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Route Distinguishers

Because a typical transit network is configured to handle more than one VPN, the provider routers are likely to have multiple VRF instances configured. As a result, depending on the origin of the traffic and any filtering rules applied to the traffic, the BGP routing tables can contain multiple routes for a particular destination address. Because BGP requires that exactly one BGP route per destination be imported into the forwarding table, BGP must have a way to distinguish between potentially identical network layer reachability information (NLRI) messages received from different VPNs.

A route distinguisher is a locally unique number that identifies all route information for a particular VPN. Unique numeric identifiers allow BGP to distinguish between routes that are otherwise identical.

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