Cfm Protocols And Link Failures; G.8032 Ring-Supported Commands And Functionality - Cisco ASR 900 Series Configuration Manual

ITU-T G.8032 Ethernet Ring Protection Switching
A single link failure in the ring ensures a loop-free topology.
Note

CFM Protocols and Link Failures

Connectivity Fault Management (CFM) and line status messages are used to detect ring link and node failure.
During the recovery phase, when the failed link is restored, the nodes adjacent to the restored link send Ring
Automatic Protection Switching (R-APS) No Request (R-APS NR) messages. On obtaining this message, the
ring protection link (RPL) owner blocks the RPL port and sends R-APS NR and R-APS RPL (R-APS NR,
RB) messages. These messages cause all other nodes, other than the RPL owner in the ring, to unblock all
blocked ports. The Ethernet Ring Protection (ERP) protocol works for both unidirectional failure and multiple
link failure scenarios in a ring topology.
The G.8032 Ethernet Ring Protection (ERP) protocol uses CFM Continuity Check Messages (CCMs) at
Note
an interval of 3.3 milliseconds (ms). At this interval (which is supported only on selected platforms),
SONET-like switching time performance and loop-free traffic can be achieved.

G.8032 Ring-Supported Commands and Functionality

A G.8032 ring supports these basic operator administrative commands:
• Force switch (FS)—Allows the operator to forcefully block a particular ring port. Note the following
• Manual switch (MS)—Allows the operator to manually block a particular ring port. Note the following
• Clear—Cancels an existing FS or MS command on the ring port. The Clear command is used at the ring
A G.8032 ring can support multiple instances. An instance is a logical ring running over a physical ring. Such
instances are used for various reasons, such as load-balancing VLANs over a ring. For example, odd-numbered
VLANs may go in one direction of the ring, and even-numbered VLANs may go in the other direction. Specific
VLANs can be configured under only one instance. They cannot overlap multiple instances. Otherwise, data
traffic or Ring Automatic Protection Switching (R-APS) messages may cross logical rings, which is not
desirable.
points about FS commands:
• Effective even if there is an existing SF condition
• Multiple FS commands for ring are supported
• May be used to allow immediate maintenance operations
points about MS commands:
• Ineffective in an existing FS or signal failure (SF) condition
• Overridden by new FS or SF conditions
• Multiple MS commands cancel all MS commands
protection link (RPL) owner to clear a nonrevertive mode condition.
LAN Switching Configuration Guide Cisco IOS XE Everest 16.5.1 (Cisco ASR 900 Series)
CFM Protocols and Link Failures
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