BGP Next-Hop-Self Overview

When a BGP router reports itself as the next hop, whether because of an explicit neighbor next-hop-self configuration or implicitly as a result of participating in an EBGP session, BGP allocates a new in label and adds an entry to the MPLS forwarding table, creating a label-to-next-hop mapping.

When a BGP router does not report itself as the next hop, whether because of an explicit neighbor next-hop-unchanged configuration or implicitly as a result of a participating in an IBGP session, BGP does not allocate a new in label. Instead, if the route is advertised as a labeled route, BGP uses the existing out label. This feature is used mainly on route reflectors.

The determination to allocate an in label is made only after the outbound route map has been processed. Therefore, the in label allocation and the creation of the label-to-next-hop mapping are performed after the need is apparent, conserving the number of in labels allocated.

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