LDP uses LDP messages to communicate between LDP peers for the correct functioning of LDP. All LDP messages contains a LDP header which is composed of LDP version, length of message, LDP ID, and is followed by a message. The LDP ID for LDP is composed of the LSR-ID and label space. The LSR ID is the first available loopback interface address. However, you can specify an IP address of your choice to use as the LSR ID for the LDP identifier.
By default, Extreme routers select the first valid and operationally UP IP address among all the enabled loopback interfaces as LSR ID for LDP. When the IP address or loopback interface that is used as the LSR ID goes down, LDP selects the next operationally UP IP address among all enabled loopback interface as the LSR ID. Otherwise, LDP will be down.
When no valid IP address is available to be selected as the LSR ID, LDP continues to remain disabled until a valid IP address is configured on an enabled loopback interface.
LDP uses a configured IP address as the LSR ID only when this IP address is configured on one of the enabled loopback interfaces. After you configure an LSR ID with a valid IP address, LDP must use the configured value as the LSR ID and restarts to use the new address.
When this IP address is not configured in the enabled state on any of the loopback interfaces, LDP continues in the disabled state. LDP is enabled as soon as this IP address is configured on one of the enabled loopback interfaces.
When you disable the feature, the LSR ID selection procedure falls back to default behavior of selecting an LSR ID for LDP when LDP is enabled.