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Configuring ERP
ERP Overview
Ethernet Ring Protection (ERP) is a protection switching mechanism for Ethernet ring topologies, such as
multi-ring and ladder networks. This implementation of ERP is based on Recommendation ITU-T G.8032/
Y.1344 and uses the ring Automatic Protection Switching (APS) protocol to coordinate the prevention of
network loops within a bridged Ethernet ring.
Loop prevention is achieved by allowing the traffic to flow on all but one of the links within the protected
Ethernet ring. This link is blocked and is referred to as the Ring Protection Link (RPL). When a ring fail-
ure condition occurs, the RPL is unblocked to allow the flow of traffic to continue through the ring.
One designated node within the ring serves as the RPL owner and is responsible for blocking the traffic
over the RPL. When a ring failure condition occurs, the RPL owner is responsible for unblocking the RPL
so that the link can forward traffic to maintain ring connectivity.
ERP Terms
Ring Protection Link (RPL)—A designated link between two ring nodes that is blocked to prevent a
loop on the ring.
RPL Owner—A node connected to an RPL. This node blocks traffic on the RPL during normal ring oper-
ations and activates the link to forward traffic when a failure condition occurs on another link in the ring.
Link Monitoring—Ring links are monitored using standard ETH CC OAM messages (CFM). Note that
for improved convergence times, this implementation also uses Ethernet link up and link down events.
Signal Fail (SF)—Signal Fail is declared when a failed link or node is detected.
No Request (NR)—No Request is declared when there are no outstanding conditions (e.g., SF) on the
node.
Ring APS (R-APS) Messages—Protocol messages defined in Y.1731 and G.8032 that determine the
status of the ring.
ERP Service VLAN—Ring-wide VLAN used exclusively for transmission of messages, including R-APS
messages.
ERP Protected VLAN—A VLAN that is added to the ERP ring. ERP determines the forwarding state of
protected VLANs.
ERP Timers
Wait To Restore (WTR) Timer. To prevent link flapping, this timer is used by the RPL to verify that the
ring has stabilized. This timer determines the number of minutes the RPL switch waits before returning
the RPL ports to a blocked state after the ring has recovered from a link failure.
Some important points about the WTR Timer are as follows:
•
The timer is started when the RPL node receives an R-APS (NR) message that indicates ring protec-
tion is no longer required.
•
The timer is stopped when the RPL owner receives an R-APS (SF) message while WTR is running,
which indicates that an error still exists in the ring.
•
When the time runs out, the RPL port is blocked and an R-APS (NR, RB) message is transmitted from
both the ring ports to indicate that the RPL is blocked.
OmniSwitch AOS Release 7 Network Configuration Guide
March 2011
ERP Overview
page 9-3
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