MPLS - LDP source interface

When implementing MPLS using LDP, Label Switch Routers will become LDP neighbors and will exchange LDP messages. By default, the transport address (the source address of the LDP messages) is selected based on the highest IP address on a loopback interface. If there is no loopback interface, it is the highest address on an active physical interface.

LDP prefers a loopback interface IP over a physical interface IP for stability reasons. In a typical network, physical interfaces can go down due to various reasons (cable issues, hardware failure, etc.), but a loopback interface is a virtual interface that is always up unless it is manually shut down. Using a loopback interface ensures that the LDP session remains stable even if a physical interface goes down. However, it is possible to change the source of LDP sessions if you have a specific requirement or reason for that.

You can explicitly configure the interface that LDP should use as its source for establishing sessions. On a Cisco IOS device, you’d use the following command in global configuration mode:

mpls ldp discovery transport-address GigabitEthernet0/0

The above command makes the Gi0/0 interface the source for its the exchange of LDP messages with neighbors.

Note that in all cases, the IP used for the source of LDP messages must be reachable using the appropriate routing configuration.