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    Understanding BGP Processing of Received Routes

    BGP processes received routes differently depending on whether the route is labeled or unlabeled, unicast or VPN.

    Labeled Unicast Routes

    When BGP receives a labeled route from a directly connected peer, BGP uses the MPLS major interface that is next to the peer IP interface to resolve the route's BGP next hop. If the MPLS major interface exists and is up, then the next hop is reachable.

    When the received labeled route is not from a directly connected peer, BGP attempts to resolve the BGP indirect next hop of the route in the IP tunnel routing table. When the BGP indirect next hop is reachable, BGP adds the route to both the IP routing table and to the IP tunnel routing table. The route is added as a U-T (unicast-tunnel-usable) route.

    Unlabeled Unicast Routes

    When BGP receives an unlabeled route from a directly connected peer, the route's next hop is resolved to the directly connected interface.

    When the received unlabeled route is not from a directly connected peer, BGP resolves the BGP indirect next hop of the route in the IP routing table. If the BGP indirect next hop is reachable, BGP adds the route to the IP routing table as a U (unicast) route.

    Resolving IPv6 Indirect Next Hops

    When the address of the indirect next hop is an IPv4-mapped IPv6 address, BGP resolves the indirect next hop in the IPv4 routing table and IPv4 tunnel routing table. When the indirect next hop is a native IPv6 address, the indirect next hop is resolved in the IPv6 routing table and IPv6 tunnel routing table.

    Labeled VPN Routes

    In the core VRF, when BGP receives a BGP-labeled VPN route from a multihop VPN peer, it attempts to resolve the BGP indirect next hop in the IP tunnel routing table. If the labeled VPN route is received from a nonmultihop peer, then the BGP indirect next hop is always resolved, because a connected route to that peer exists in the IP tunnel routing table.

    Table 1 summarizes indirect next hop resolution.

    Table 1: Resolution of Indirect Next Hops

    Route Type

    Table in Which BGP Indirect Next Hop Resolves

    Unlabeled unicast

    IP routing table

    Labeled unicast

    IP tunnel routing table, IP routing table

    Labeled VPN

    IP tunnel routing table

    Published: 2014-08-18