Understanding Forwarding Adjacencies
A forwarding adjacency is a traffic engineering label-switched path (LSP) that is configured between two nodes and that is used by the interior gateway protocol (IGP) to forward traffic.
When you set up MPLS traffic-engineering tunnels between sites, by default the IGP does not consider those tunnels for traffic forwarding. Forwarding adjacencies allow you to treat a traffic engineering LSP tunnel as a link in an IGP topology. The link is used in the shortest-path-first (SPF) algorithm and is advertised to the IGP peers. A forwarding adjacency can be created between routing devices regardless of their location in the network.