LevelOne GTL-2691 - Manual (Page 465)

C HAPTER 14 | Basic Administration Protocols
Ethernet Ring Protection Switching
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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.
Figure 258: ERPS Ring Components
East Port West Port
RPL Owner
CC Messages
RPL
x
CC Messages
(Idle State)