MPLS LDP-IGP Synchronization

How MPLS LDP-IGP Synchronization Works

Packet loss can occur because the actions of the IGP and LDP are not synchronized. Packet loss can occur in the following situations:

•When an IGP adjacency is established, the router begins forwarding packets using the new adjacency before the LDP label exchange completes between the peers on that link.

•If an LDP session closes, the router continues to forward traffic using the link associated with the LDP peer rather than an alternate pathway with a fully synchronized LDP session.

The MPLS LDP-IGP Synchronization feature:

•Provides a means to synchronize LDP and IGPs to minimize MPLS packet loss.

•Enables you to globally enable LDP-IGP Synchronization on each interface associated with an IGP Open Shortest Path First (OSPF) or IS-IS process.

•Provides a means to disable LDP-IGP Synchronization on interfaces that you do not want enabled.

•Prevents MPLS packet loss due to synchronization conflicts.

•Works when LDP is enabled on interfaces using either the mpls ip or mpls ldp autoconfig command.

To enable LDP-IGP Synchronization on each interface that belongs to an OSPF or IS-IS process, enter the mpls ldp sync command. If you do not want some of the interfaces to have LDP-IGP Synchronization enabled, issue the no mpls ldp igp sync command on those interfaces.

If the LDP peer is reachable, the IGP waits indefinitely (by default) for synchronization to be achieved. To limit the length of time the IGP session must wait, enter the mpls ldp igp sync holddown command. If the LDP peer is not reachable, the IGP establishes the adjacency to enable the LDP session to be established.

When an IGP adjacency is established on a link but LDP-IGP Synchronization is not yet achieved or is lost, the IGP advertises the max-metric on that link.

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Synchronized here means that the local label bindings have been sent over the Label Distribution Protocol session to the Label Distribution Protocol peer. However, when the synchronization is turned on at router A and that router has only one link to router B and no other IP connectivity to router B via another path (this means via other routers), the OSPF adjacency never comes up. OSPF waits for the Label Distribution Protocol session to come up, but the Label Distribution Protocol session cannot come up because router A cannot have the route for the Label Distribution Protocol router ID of router B in its routing table.
The OSPF and Label Distribution Protocol adjacency can stay down forever in this situation! If router A has only router B as a neighbor, the Label Distribution Protocol router ID of router B is not reachable; this means that no route exists for it in the routing table of router A. In that case, the Label Distribution Protocol-IGP synchronization detects that the peer is not reachable and lets OSPF bring up the adjacency anyway. In this instance, the link is advertised with maximum metric until the synchronization occurs.
This makes the path through that link a path of last resort. In some instances, the problem with the Label Distribution Protocol session might be a persistent one; therefore, it is probably not desirable to keep waiting for the IGP adjacency to be established. The solution for this is to configure a Holddown timer for the synchronization. If the timer expires before the Label Distribution Protocol session is established, the OSPF adjacency is built anyway. If everything is fine with Label Distribution Protocol across that link, Label Distribution Protocol also forms a session across the link. While OSPF is waiting to bring up its adjacency until Label Distribution Protocol synchronizes, the OSPF interface state is down and OSPF does not send Hellos onto that link.

Reference: MPLS LDP-IGP Synchronization
http://salfarisi25.wordpress.com/2011/01/18/multiprotocol-label-switching-label-distribution-protocol/