Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
– 464 –
Operational Concept
Loop avoidance in the ring is achieved by guaranteeing that, at any time, traffic
may flow on all but one of the ring links. This particular link is called the ring
protection link (RPL), and under normal conditions this link is blocked to traffic. One
designated node, the RPL owner, is responsible for blocking traffic over the RPL.
When a ring failure occurs, the RPL owner is responsible for unblocking the RPL,
allowing this link to be used for traffic.
Ring nodes may be in one of two states:
Idle – normal operation, no link/node faults detected in ring
Protection – Protection switching in effect after identifying a signal fault
In Idle state, the physical topology has all nodes connected in a ring. The logical
topology guarantees that all nodes are connected without a loop by blocking the
RPL. Each link is monitored by its two adjacent nodes using Connectivity Fault
Management (CFM) protocol messages.
Protection switching (opening the RPL to traffic) occurs when a signal failure
message generated by the Connectivity Fault Management (CFM) protocol is
declared on one of the ring links, and the detected failure has a higher priority than
any other request; or a Ring – Automatic Protection Switching protocol request
(R-APS, as defined in Y.1731) is received which has a higher priority than any other
local request.
A link/node failure is detected by the nodes adjacent to the failure. These nodes
block the failed link and report the failure to the ring using R-APS (SF) messages.
This message triggers the RPL owner to unblock the RPL, and all nodes to flush their
forwarding database. The ring is now in protection state, but it remains connected
in a logical topology.
When the failed link recovers, the traffic is kept blocked on the nodes adjacent to
the recovered link. The nodes adjacent to the recovered link transmit R-APS (NR - no
request) message indicating they have no local request. When the RPL owner
receives an R-APS (NR) message it starts the Wait-To-Recover (WTR) timer. Once
WTR timer expires, the RPL owner blocks the RPL and transmits an R-APS (NR, RB -
ring blocked) message. Nodes receiving this message flush the forwarding
database and unblock their previously blocked ports. The ring is now returned to
Idle state.
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