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LDP Operation

1) LDP Discovery:

There are two types of LDP discoveries-

a) A basic discovery mechanism used to discover directly connected LSR neighbors

In basic discovery mechanism, an LSR sends out periodic Hello messages as UDP packets to “all routers on this subnet” i.e. to discover neighbors. The Hello messages contain the LDP Identifier (Router ID: Label Space) it intends to use with other information. The LDP ID is a 6-byte value with 4-byte Router ID and 2-byte Label Space. The 4-byte Router ID is configurable. By default, the Router ID is the highest Loopback interface on the router or in case the Loopback interface is not present, the highest active interface. Router ID can be configured using mplsldp router-id [force] command.

The router determines the LDP router ID as follows, if the mplsldprouter-id command is not executed,

1. The router examines the IP addresses of all operational interfaces.
2. If these IP addresses include loopback interface addresses, the router selects the largest loopbackaddress as the LDP router ID.
3. Otherwise, the router selects the largest IP address pertaining to an operational interface as the LDProuter ID.

The normal (default) method for determining the LDP router ID may result in a router ID that is not usablein certain situations. For example, the router might select an IP address as the LDP router ID that therouting protocol cannot advertise to a neighboring router. The mplsldprouter-id command allows you tospecify the IP address of an interface as the LDP router ID. Make sure the specified interface is operationalso that its IP address can be used as the LDP router ID.

When you issue the mplsldprouter-id command without the force keyword, the router select selects theIP address of the specified interface (provided that the interface is operational) the next time it is necessaryto select an LDP router ID, which is typically the next time the interface is shut down or the address isconfigured.

When you issue the mplsldprouter-idcommand with the force keyword, the effect of the mplsldprouter-idcommand depends on the current state of the specified interface:

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