Describes MPLS LDP IGP synchronization, providing instructions for configuring MPLS LDP IS-IS synchronization to ensure label distribution protocol and IGP alignment, preventing traffic loss during state changes.
MPLS label distribution protocol IGP synchronization is a network feature that
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ensures Label Distribution Protocol (LDP) completes label exchange before an Interior Gateway Protocol (IGP) path is used for MPLS switching
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coordinates LDP session status with IS-IS link metrics to minimize MPLS packet loss, and
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improves network stability and reliability by preventing traffic loss during adjacency changes.
| MPLS Label Distribution Protocol IGP Synchronization |
Release 25.4.1 |
Introduced in this release on: Fixed Systems (8700 [ASIC: K100])(select variants only*) *This feature is supported on Cisco 8711-48Z-M routers. |
| MPLS Label Distribution Protocol IGP Synchronization |
Release 25.1.1 | Introduced in this release on: Fixed Systems (8010 [ASIC: A100])(select variants only*) *This feature is supported on Cisco 8011-4G24Y4H-I routers. |
| MPLS Label Distribution Protocol IGP Synchronization |
Release 24.4.1 | Introduced in this release on: Fixed Systems (8200 [ASIC: P100], 8700 [ASIC: P100, K100])(select variants only); Modular Systems (8800 [LC ASIC: P100])(select variants only*) You can now prevent traffic loss during IGP adjacency establishment or LDP session closure by synchronizing LDP with IS-IS. This feature ensures that LDP completes its label exchange before the IGP path is used for switching, thus preventing MPLS packet loss. This synchronization enhances network stability and reliability by coordinating LDP and IGP operations, particularly in dynamic network environments. *This feature is supported on:
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Causes of MPLS traffic loss
MPLS traffic loss can happen in two main situations:
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When an IGP adjacency is established, a router might start forwarding packets over the new link before LDP has exchanged labels with the peer on that link.
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When an LDP session closes, the router may keep forwarding traffic across the affected link, instead of switching to an alternate path with an established LDP session.
Synchronization mechanism
This feature provides a synchronization mechanism between LDP and IS-IS by dynamically changing the link metric in IS-IS according to the LDP session state. When an IS-IS adjacency is present but the LDP session is down or label exchange is incomplete, IS-IS advertises the maximum metric for that link. Synchronization is considered achieved when IS-IS returns to advertising the regular configured or default metric.
Metric advertisement behavior
When an IS-IS adjacency is established on a link but the LDP session is lost or label exchange is incomplete, IS-IS advertises the maximum metric for that link. Synchronization is considered achieved once IS-IS resumes advertising the normal configured metric.
In IS-IS, a link with the maximum wide metric (0xFFFFFF) is not considered for shortest path first (SPF). For MPLS LDP IGP synchronization, a maximum wide metric value of -1 (0xFFFFFE) is used with this feature.