Cisco Systems 12.4 How to Configure MPLS LDP

• Platform-wide--An LDP platform supports a single platform-wide label space for use by interfaces that

can share the same labels. For Cisco platforms, all interface types, except LC-ATM, use the platform-

wide label space.

LDP uses a 6-byte quantity called an LDP Identifier (or LDP ID) to name label spaces. The LDP ID is

made up of the following components:

• The first four bytes, called the LPD router ID, identify the LSR that owns the label space.

• The last two bytes, called the local label space ID, identify the label space within the LSR. For the

platform-wide label space, the last two bytes of the LDP ID are always both 0.

The LDP ID takes the following form:

The following are examples of LPD IDs:

• 172.16.0.0:0

• 192.168.0.0:3

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

1The router examines the IP addresses of all operational interfaces.

2If these IP addresses include loopback interface addresses, the router selects the largest loopback

address as the LDP router ID.

3Otherwise, the router selects the largest IP address pertaining to an operational interface as the LDP

router ID.

The normal (default) method for determining the LDP router ID may result in a router ID that is not usable

in certain situations. For example, the router might select an IP address as the LDP router ID that the

routing protocol cannot advertise to a neighboring router. The mpls ldp router-id command allows you to

specify the IP address of an interface as the LDP router ID. Make sure the specified interface is operational

so that its IP address can be used as the LDP router ID.

When you issue the mpls ldp router-id command without the force keyword, the router select selects the

IP address of the specified interface (provided that the interface is operational) the next time it is necessary

to select an LDP router ID, which is typically the next time the interface is shut down or the address is

configured.

When you issue the mpls ldp router-idcommand with the force keyword, the effect of the mpls ldp

router-idcommand depends on the current state of the specified interface:

• If the interface is up (operational) and if its IP address is not currently the LDP router ID, the LDP

router ID changes to the IP address of the interface. This forced change in the LDP router ID tears

down any existing LDP sessions, releases label bindings learned via the LDP sessions, and interrupts

MPLS forwarding activity associated with the bindings.

•If the interface is down (not operational) when the mpls ldp router-idinterface force command is

issued, when the interface transitions to up, the LDP router ID changes to the IP address of the

interface. This forced change in the LDP router ID tears down any existing LDP sessions, releases

label bindings learned via the LDP sessions, and interrupts MPLS forwarding activity associated with

the bindings.

How to Configure MPLS LDP

• Enabling Directly Connected LDP Sessions, page 6

MPLS Label Distribution Protocol (LDP)

How to Configure MPLS LDP

MPLS LDP Configuration Guide, Cisco IOS Release 12.4