Interoperability With Proprietary Lockout - Cisco ASR 900 Series Manual

MPLS Transport Profile
transport entity. Each endpoint also sends a PSC message to inform the peer endpoint of the state condition.
The PSC mechanism is necessary to coordinate the two transport entity endpoints and implement 1:1
bidirectional protection switching even for a unidirectional failure. The switching of the transport path from
working path to protected path can happen because of various failure conditions (such as link down indication
(LDI), remote defect indication (RDI), and link failures) or because administrator/operator intervention (such
as shutdown, lockout of working/forced switch (FS), and lockout of protection).
Each endpoint LER implements a PSC architecture that consists of multiple functional blocks. They are:
• Local Trigger Logic: This receives inputs from bidirectional forwarding detection (BFD), operator
commands, fault operation, administration, and maintenance (OAM) and a wait-to-restore (WTR) timer.
It runs a priority logic to decide on the highest priority trigger.
• PSC FSM: The highest priority trigger event drives the PSC finite state machine (FSM) logic to decide
what local action, if any, should be taken. These actions may include triggering path protection at the
local endpoint or may simply ignore the event.
• Remote PSC Signaling: In addition to receiving events from local trigger logic, the PSC FSM logic
also receives and processes PSC signaling messages from the remote LER. Remote messages indicate
the status of the transport path from the viewpoint of the far end LER. These messages may drive state
changes on the local entity.
• PSC Message Generator: Based on the action output from the PSC control logic, this functional block
formats the PSC protocol message and transmits it to the remote endpoint of the protected domain. This
message may either be the same as the previously transmitted message or change when the PSC control
has changed. The messages are transmitted as an initial burst followed by a regular interval.
• Wait-to-Restore Timer: The (configurable) WTR timer is used to delay reversion to a normal state
when recovering from a failure condition on the working path in revertive mode. The PSC FSM logic
starts/stops the WTR timer based on internal conditions/state. When the WTR expires, it generates an
event to drive the local trigger logic.
• Remote Event Expire Timer: The (configurable) remote-event-expire timer is used to clear the remote
event after the timer is expired because of remote inactivity or fault in the protected LSP. When the
remote event clear timer expires, it generates a remote event clear notification to the PSC FSM logic.

Interoperability With Proprietary Lockout

An emulated protection (emulated automatic protection switching (APS)) switching ensures synchronization
between peer entities. The emulated APS uses link down indication (LDI)message (proprietary) extensions
when a lockout command is issued on the working or protected LSP. This lockout command is known as
emLockout. A lockout is mutually exclusive between the working and protected LSP. In other words, when
the working LSP is locked, the protected LSP cannot be locked (and vice versa).
The emLockout message is sent on the specified channel from the endpoint on the LSP where the lockout
command (working/protected) is issued. Once the lockout is cleared locally, a Wait-To-Restore (WTR) timer
(configurable) is started and the remote end notified. The local peer continues to remain in lockout until a
clear is received from the remote peer and the WTR timer has expired and only then the LSP is considered
to be no longer locked out. In certain deployments, you use a large WTR timer to emulate a non-revertive
behavior. This causes the protected LSP to continue forwarding traffic even after the lockout has been removed
from the working LSP.
The PSC protocol as specified in RFC-6378 is incompatible with the emulated APS implementation in certain
conditions. For example, PSC implements a priority scheme whereby a lockout of protection (LoP) is at a
higher priority than a forced switch (FS) issued on a working LSP. When an FS is issued and cleared, PSC
MPLS Basic Configuration Guide, Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
MPLS-TP Linear Protection with PSC Support
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