LevelOne GTL-2881, User Manual

Page: 450 / 706

Chapter 13
| Basic Administration Protocols
Ethernet Ring Protection Switching
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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.
A link/node failure is detected by the nodes adjacent to the failure. These nodes
block the failed link and report the failure to the ring using R-APS (SF) messages.
This message triggers the RPL owner to unblock the RPL, and all nodes to flush their
forwarding database. The ring is now in protection state, but it remains connected
in a logical topology.
When the failed link recovers, the traffic is kept blocked on the nodes adjacent to
the recovered link. The nodes adjacent to the recovered link transmit R-APS (NR - no
request) message indicating they have no local request. When the RPL owner
receives an R-APS (NR) message it starts the Wait-To-Recover (WTR) timer. Once
WTR timer expires, the RPL owner blocks the RPL and transmits an R-APS (NR, RB -
ring blocked) message. Nodes receiving this message flush the forwarding
database and unblock their previously blocked ports. The ring is now returned to
Idle state.
Figure 294: ERPS Ring Components
Multi-ring/Ladder Network – ERPSv2 also supports multipoint-to-multipoint
connectivity within interconnected rings, called a “multi-ring/ladder network”
topology. This arrangement consists of conjoined rings connected by one or more
interconnection points, and is based on the following criteria:
◆
The R-APS channels are not shared across Ethernet Ring interconnections.
◆
On each ring port, each traffic channel and each R-APS channel are controlled
(e.g., for blocking or flushing) by the Ethernet Ring Protection Control Process
(ERP Control Process) of only one ring.
E ast Port
W est Port
RPL Owner
CC Messages
RPL
x
CC Messages
( I d l e State)