Wtr Synchronization - Cisco ASR 900 Series Manual

MPLS Transport Profile
MPLS-TP Linear Protection with PSC Support
When a lockout (working) command is issued, the new-LER implementation sends an emLockout command
on the working LSP and PSC(FS) on the protected LSP. The remote peer receives two commands in either
order. A priority scheme for local events is modified slightly beyond what is defined in order to drive the PSC
FSM to a consistent state despite the order in which the two messages are received.
In the new implementation, it is possible to override the lockout of the working LSP with the lockout of the
protected LSP according to the priority scheme. This is not allowed in the existing implementation. Consider
the following steps between old (O) and new (N) node setup:
Time T1: Lockout (on the working LSP) is issued on O and N. Data is switched from the working to the
protected LSP.
Time T2: Lockout (on the protected LSP) is issued on O and N. The command is rejected at O (existing
behavior) and accepted at N (new behavior). Data in O->N continues on the protected LSP. Data in N->O
switches to the working LSP.
You must issue a clear lockout (on the working LSP) and re-issue a lockout (on the protected LSP) on the old
node to restore consistency.

WTR Synchronization

When a lockout on the working label switched path (LSP) is issued and subsequently cleared, a WTR timer
(default: 10 sec, configurable) is started. When the timer expires, the data path is switched from protected to
working LSP.
The PSC protocol indicates that the switch should happen immediately when a lockout (FS) is cleared.
When a new node is connected to the old node, for a period of time equal to the WTR timer value, the data
path may be out-of-sync when a lockout is cleared on the working LSP. You should configure a low WTR
value in order to minimize this condition.
Another issue is synchronization of the WTR value during stateful switchover (SSO). Currently, the WTR
residual value is not checkpointed between the active and standby. As a result, after SSO, the new active
restarts the WTR with the configured value if the protected LSP is active and the working LSP is up. As part
of the PSC protocol implementation, the residual WTR is checkpointed on the standby. When the standby
becomes active, the WTR is started with the residual value.
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
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