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    Traffic Flow from the VPN to the Internet Overview

    Traffic from a CE router arrives on a PE interface that exists in the context of a VRF. The PE router then looks up the destination address of the IP packet in the context of the VRF routing table rather than the VR routing table.


    The VRF routing table lookup introduces the following complication.

    • The size of the Internet routing table. Placing a full default-free Internet routing table in the VRF routing table is not feasible because it does not scale. The PE router would have to support more than 100,000,000 routes, because the full default-free Internet routing table is currently about 120,000 routes and the router must support up to 1,000 VRFs.


    The following methods enable advertising of Internet routes to VPN sites and thus enable traffic flow from the VPNs to the Internet:

    • Configure default routes instead of a full default-free Internet routing table in the VRF. The default routes must point to a shared IP interface that you create on top of the layer 2 interface that points to the Internet gateway.
    • Configure a single full default-free Internet routing table in the context of the parent VR and share this one table among all VRFs with the fallback global feature. Fallback global enables an additional lookup in the IP routing table of the parent VR in the event that the IP route lookup in the child VRF fails.
    • When reachability to a small number of networks in the Internet is required, then configure a global import map to import only the specific route to these networks into the VRF.

    You can create multiple IP interfaces on top of a single layer 2 interface. One of those interfaces is the primary IP interface for receiving and sending IP packets. The other interfaces are shared IP interfaces that are used only to send traffic.

    Published: 2014-08-18