Path selection for signaling occurs in the outbound direction (ST-PE to TT-PE) for an MS-PW. In the TT-PE to ST-PE direction, a label mapping message follows the reverse of the path already taken by the outgoing label mapping.
A node can use explicit paths, static routes, or BGP routes to select the next hop S-PE or T-PE. The order of preference used in selecting these routes is:
Explicit Path
Static route
BGP route
To use an explicit path for an MS-PW, an explicit path must have been configured in the config>services>pw-routing>path path-name context. The user must then configure the corresponding path path-name under spoke-sdp-fec.
If an explicit path name is not configured, the TT-PE or S-PE performs a longest match lookup for a route (static if it exists, and BGP if not) to the next hop S-PE or T-PE to reach the TAII.
Pseudowire routing chooses the MS-PW path in terms of the sequence of S-PEs to use to reach a specific T-PE. It does not select the SDP to use on each hop, which is instead determined at signaling time. When a label mapping is sent for a specific pseudowire segment, an LDP SDP is used to reach the next-hop S-PE/T-PE if such an SDP exists. If not, and a RFC 3107 labeled BGP SDP is available, then that is used. Otherwise, the label mapping fails and a label release is sent.