Link failure is often an unavoidable part of networking. However, there are methods of
improving the reliability of a router or bridge network even when link failures occur. For
example, SONET/SDH seal-healing rings are frequently used to add a level of robustness
to router networks. This ring protection switching is now extended to Ethernet links. You
can configure Ethernet ring protection for a series of two or more systems so that if one
link fails, traffic is rerouted around the failure on the ring.

The basic idea of Ethernet ring protection is to use one specific link to protect the whole
ring. This special link is the ring protection link (RPL). When all links are up and running,
the RPL blocks traffic and remains idle. The RPL itself is controlled by the designated
RPL owner node. There is only one RPL owner node on the ring and the RPL owner node
is responsible for blocking the RPL interface under normal operating conditions. However,
if a link failure occurs on the ring, the RPL owner node is responsible for unblocking the
RPL interface and protection–switching the traffic on the alternate path around the ring.
An Ethernet ring automatic protection switching (R-APS) messaging protocol coordinates
the protection activities of all nodes on the ring. The APS blocks traffic over the failed
link and unblocks traffic over the RPL.

When the failed link is repaired, the traffic reverts to its normal pattern. That is, the RPL
owner blocks the RPL link and unblocks traffic over the cleared link.

Two or more nodes form a ring. Links between the nodes form a chain, with the last node
also connecting the first. Every ring node therefore has two ports related to the ring, one
in each direction. In this chapter, these directions are referred to as east and west.

151