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Example: Configuring Synchronization Between IS-IS and LDP


This example shows how to enable synchronization between IS-IS and LDP.


Before you begin, configure IS-IS and LDP. For an example, see Example: Configuring a Layer 3 VPN with Route Reflection and AS Override.


LDP distributes labels in non-traffic-engineered applications. Labels are distributed along the best path determined by IS-IS. If the synchronization between LDP and IS-IS is lost, the label-switched path (LSP) goes down. Therefore, LDP and IS-IS synchronization is beneficial. When LDP synchronization is configured and when LDP is not fully operational on a given link (a session is not established and labels are not exchanged), IS-IS advertises the link with the maximum cost metric. The link is not preferred but remains in the network topology.

LDP synchronization is supported only on point-to-point interfaces and LAN interfaces configured as point-to-point interfaces under IS-IS. LDP synchronization is not supported during graceful restart.

To advertise the maximum cost metric until LDP is operational for LDP synchronization, include the ldp-synchronization statement:

To disable synchronization, include the disable statement. To configure the time period to advertise the maximum cost metric for a link that is not fully operational, include the hold-time statement.


When an interface has been in the holddown state for more than 3 minutes, a system log message with a warning level is sent. This message appears in both the messages file and the trace file.

Figure 1 shows the topology used in this example.

Figure 1: IS-IS and LDP Synchronization Topology
IS-IS and LDP Synchronization

This example describes the steps on Device P1.


CLI Quick Configuration

To quickly configure this example, copy the following commands, paste them into a text file, remove any line breaks, change any details necessary to match your network configuration, and then copy and paste the commands into the CLI at the [edit] hierarchy level.

Device P1

Step-by-Step Procedure

The following example requires you to navigate various levels in the configuration hierarchy. For information about navigating the CLI, see Using the CLI Editor in Configuration Mode in the CLI User Guide.

To configure synchronization between IS-IS and LDP:

  1. Enable MPLS on the interfaces
  2. Enable IS-IS on the interfaces.
  3. Enable LDP on the interfaces.
  4. Enable LDP synchronization on the IS-IS interfaces.
  5. Configure the IS-IS interfaces to behave like point-to-point interfaces.


From configuration mode, confirm your configuration by entering the show protocols command. If the output does not display the intended configuration, repeat the instructions in this example to correct the configuration.

If you are done configuring the device, enter commit from configuration mode. Repeat the configuration on Device R2.


Confirm that the configuration is working properly.

Verifying LDP Synchronization


Check LDP synchronization setting on the IS-IS interfaces.


From operational mode, enter the show isis interface extensive command.

user@P1> show isis interface extensive


The output shows that LDP is synchronized with IS-IS.