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BGP Destinations

 

You can configure MPLS to control the paths that traffic takes to destinations outside an AS.

Both IBGP and EBGP take advantage of the LSP host routes without requiring extra configuration. BGP compares the BGP next-hop address with the LSP host route. If a match is found, the packets for the BGP route are label-switched over the LSP. If multiple BGP routes share the same next-hop address, all the BGP routes are mapped to the same LSP route, regardless of which BGP peer the routes are learned from. If the BGP next-hop address does not match an LSP host route, BGP routes continue to be forwarded based on the IGP routes within the routing domain. In general, when both an LSP route and an IGP route exist for the same BGP next-hop address, the one with the lowest preference is chosen.

Figure 1 shows an MPLS topology that illustrates how MPLS and LSPs work. This topology consists of a single domain with four routers. The two routers at the edges of the domain, Router 1 and Router 4, are running EBGP to communicate with peers outside the domain and IBGP to communicate between themselves. For intradomain communication, all four routers are running an IGP. Finally, an LSP tunnel exists from Router 1 to Router 4.

Figure 1: MPLS Application Topology
MPLS Application Topology

When BGP on Router 1 receives prefixes from Router 4, it must determine how to reach a BGP next-hop address. Typically, when traffic engineering is not enabled, BGP uses IGP routes to determine how to reach next-hop addresses. (See the left side of Figure 2.) However, when traffic engineering is enabled, if the BGP next-hop matches the LSP tunnel endpoint (that is, the MPLS egress router), those prefixes enter the LSP tunnel. (To track these prefixes, look at the Active Route field in the show mpls lsp command output or at the output of the show route label-switched-path path-name command.) If the BGP next hop does not match an LSP tunnel endpoint, those prefixes are sent following the IGP’s shortest path. (See Figure 2.)

Figure 2: How BGP Determines How to Reach Next-Hop Addresses
How BGP Determines How to Reach Next-Hop
Addresses