Use case

In today's post we will explain Cisco configuration for Inter-AS MPLS L3VPN Option B. Here are the main characteristics of this option:

  1. VPNv4 eBGP session is established between ASBR's, thus they have to hold all VPNv4 routes that are exchanged between Service Providers. It makes this option less scalable than Option C, where VPNv4 routes are stored on Route Reflectors only.
  2. Given the fact that ASBR has to accept iBGP VPNv4 routes, one of the following conditions have to be met: route target filtering has to be disabled, VRF with "route-target import" has to be configured or ASBR has to be configured as Route Reflector (which essentially disables route target filtering in the background).
  3. "Next-hop-self" has to be configured for iBGP VPNv4 session on ASBR because, even though the address family is VPNv4, next hop is not changed automatically as in case of PE advertising VRF route.
  4. Packets betweem CE routers travel via 3 LSP's: PE to ASBR1, ASBR1 to ASRB2 and ASBR2 to PE2. Remember that each LSP terminates where next hop changes.

Configuration

ASBR1

Routing protocols configuration

router bgp 65001
bgp log-neighbor-changes
no bgp default ipv4-unicast
neighbor 2.2.2.2 remote-as 65001
neighbor 2.2.2.2 update-source Loopback0
neighbor 3.3.3.3 remote-as 65001
neighbor 3.3.3.3 update-source Loopback0
neighbor 192.168.0.11 remote-as 65002
!
address-family ipv4
exit-address-family
!
address-family vpnv4
no bgp default route-target filter
neighbor 3.3.3.3 activate
neighbor 3.3.3.3 send-community extended
neighbor 3.3.3.3 next-hop-self
neighbor 192.168.0.11 activate
neighbor 192.168.0.11 send-community extended
exit-address-family

ASBR2

Routing protocols configuration

router bgp 65002
address-family vpnv4 unicast
retain route-target all
!
neighbor 12.12.12.12
remote-as 65002
update-source Loopback0
address-family vpnv4 unicast
next-hop-self
!
!
neighbor 192.168.0.4
remote-as 65001
address-family vpnv4 unicast
route-policy ACCEPT_ALL in
route-policy ACCEPT_ALL out
!
!
!