LevelOne GEL-5261, User Manual

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Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
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.