Operational BGP Communities

Prefix redistribution controlling
Route server peers will be able to manipulate outbound routing policies via an in-band mechanism using BGP communities,
Currently, we offer the following options:
BGP Community BGP Extended Community BGP Large Community Description
0:peer-as RT:0:peer-as 63529:0:peer-as Do not announce a prefix to a certain peer
63529:peer-as RT:63529:peer-as 63529:1:peer-as Announce a prefix to a certain peer
0:63529 RT:0:63529 63529:0:0 Do not announce a prefix to any peer
63529:63529 RT:63529:63529 63529:1:0 Announce a prefix to all peers
65003:peer-as RT:65003:peer-as 63529:103:peer-as Prepend 3 time to peer-as
65002:peer-as RT:65002:peer-as 63529:102:peer-as Prepend 2 time to peer-as
65001:peer-as RT:65001:peer-as 63529:101:peer-as Prepend 1 time to peer-as
For destination peers employing a 32-bit ASN, please consider using BGP Extended Communities or BGP Large Communities

With BGP Large Communities Attribute [RFC8092], Network operators can continue to build upon well established practises in the tradition of [RFC1997]

The Use of BGP Large Communities is also described in [RFC8195]

{63529:63529} or no community
This route will be advertised to anyone (Default)
{63529:63529 0:63529} or {0:63529}
This route will not be advertised to anyone

This route will be advertised to everyone except AS 65001
{0:65001 0:65002}
This route will be advertised to everyone except AS 65001 and AS 65002
{0:65003 63529:65003}
This route will be advertised to everyone except AS 65003

{0:63529 63529:65001}
This route will be advertised toward AS 65001 only
{0:63529 63529:65001 63529:65002}
This route will be advertised toward AS 65001 and AS 65002 only
{0:63529 63529:65001 63529:65002 63529:65003 0:65002}
This route will be advertised toward AS 65001 and AS 65003 only
Config guide
ip classless
ip bgp-community new-format
router bgp (your AS)
address-family ipv4
  neighbor activate
  neighbor activate
  neighbor BKNIX-RS-v4 send-community
  neighbor BKNIX-RS-v4 next-hop-self
  neighbor BKNIX-RS-v4 route-map BKNIX-out out
  neighbor BKNIX-RS-v4 route-map BKNIX-in in
route-map BKNIX-in permit 10				#Accept all prefixes from RS
route-map BKNIX-out permit 10				#Do not announce this prefixes to everyone
   match ip address prefix-list filter-out
   set community 0:63529 additive
route-map BKNIX-out permit 20
   match community .....				#Announce all route matched these community to everyone
   set community 63529:63529

route-map BKNIX-out permit 30
   match community .....				#Deny route matched these community to ASN131071 (32 bits ASN)
   set extcommunity rt 0:131071

ip prefix-list filter-out seq 10 permit x.x.x.x/y

Graceful BGP Session Shutdown
BKNIX Route servers accept this well-known BGP Community, RFC 8326 (Graceful BGP Session Shutdown) which indicate our route servers to recalculate and redistribute an alternate path (if available) for your prefixes announcement.
*It is recommended that all RS clients should apply GRACEFUL_SHUTDOWN at the inbound direction towards your RS BGP sessions. NLNOG has a good example here.

Example : Operations at Maintenance Time
  • Member announces all the prefixes with BGP Community GRACEFUL_SHUTDOWN (65535:0) to route servers.
  • Route servers will set BGP local_preference of these prefixes to 0.
  • Route servers will recalculate the alternative path of these prefixes.
    • if prefixes have the alternative path, new advertised prefixes will be redistributed to clients.
    • if prefixes do not have the alternative path, same prefixes remain advertised with BGP Community GRACEFUL_SHUTDOWN.
  • Other RS clients have learned alternate paths and forward traffic through the new paths. At this point, Member can safely shutdown your BGP session.

21 July 2021 | Posted by Admin