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Configuring Label Distribution Protocol Synchronization

The Label Distribution Protocol (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 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:




ldp-synchronization {

disable;

hold-time seconds;
}

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.

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

For a list of hierarchy levels at which you can configure these statements, see the statement summary section for these statements.

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