When the Multiprotocol Label Switching Label Distribution Protocol-IGP synchronization is active for an interface, the IGP announces that link with maximum metric until the synchronization is achieved, or until the Label Distribution Protocol session is running across that interface. The maximum link metric for OSPF is 65536 (hex 0xFFFF). No path through the interface where Label Distribution Protocol is down is used unless it is the only path. (No other paths have a better metric.) After the Label Distribution Protocol session is established and label bindings have been exchanged, the IGP advertises the link with its normal IGP metric.

At that point, the traffic is label-switched across that interface. Basically, OSPF does not form an adjacency across a link if the Label Distribution Protocol session is not established first across that link. (OSPF does not send out Hellos on the link.) Until the Label Distribution Protocol session is established or until the synchronization Holddown timer has expired, the OSPF adjacency is not established. 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 case, 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 cases, the problem with the Label Distribution Protocol session might be a persistent one; therefore, it might not be 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.

Multiprotocol Label Switching Label Distribution Protocol-IGP Synchronization Configuration

Multiprotocol Label Switching Label Distribution Protocol-IGP Synchronization is enabled for the IGP process. This means that it is configured for an IGP, and it applies to all the interfaces on which the IGP is running. The command to enable it for the IGP is Multiprotocol Label Switching ldp sync, and it is configured under the router process. You can disable Multiprotocol Label Switching Label Distribution Protocol-IGP Synchronization on one particular interface with the command no Multiprotocol Label Switching ldp igp sync. By default, if synchronization is not achieved, the IGP waits indefinitely to bring up the adjacency. You can change this with the global command Multiprotocol Label Switching ldp igp sync holddown msecs, which instructs the IGP to wait only for the configured time. After the synchronization Holddown timer expires, the IGP forms an adjacency across the link. As long as the IGP adjacency is up, while the Label Distribution Protocol session is not synchronized, the IGP advertises the link with maximum metric.

Multiprotocol Label Switching Label Distribution Protocol Session Protection

A common problem in networks is flapping links. The flapping of links can have several causes, but it is not the goal of this article to look deeper into this. Flapping links do have an important impact on the convergence of the network. Because the IGP adjacency and the Label Distribution Protocol session are running across the link, they go down when the link goes down. This is unfortunate, especially because the link is usually not down for long. The impact is pretty severe though, because the routing protocol and Label Distribution Protocol can take time to rebuild the neighborship. Label Distribution Protocol has to rebuild the Label Distribution Protocol session and must exchange the label bindings again. To avoid having to rebuild the Label Distribution Protocol session altogether, you can protect it. When the Label Distribution Protocol session between two directly connected Label Switch Routers is protected, a targeted Label Distribution Protocol session is built between the two Label Switch Routers. When the directly connected link does go down between the two Label Switch Routers, the targeted Label Distribution Protocol session is kept up as long as an alternative path exists between the two Label Switch Routers. The Label Distribution Protocol link adjacency is removed when the link goes down, but the targeted adjacency keeps the Label Distribution Protocol session up. When the link comes back up, the Label Switch Router does not need to re-establish the Label Distribution Protocol session; therefore, the convergence is better.

The access list (acl) you can configure lets you specify the Label Distribution Protocol peers that should be protected. It should hold the Label Distribution Protocol Router Identifier of the Label Distribution Protocol neighbors that need protection. The duration is the time that the protection (the targeted Label Distribution Protocol session) should remain in place after the Label Distribution Protocol link adjacency has gone down. The default value is infinite. For the protection to work, you need to enable it on both the Label Switch Routers. If this is not possible, you can enable it on one Label Switch Router, and the other Label Switch Router can accept the targeted Label Distribution Protocol Hellos by configuring the command Multiprotocol Label Switching ldp discovery targeted-hello accept. Label Distribution Protocol Session Protection is enabled on all four routers. The Label Switch Router madrid has two Label Distribution Protocol sessions: one with london and one with sydney. When the link madridsydney fails, the targeted Label Distribution Protocol session is held up as it reroutes over the path madrid-london-romesydney. Example 4-30 shows the Label Distribution Protocol session on madrid to router sydney before the link went down. The link madrid-sydney then goes down. You can see a logging message for the Label Distribution Protocol session when the link goes down and when the link comes back up. The first logging message indicates that the Label Distribution Protocol session has gone into protecting state; the second indicates that the Label Distribution Protocol session has been recovered successfully.