Management Guide
CLI
TigerAccess™ EE
SMC7824M/VSW
289
8.4 Ethernet Ring Protection (ERP)
The ERP is a protection protocol for Ethernet ring topology to prevent Loop from a link
failure or recovery. It is designed to minimize the time for removing Loop within 50 milli-
seconds while there is an enormous amount of traffic flow in Metro Ethernet network.
It is a unique robustness functionality, which runs on every network element involved in
the ring configurations. It means that each system is active part of the ring protection
mechanism. Therefore, it guarantees to switch over towards a new topology after link or
system failure within 50 milliseconds.
8.4.1 ERP
Mechanism
The purpose of Ethernet Ring Protection (ERP) is to prevent the Loop by performing the
Redundancy Manager Node (RM Node) to detect a link failure and recover from it. An
Ethernet ring consists of one or more ERP domains. ERP domain is an identifier of a sin-
gle ring topology to be controlled by ERP mechanism. A node is one of the switches on
the ERP ring. Each switch is configured as either RM node or normal node. RM node is
responsible for keeping an open loop whenever all nodes and links are operating correctly.
One ERP domain should have one RM node. Normal nodes are responsible to inform RM
node of Link failures/recovery.
Both RM node and normal node have a primary and secondary port. You need to specify
primary and secondary port which is directly connected to the node within an Ethernet
ring. A secondary port of RM node is blocked as unused link for traffic while it runs without
the link failure detection.
ERP Operation
If a link failure occurs, the normal nodes adjacent to the failure block their ports that de-
tecting the link failure and send Link Down message to RM node. After RM node receives
Link Down messages from the normal nodes, it unblocks its secondary port for traffic
transmission. RM node responds to those messages using RM Link Down message
which informs the other nodes that its secondary port has been unblocked.
If the link failure is recovered, the normal nodes send Link Up message to RM node. And
they keep the blocking status of those failed ports. If the blocked ports of the normal
nodes start to forward right after a Link Failure is recovered, a temporary loop can occur.
If RM node receives Link Up message, it blocks its own secondary port and sends RM
Link UP message which informs the nodes of the secondary port’s blocking status. If the
nodes receive RM Link Up message, they unblocks the ports which are detected a Link
Failure recovery. The Ethernet ring is back to normal state.
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