Barry