The network administrator configures the egress ASBR to perform translation of RTs to a VPN distinguisher extended community
attribute, and configures the ingress ASBR to perform translation of the VPN distinguisher to RTs. More specifically, the
translation is achieved as follows:
On the Egress ASBR
An outbound route map specifies a match excommunity clause that determines which VPN routes are subject to mapping, based on the route’s RT values.
A set extcommunity vpn-distinguisher command sets the VPN distinguisher that replaces the RTs.
The set extcomm-list delete command that references the same set of RTs is configured to remove the RTs, and then the route is sent to the neighboring
On the Ingress ARBR
An inbound route map specifies a match excommunity vpn-distinguisher command that determines which VPN routes are subject to mapping, based on the route’s VPN distinguisher.
The set extcommunity rt command specifies the RTs that replace the VPN distinguisher.
For routes that match the clause, the VPN distinguisher is replaced with the configured RTs.
Additional Behaviors Related to the VPN Distinguisher
On the egress ASBR, if a VPN route matches a route map clause that does not have the set extcommunity vpn-distinguisher command configured, the RTs that the VPN route is tagged with are retained.
The VPN distinguisher is transitive across the AS boundary, but is not carried within the iBGP cloud. That is, the ingress
ASBR can receive the VPN distinguisher from an eBGP peer, but the VPN distinguisher is discarded on the inbound side after
it is mapped to the corresponding RTs.
On the ingress ASBR, if a VPN route carrying the VPN distinguisher matches a route map clause that does not have a set extcommunity rt command configured in the inbound route map, the system does not discard the attribute, nor does it propagate the attribute
within the iBGP cloud. The VPN distinguisher for the route is retained so that the network administrator can configure the
correct inbound policy to translate the VPN distinguisher to the RTs that the VPN route should carry. If the route is sent
to eBGP peers, the VPN distinguisher is carried as is. The network administrator could configure a route-map entry to remove
the VPN distinguisher from routes sent to eBGP peers.
Configuring a set extcommunity vpn-distinguisher command in an outbound route map or a match excommunity command in an inbound route map results in an outbound or inbound route refresh request, respectively, in order to update
the routes being sent or received.