BGP VPN Route Leaking

Route leaking refers to the process of adding a BGP VPN route to the routing table of the local or remote VPN instance in a BGP/MPLS IP VPN scenario. Route leaking can be classified as local route leaking or remote route leaking based on the source of the BGP VPN route.

Remote route leaking: After a PE receives a BGP VPNv4/VPNv6 route from a remote PE, the local PE matches the export target (ERT) of the route against the import targets (IRTs) configured for local VPN instances. If the ERT matches the IRT of a local VPN instance, the PE converts the BGP VPNv4/VPNv6 route to a BGP VPN route and adds the BGP VPN route to the routing table of this local VPN instance.
Local route leaking: A PE matches the ERT of a BGP VPN route in a local VPN instance against the IRTs configured for other local VPN instances. If the ERT matches the IRT of a local VPN instance, the PE adds the BGP VPN route to the routing table of this local VPN instance. Locally leaked routes include locally imported routes or routes learned from VPN peers.

After a PE receives VPNv4 routes destined for the same IP address from another PE or VPN routes from a CE, the local PE implements route leaking by following the steps shown in Figure 1.

Figure 1 Flowchart for BGP VPN route leaking

In Figure 2, PEs have the same VPN instance (vpna) and the RTs among VPN instances match each other. The RD configured for PE2 and PE3 is 2:2, and that configured for PE4 is 3:3. Site 2 has a route destined for 10.1.1.0/24. The route is sent to PE2, PE3, and PE4, which then convert this route to multiple BGP VPNv4 routes and send them to PE1. Upon receipt of the BGP VPNv4 routes, PE1 implements route leaking as shown in Figure 3. The detailed process is as follows:

After receiving the BGP VPNv4 routes from PE2, PE3, and PE4, PE1 adds them to the BGP VPNv4 routing table.
PE1 converts the BGP VPNv4 routes to BGP VPN routes by removing their RDs, adds the BGP VPN routes to the routing table of the VPN instance, selects an optimal route from the BGP VPN routes based on BGP route selection rules, and adds the optimal BGP VPN route to the IP VPN instance routing table.

Figure 2 Route leaking networking
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Figure 3 BGP VPN route leaking process