IEEE 802.1ag supports fault detection through Continuity Check Messages (CCM). CCM is
somewhat like the “keep-alive” message. By default, every 802.1ag-capable network device
(ND) sends out “heart-beat” style CCM periodically. Hence, by configuring a list of expected
existent remote 802.1ag-capable NDs, the local 802.1ag-capable ND can detect the health
status of its connection to the remote ND.
Fault verification
IEEE 802.1ag supports fault verification through Loopback Messages (LBM) and Loopback
Reply (LBR). The LBM/LBR mechanism is similar to ICMP “ping” function in the IP network.
However, since the interfaces of an ND in an Ethernet network may not possess their
individual IP address, the operator can not achieve fault verification by ICMP “ping” in such
a network environment.
Fault isolation
In addition to the LBM/LBR mechanism, IEEE 802.1ag supports fault isolation through
Linktrace Messages (LTM) and Linktrace Reply (LTR). The LTM/LTR mechanism is similar to
the “trace route” function in the IP network. However, since the interfaces of an ND in an
Ethernet network may not possess their individual IP address, the operator can not achieve
fault isolation by “trace route” in such a network environment.
IEEE 802.1ag also defines the following network error status to be conveyed in the CCM.
MACstatus
Either some remote ND is reporting its interface as not ready to pass both the data packets
and BPDU, or all remote NDs are reporting their interfaces as not able to pass ordinary data
packets.
RemoteCCM
The ND is not receiving valid CCMs from at least one of the remote NDs.
ErrorCCM
The ND has received at least one invalid CCM whose CCM Interval has not yet timed out.
XconCCM
The ND has received at least one CCM from either another VLAN or a lower MD Level
whose CCM Interval has not yet timed out.
4.5.12.1 CFM Stack
Select CFM Stack from the CFM Information menu and then the following screen page
appears.
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