Brocade Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide (Supporting R05.6.00) - Manual (Page 352)

328 Multi-Service IronWare Multiprotocol Label Switch (MPLS) Configuration Guide
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Label withdrawal delay
2
Setting the secs variable to 0 disables the label withdrawal delay feature for subsequent events.
Any FEC which has already started the label withdrawal delay timer continues to run the timer and
to delay sending its label withdrawal messages upstream
The [no] form of the command restores the default behavior. The value specified for the secs
variable must match the configured value at the time that the no form of the command is executed
NOTE
A typical convergence period in a heavily scaled network (50 LDP sessions and 4k FECs) on the XMR
platform is less than four seconds for a single link up or session down event.
To display information on the label withdrawal delay configuration refer to “Displaying FEC
information” .
Label withdrawal delay and LDP Graceful Restart (GR)
Helper node
Graceful restart helper procedures are initiated when a session to a peer goes down. During the
session down processing, the label mappings exchanged with a peer are preserved while the peer
is reconnecting. If the peer session is reconnected within a configurable time limit then the label
mappings previously exchanged with the peer are refreshed with new label mappings. Any
mappings that are not refreshed are released.
When both label withdrawal delay and GR are enabled, the label withdrawal delay timer is not
initiated when the session goes down because the session is considered to be in a special
restarting state and not actually down. If the session is not re-established within the reconnect
time for GR then the session is considered to be down and the label withdrawal delay timer may be
started for any FECs which meet the criteria for label withdrawal delay. If the session is
re-established within the reconnect time for GR then the label withdrawal delay timer is not started.
Individual FECs may experience some transition as the label mappings from the peer are
refreshed. For example, if a label mapping is not refreshed during the restart window, then that
label mapping will be removed. This will affect the FEC, especially if it was associated with an
installed downstream mapping. If an alternate route exists, the FEC will reconverge on the alternate
route much earlier and again the label withdrawal delay timer is not started for the FEC
Restarting node
The GR restarting procedures are executed by a node which is starting the LDP process and has
retained the forwarding state for LDP connections from an earlier instance of LDP. The restarting
procedures are executed during a management module switchover. During the forwarding state
hold period, connections in the forwarding state are marked stale until they are refreshed by an
additional label mapping. Stale connections are removed after this period.
This scenario does not affect label withdrawal delay since it involves a complete restart of the LDP
control plane. In other words, there are no FECs or sessions to apply the label withdrawal delay
timer to at restart. Other procedures for label withdrawal delay as described above may occur in the
same way during the forwarding state hold period.
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 some time after the route
for the FEC has been established on the new link.