Label withdrawal delay and LDP-IGP synchronization

LDP-IGP synchronization aims to prevent traffic loss due to the introduction of a new link into the network. Appropriate label mappings for a FEC may not be available for a period of time after the route for the FEC has been established on the new link.

When LDP-IGP synchronization is enabled, the IGP metric for the new link is temporarily advertised at a maximum value to force traffic to use an alternate route, if one is available. After all label mappings are received on the link, the IGP metric is adjusted on the link to the normal value and route updates may occur as the cost of the link has been reduced.

When both label withdrawal delay and LDP-IGP synchronization are enabled, the label withdrawal delay timer will not be started if there are alternate routes for the FEC. For example, the following sequence of events is possible:

  1. A FEC has an installed downstream mapping over link 1.
  2. Link 2 is introduced to the network. The IGP metric for link 2 is advertised at the maximum value and there is no route update for any FEC with an already established route.
  3. Label mappings are received from the new peer and a new retained downstream mapping is established for the FEC.
  4. When all label mappings for the session on link 2 have been received, the IGP metric is adjusted to the normal value. By default, the LDP-IGP synchronization hold down time is disabled and IGP will wait until LDP gives an in-sync indication for the link before advertising it with the normal metric. If the LDP-IGP synchronization hold-down time is enabled and label mappings are not received from the new peer within the configured synchronization hold-down time period, then the label withdrawal delay timer will start for the FEC. In this case, it is the time at which the label withdrawal delay timer starts that is affected. Instead of starting almost immediately after the new link becomes operational, it is delayed by the configured time to allow for LDP-IGP synchronization.
  5. This may result in a route change at the FEC. The FEC may install the downstream mapping associated with link 2 and transition the downstream mapping associated with link 1 to retained.
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