Foundry NetIron M2404C and M2404F Metro Access Switches
Network Administration Tools (Rev. 03)
802.1ag Connectivity Fault Management (CFM)
© 2008 Foundry Networks, Inc.
Page 82 of 155
corresponding timer for the CCMs. Once MEPs have received and processed the CCMs, each MEP
will have a view of all other peer MEPs for a given Virtual Bridge LAN Service.
A CCM validity timer is used to detect the loss of CCMs. A CCM loss is assumed when the next
CCM from a remote MEP is not received within the timeout of this validity timer. If three
consecutive CCMs are lost, continuity to that remote MEP is assumed to have failed and a fault is
detected. Subsequent fault verification and isolation procedures can be exercised. A hard fault may
possibly result in network isolation which leads to loss of CCMs for many Virtual Bridge LAN
services. If the hard fault can be detected and notified to the Management application, additional
notifications by each MEP may not be needed. Therefore, CFM provides an alarm suppression
mechanism for notifications that get generated as the result of CCM timeouts.
A CCM does not require a response and a multicast CCM; it requires only N transmissions within
its member group, where N is the number of members within the member group. In other words, if
a Virtual Bridge LAN Service has N member MEPs, only N CCMs need to be transmitted
periodically – one from each MEP.
Continuity Check (CC) messages are periodic hello messages multicast by a MEP within the
maintenance domain, at the rate of X per second; X can be, say, 1, 10, 20, 100, etc. All
Maintenance association Intermediate Points (MIPs) and MEPs in that domain will receive it but
will not respond to it. The receiving MEPs will build a MEP database that has entities of the format
[MEP DA, Port]. MEPs receiving this CC message will catalog it and know that the various
maintenance associations (MAs) are functional, including all intermediate MIPs.
Figure 14
and
Figure 15
show CCMs being exchanged within a domain. MEPs must be configured to expect a set
of MEP SAs. This MEP will compare the received CCs with the expected set.
Figure 14: MEP1 and MEP3 Send a Multicast CC Frame