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This article may require cleanup to meet Wikipedia's quality standards. Please improve this article if you can. (November 2008) |
Ethernet Ring Protection Switching, or ERPS, is an effort at ITU-T to provide sub-50ms protection for Ethernet traffic in a ring topology and at the same time ensuring that there are no loops formed at the Ethernet layer. This is defined in ITU-T G.8032. The first version supported a single ring architecture and the second version is expected to address multiple rings.
Principle of operation
In ERPS there is a central node called RPL owner node which blocks one of the ports to ensure that there is no loop formed for the Ethernet traffic. The link which gets blocked by the RPL is called Ring Protection Link or RPL. The other node which is connected to the RPL is known as just RPL node. It uses control messages called R-APS to coordinate the activities of switching on/off the RPL link.
Any failure along the ring triggers a R-APS(SF) (R-APS signal fail) message along both directions from the nodes adjacent to the failed link after these nodes have blocked the port facing the failed link. On obtaining this message, RPL owner unblocks the RPL port. (Note that a single link failure anywhere in the ring ensures a loop free topology.)
During the recovery phase when the failed link gets restored the nodes adjacent to the restored link send R-APS(NR) (R-APS no request) messages. On obtaining this message, the RPL owner block the RPL port and then sends a R-APS(NR,RB) (R-APS no request, root blocked) messages. This will cause all other nodes other than RPL owner in the ring to unblock all the blocked ports.
This protocol is robust enough to work for unidirectional failure and multiple link failure scenarios in a ring topology. It allows mechanism to force switch (FS) or manual switch (MS) to take care of field maintenance scenario.
During the Interoperability event at CEWC Berlin 2008, the convergence time for switchover was shown to be 35ms.[1]
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External links
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