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    Example: Simple ECMP Scenario for BGP/MPLS VPN

    Consider the simple ECMP scenario for a BGP/MPLS VPN shown in Figure 1.

    Figure 1: ECMP BGP/MPLS VPN Scenario

    ECMP BGP/MPLS VPN Scenario

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

    • 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 detail 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
            MPLS next-hop: 385, label 24 on GigabitEthernet1/1/0.2
    (ip19000002.mpls.ip [V:pe1]), nbr
        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
            MPLS next-hop: 737, label 27 on GigabitEthernet1/1/0.1
    (ip19000001.mpls.ip [V:pe1]), nbr

    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.

    Published: 2014-08-18