CFM Messages
CFM uses standard Ethernet frames. CFM frames are distinguishable by EtherType and for multicast messages
by MAC address. CFM frames are sourced, terminated, processed, and relayed by bridges. Routers can support
only limited CFM functions.
Bridges that cannot interpret CFM messages forward them as normal data frames. All CFM messages are
confined to a maintenance domain and to an S-VLAN (PE-VLAN or Provider-VLAN). Three types of messages
are supported:
•
Continuity Check
•
Loopback
•
Traceroute
Continuity Check Messages
CFM CCMs are multicast heartbeat messages exchanged periodically among MEPs. They allow MEPs to
discover other MEPs within a domain and allow MIPs to discover MEPs. CCMs are confined to a domain
and S-VLAN.
CFM CCMs have the following characteristics:
•
Transmitted at a configurable periodic interval by MEPs. The interval can be from 10 seconds to 65535
seconds, the default is 30.
•
Contain a configurable hold-time value to indicate to the receiver the validity of the message. The default
is 2.5 times the transmit interval.
•
Catalogued by MIPs at the same maintenance level.
•
Terminated by remote MEPs at the same maintenance level.
•
Unidirectional and do not solicit a response.
•
Carry the status of the port on which the MEP is configured.
Carrier Ethernet Configuration Guide (Cisco ASR 920 Series)
131
Configuring Ethernet Connectivity Fault Management in a Service Provider Network
CFM Messages
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