MPLS-TP Maintenance Identifiers
Page 74 7450 ESS MPLS Guide
If a PE with a non-zero refresh timer configured does not receive control channel status refresh
messages for 3.5 time the specified timer value, then by default it will time out and assume a PW
status of zero.
A trap is generated if the refresh timer times-out.
If PW redundancy is configured, the system will always consider the literal value of the PW status;
a time-out of the refresh timer will not impact the choice of the active transit object for the VLL
service. The result of this is that if the refresh timer times-out, and a given PW is currently the
active PW, then the system will not fail-over to an alternative PW if the status is zero and some
lower-layer OAM mechanism e.g. BFD has not brought down the LSP due to a connectivity
defect. It is recommended that the PW refresh timer be configured with a much longer interval
than any proactive OAM on the LSP tunnel, so that the tunnel can be brought down before the
refresh timer expires if there is a CC defect.
Note that a unidirectional continuity fault on a RSVP TE LSP may not result in the LSP being
brought down before the received PW status refresh timer expires. It is therefore recommended
that either bidirectional static MPLS-TP LSPs with BFD CC, or additional protection mechanisms
e.g. FRR be used on RSVP-TE LSPs carrying MPLS-TP PWs. This is particularly important in
active/standby PW dual homing configurations, where the active / standby forwarding state or
operational state of every PW in the redundancy set must be accurately reflected at the redundant
PE side of the configuration.
Note that a PW with a refresh timer value of zero is always treated as having not expired.
The system implements a hold-down timer for control-channel-status PW-status bits in order to
suppress bouncing of the status of a PW. For a specific spoke-SDP, if the system receives 10 PW-
status change events in 10 seconds, the system will hold-down the spoke-SDP on the local node
with the last received non-zero PW-status bits for 20 seconds. It will update the local spoke with
the most recently received PW-status. This hold down timer is not persistent across shutdown/no-
shutdown events.
PW Control Channel Status Request Mechanism
The system implements an optional PW control channel status request mechanism. This enhances
the existing control channel status mechanism so that a peer that has stale PW status for the far-
end of a PW can request that the peer PE send a static PW status update. Accurate and current
information about the far end status of a PW is important for proper operation of PW redundancy.
This mechanism ensures a consistent view of the control plane is maintained, as far as possible,
between peer nodes. It is not intended to act as a continuity check between peer nodes.
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