Configuring LDP-IGP Synchronization

LDP-IGP synchronization helps minimize traffic interruptions when the traffic is switched from the backup link to the primary link and implement millisecond-level switchovers.

Usage Scenario

You can configure LDP-IGP synchronization to prevent traffic loss after the primary LSP fails on the network where primary and backup links exist. The details are as follows:

When the primary link is restored and an LDP session or an LDP adjacency between nodes along the primary link fails, LSP traffic is discarded because LSP traffic is switched from the primary link to the backup link, whereas IGP traffic is still transmitted through the primary link.
When the primary link fails, IGP traffic and LSP traffic are switched to the backup link. When the primary link recovers from a fault, LSP traffic is discarded because IGP converges faster than LDP, and IGP traffic is switched to the primary LSP earlier than LDP traffic.

Both OSPF and IS-IS support LDP-IGP synchronization.

Pre-configuration Tasks

Before configuring LDP-IGP synchronization, complete the following tasks:

Configure basic IGP (OSPF or IS-IS) functions.
Enable MPLS globally and on each interface.
Enable MPLS LDP globally and on each interface.

Enabling LDP-IGP Synchronization: LDP-IGP synchronization needs to be enabled on interfaces on both ends of the link between the node where a primary LSP and a backup LSP diverge from each other and its LDP peer on the primary LSP. LDP-IGP synchronization is enabled either on an interface or in an IGP process.

(Optional) Blocking LDP-IGP Synchronization on an Interface: If you do not want to run LDP-IGP synchronization on some interfaces, you can block the function on these interfaces.

(Optional) Setting a Value for the Hold-down Timer: This section describes how to set a value for the hold-down timer. Before the timer expires, an interface waits for the setup of an LDP session and an LDP adjacency without setting up an IGP neighbor relationship.

(Optional) Setting the Hold-max-cost Timer Value: When an LDP session or an LDP adjacency on a primary LSP fails, traffic is transmitted along a backup LSP within the timeout period of the hold-max-cost timer before the LDP session and LDP adjacency on the primary LSP recovers.

(Optional) Setting the Value of the igp-sync-delay Timer: When an LDP session is reestablished on a faulty link, LDP starts the igp-sync-delay timer to wait for the establishment of an LSP. After the igp-sync-delay timer expires, LDP-IGP synchronization is complete.

(Optional) Configuring LDP Graceful Deletion: LDP graceful deletion can be configured to speed up traffic switching using LDP-IGP synchronization, which improves network reliability.

Verifying LDP-IGP Synchronization Configuration: After configuring LDP-IGP synchronization on an interface, you can view the synchronization status of interfaces with LDP-IGP synchronization enabled.