Brocade Communications Systems Brocate Ethernet Access Switch 6910 - Manual (Page 1011)

Brocade 6910 Ethernet Access Switch Configuration Guide 961
53-1002581-01
Ethernet Ring Protection Switching
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Ethernet Ring Protection Switching
NOTE
Information in this section is based on ITU-T G.8032/Y.1344.
The ITU G.8032 recommendation specifies a protection switching mechanism and protocol for
Ethernet layer network rings. Ethernet rings can provide wide-area multipoint connectivity more
economically due to their reduced number of links. The mechanisms and protocol defined in
G.8032 achieve highly reliable and stable protection; and never form loops, which would fatally
affect network operation and service availability.
The G.8032 recommendation, also referred to as Ethernet Ring Protection Switching (ERPS), can
be used to increase the availability and robustness of Ethernet rings. An Ethernet ring built using
ERPS can provide resilience at a lower cost and than that provided by SONET or EAPS rings.
ERPS is more economical than EAPS in that only one physical link is required between each node in
the ring. However, since it can tolerate only one break in the ring, it is not as robust as EAPS. ERPS
supports up to 255 nodes in the ring structure. ERPS requires a higher convergence time when
more that 16 nodes are used, but should always run under than 500 ms.
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.