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Equal-Cost Multipath Support

Equal-cost multipath (ECMP) is a traffic load-balancing feature that enables traffic to the same destination to be distributed over multiple paths that have the same cost. BGP ECMP support for BGP/MPLS VPNs enables MPLS VPN routes to be included in the list of available equal-cost paths. You can specify that up to 16 equal-cost paths be considered.

The set of ECMP legs in a network can contain MPLS indirect next hops, either as a leg itself or pointed to by a leg. If the path to any of the MPLS indirect next hops fails, then the routing protocol begins recalculating the set of viable routes as soon as it is notified of the failure. When the recalculation has finished, the protocol then updates the routing table with the new routes.

From the time the path fails until the routing table is updated, the traffic flowing over the ECMP leg that has the failed MPLS indirect next hop is lost.

To reduce the amount of lost traffic, the failed path is quickly pruned from the ECMP set as soon as the protocol is notified of the connectivity failure. Traffic for the destination is then forwarded over the remaining equal-cost paths to the destination. When the recalculated set of routes is installed in the routing table, traffic for the destination is forwarded by means of the new route.

ECMP sets can have an MPLS indirect next hop as one of the legs in the following scenarios:

In a BGP-MPLS VPN where a given VPN prefix is learned from multiple PE routers.
When multiple RSVP-TE tunnels are created over different paths to the same destination.
In a network that connects IPv6 islands across an IPv4 core, where a given IPv6 prefix is learned from multiple egress PEs running IPv6.

Consider the simple ECMP scenario for a BGP/MPLS VPN shown in Figure 68.

Figure 68: ECMP BGP/MPLS VPN Scenario

Image g016448.gif

With respect to PE 1, this network has an ECMP set of two equal-cost legs for the VPN prefix of CE 2, 192.168.0.1/32:

PE 1 -> P 1 -> PE 2 -> CE 2
PE 1 -> P 2 -> PE 3 -> CE 2

The details of these routes are displayed by the following command:

host1:pe1:pe1-ce1#show ip route 192.168.0.1 detail
192.168.0.1/32 Type: Bgp Distance: 200 Metric: 0 Tag: 0 Class: 0
  MPLS next-hop: 741, ECMP next-hop, leg count 2
    MPLS next-hop: 389, label 17, VPN traffic, resolved by MPLS next-hop 376
      MPLS next-hop: 376, resolved by MPLS next-hop 385, peer 10.3.3.3
        MPLS next-hop: 385, label 24 on GigabitEthernet1/1/0.2
(ip19000002.mpls.ip [V:pe1]), nbr 10.3.2.2
    MPLS next-hop: 740, label 18, VPN traffic, resolved by MPLS next-hop 729
      MPLS next-hop: 729, resolved by MPLS next-hop 737, peer 10.2.2.2
        MPLS next-hop: 737, label 27 on GigabitEthernet1/1/0.1
(ip19000001.mpls.ip [V:pe1]), nbr 10.3.1.2

If the connection to PE 2 fails, BGP marks the MPLS next hop 729 as a failed indirect next hop as soon as BGP is notified of the loss of connectivity. However, some traffic continues to be forwarded to CE 2 through PE 2; this traffic is lost. BGP quickly prunes the failed route from the FIB, stopping this traffic loss, and then recalculates the routes to CE 2. During this period, traffic for CE 2 is forwarded only through PE 3. When the new routes are installed in the FIB, traffic is forwarded to CE 2 by means of the newly installed route.

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