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    Creation of a BGP Session Between the CE Router and the Parent VR Overview

    The fallback global option enables traffic that arrives at a VRF from the CE router to be sent out on the uplink determined to be optimal by using the full Internet routing table present in the parent VR.

    If a CE router is multihomed to multiple PE routers, it must receive a full Internet routing table from each of the PE routers so that the CE router can determine which of the PE routers is optimal for a given Internet prefix.

    You can easily create a BGP session from the VRF to the CE router to advertise routes in the VRF to the CE router. However, doing this is insufficient because the VRF does not contain the full Internet routing table, which is present only in the parent VR.

    This situation requires a BGP session from the parent VR to the CE router (Figure 1). This BGP session in turn requires a route in the VRF to the loopback interface in the parent VR that is used for BGP peering with the CE router. To achieve this configuration, you must do both of the following:

    1. In the parent VR, create a shared IP interface for the PE-CE interface and point a static route to the loopback of the CE router to the shared interface.
    2. Use a global import map in the VRF to import into the VRF the route to the loopback interface in the parent VR.

      Figure 1: BGP Session Between CE Router and Parent VR

      BGP Session Between CE Router and Parent

    You must also configure either fallback global or a default route to a manually created shared interface in the VRF. See Example: Configuring a Fallback Global Option or Example: Configuring a Default Route to a Shared Interface for details.

    You can use the BGP session between the CE router and the parent VR to enable the CE router to advertise prefixes within the VPN site that can be reachable from the global Internet. An alternative configuration is to use a global export map as described in Understanding Route Distribution for a VRF using Maps.

    Published: 2014-08-18