It's a bit non-obvious, but you can do this with a loop. For example we basically replicate the Euro-IX routeserver announcement control communities inside our network, and we use the following function to translate them when exporting routes to a (supporting) route server

# Translate Euro-IX common communities for use by a route server
function translate_routeserver_communities(
  int dest_asn
)
{
  lclist announce_controls = filter(bgp_large_community, [(OURAS, 0..1, *)]);
  for lc c in announce_controls do {
    bgp_large_community.add((dest_asn, c.data1, c.data2));
  }

  lclist prepends = filter(bgp_large_community, [(OURAS, 101..103, *)]);
  for lc c in prepends do {
    bgp_large_community.add((dest_asn, c.data1, c.data2));
  }
}

(I think we could combine those into one with a filter(bgp_large_community, [(OURAS, 0..1, *), (OURAS, 101..103, *)])? This code originally looked slightly different and at the time combining these wasn't posible.

Note that this doesn't strip the input communities - we do that as a separate step later where we strip all non-informational communities

- Erin

On Tue, 20 Feb 2024, at 00:30, Ilham Maulana wrote:
Hi,


Is it possible in bird to copy specific value inside BGP Community?

Example:

Route 1, Community (a,b,c) -> inbound
Route 2, Community (a,b,d) -> inbound
--
Route 1, Community (k,l,c) -> outbound
Route 2, Community (k,l,d) -> outbound

The specific value I want to preserve is c and d, and it is dynamic variable.

whatever c and d inbound, copy to c and d outbound.

Thanks.
Ilham