Example: Enabling BGP ECMP for BGP/MPLS VPN IBGP

Enabling ECMP support for BGP/MPLS VPNs allows multiple VPN routes to be included in the list of available equal-cost paths. You can use the maximum-paths command with the ibgp or eibgp keywords to enable ECMP support for BGP/MPLS VPNs.

The eibgp keyword specifies that the E Series router consider both external BGP (EBGP) and internal BGP (IBGP) paths when determining the number of equal-cost paths to the same destination that BGP can submit to the IP routing table. The ibgp keyword specifies that the E Series router consider multiple internal IBGP paths, but not EBGP paths, when determining the number of equal-cost paths.

You can create an ECMP environment in which multiple IBGP paths are selected as multipaths and used for load balancing. In the example shown in Figure 94, the E Series router gives equal consideration to IBGP VPN routes learned from multiple remote PE devices when determining load balancing.

Figure 94: BGP/MPLS VPN IBGP Example

The sample BGP/MPLS network connects PE 1, PE 2, and PE 3, which are configured for VPNv4 unicast IBGP peering. CE 1 and CE 2 are configured for EBGP peering with the PE devices. CE 2 is multihomed, connected to both PE 2 and PE 3.

VRF A has two equal-cost paths through the MPLS network to get to CE 2: the IBGP path to PE 2, and the IBGP path to PE 3.

To support BGP/MPLS ECMP, PE 1 is configured with the maximum-paths ibgp command under IPv4 unicast VRF A address family. Doing this allows IBGP paths from both PE 2 and PE 3 to be selected as multipaths for use in load balancing.

Traffic from CE 1 to CE 2 that takes an IBGP route from PE 1 to either PE 2 or PE 3 is forwarded as MPLS-encapsulated packets. PE 2 and PE 3 receive the MPLS-encapsulated traffic from PE 1, remove the MPLS encapsulation, and then forward the traffic as IP packets by means of their EBGP route to CE 2.