Specifying the Transport Address Used by LDP

Routers must first establish a TCP session between each other before they can establish an LDP session. The TCP session enables the routers to exchange the label advertisements needed for the LDP session. To establish the TCP session, each router must learn the other router's transport address. The transport address is an IP address used to identify the TCP session over which the LDP session will run.

To configure the LDP transport address, include the transport-address statement:

For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.

If you specify the router-id option, the address of the router identifier is used as the transport address (unless otherwise configured, the router identifier is typically the same as the loopback address). If you specify the interface option, the interface address is used as the transport address for any LDP sessions to neighbors that can be reached over that interface. Note that the router identifier is used as the transport address by default.

Note:

For proper operation the LDP transport address must be reachable. The router-ID is an identifier, not a routable IP address. For this reason its recommended that the router-ID be set to match the loopback address, and that the loopback address is advertised by the IGP.

You cannot specify the interface option when there are multiple parallel links to the same LDP neighbor, because the LDP specification requires that the same transport address be advertised on all interfaces to the same neighbor. If LDP detects multiple parallel links to the same neighbor, it disables interfaces to that neighbor one by one until the condition is cleared, either by disconnecting the neighbor on an interface or by specifying the router-id option.