The Connectivity Fault Management (hereinafter also CFM) functionality allows detecting unavailable links between CPE devices. When CFM is enabled, the CPE device sends Continuity Check Message (CCM) control packets over its links at the specified intervals, and listens for response control packets on opposite-direction links. If response control packets do not arrive, the CPE device considers the link unavailable and starts transmitting traffic over a randomly selected available link.
The response timeout is equal to the control packet sending interval times 3.5. The time it takes to switch a CPE device over to a new connection when no response packets are received is equal to the control packet sending interval multiplied by a number in the range from 3.5 to 7. For example, if you specified 300 milliseconds as the time interval for sending control packets, the response timeout is 1.05 seconds, and the time it takes to switch the CPE device to a new link is 1.05 to 2.1 seconds.
You can enable CFM on a CPE device or link:
When CFM is enabled on one CPE device and disabled on the other CPE device, the CPE device with CFM enabled takes precedence. In this case, CPE devices send control packets to each other over all of their links.
For example, you can enable CFM on a CPE device and disable CFM on one of the links of that CPE device. In this case, the CPE device sends control packets over all its links, except for the link on which CFM is disabled. By default, CFM is disabled on CPE devices and links.