BGP Additional-paths

BGP additional-paths enables the advertisement of multiple paths for the same prefix without new paths implicitly replacing the previous paths. Path diversity is achieved rather than path hiding.

A BGP device generally advertises only its best path to neighboring devices, even when multiple paths exist; the advertisement of the same prefix from the same neighbor replaces the previous announcement of that prefix. This path hiding is an implicit withdrawal behavior, which achieves better scaling but at the cost of path diversity. Path hiding can affect the efficient use of BGP multipath and path diversity, and prevent hitless planned maintenance. Upon next-hop failures, path hiding also inhibits fast and local recovery because the network must wait for BGP control plane convergence to restore traffic.

The following figure shows two types of path hiding:

• Paths P1 and P2 (for prefix P) are advertised to RR1. P1 is advertised from BR1 and P2 is advertised from BR2. RR1 selects P1 as the best path and advertises only P1 to PE.
• Paths X1 and X2 (for prefix X) are advertised to BR4. X1 is advertised from BR3 with a local preference of 100. BR4 also has path X2 with a local preference 50. However, only the best path (X1) is selected. BR3 advertises X1 to the RRs and X2 is suppressed.

BGP path hiding

BGP additional-paths enables BGP to advertise even secondary best routes, so that multiple paths for the same prefix can be advertised without the new paths implicitly replacing previous paths. BGP additional-paths provides a generic way of offering path diversity.

BGP additional-paths is implemented by including an additional four-octet value known as a path identifier (ID) for each path in the Network Layer Reachability Information (NLRI). Path IDs are unique to a peering session and are generated for each network. That is, a generated path ID is unique for each peer for each prefix.

To send or receive additional paths, the additional-paths capability must be negotiated. When it is not negotiated, only the best path is sent.

When the additional-paths capability is negotiated, BGP updates carry the path ID. To carry the path ID in an update message, the existing NLRI encodings are extended by prepending the path ID field, which consists of four octets.

The BGP device uses the prefix and path ID to uniquely identify a path advertised to a neighbor so as to continue to send updates for that path. The receiving BGP neighbor that re-advertises a route regenerates its own path ID that is to be associated with the re-advertised route.

The set of additional paths advertised to each neighbor can be different, and advertisement filters are provided for each neighbor.

The following figure shows a BGP additional-paths configuration that supports path diversity.

BGP additional-paths

• Although route reflector RR1 has two sources for prefix N, it advertises only the best route to its peers when the additional-paths capability is not configured.
• When the additional-paths capability is configured at RR1 and PE3, PE3 receives two routes to prefix N. Receiving two routes facilitates quick network convergence and recovery:
  • Using PE1 and PE2
  • ECMP paths to PE1 and PE2