When an LDP session fails, the label associated with a FEC is withdrawn from all upstream peers. In addition, if the IGP adjusts the route for the FEC such that the current neighbor is no longer the next hop for the FEC, then the associated FEC label is withdrawn from all upstream peers.
The label withdrawal delay timer allows you to configure a delay to allow the IGP and LDP to converge after these events. The delay helps avoid sending the label withdraw message to the upstream peers. For example, after a link failure, instead of immediately sending a label withdraw to all upstream peers for the FEC, the delay allows the IGP to install a route which may match another existing downstream session. Label withdrawal from all upstream peers can be avoided if the FEC achieves a downstream label mapping which is consistent with the IGP routing table.
If the timer expires, the FEC label is withdrawn from all upstream peers.
Session down event
When a session becomes inactive, each DM for the session is deleted. This may cause LW to be sent upstream if the last installed DM is removed for a FEC. The behavior is as follows:
If a DM is a candidate to be removed, the system evaluates whether or not its FEC is a candidate for delaying the LW upstream, based on whether there are other installed DMs
If there are no other installed DMs, the system proceeds as follows:
A timer is started on the FEC for the LW delay period.
The DM remains installed until either a new DM becomes installed for the FEC, the route for the FEC is deleted, or the FEC timer expires.
If there are other installed DMs, then the DM is removed as per normal.
Route update event
When a route update for the FEC occurs, the installed DM may transition to retained if the next hop address for the FEC has changed. The behavior is as follows:
The route update may result in a FEC with the following criteria:
The LW delay timer is not already running for the FEC.
The FEC would reference only retained DMs after processing the route update. Note there must be at least one DM.
If these criteria are met, the system proceeds as follows:
A timer is started on the FEC for the LW delay period.
The current installed DM remains installed.
The current route information is updated. The route is not applied until there is a LM which matches the route, or the LW delay timer expires.
If these criteria are not met, then LWD is not activated and normal procedures for the FEC are done.
The existing behavior for link update or route update is not changed if the LWD is not activated for the FEC.
