22.1.4 Continuity Check

Continuity Check(CC) is a function that constantly monitors connectivity between MEP. All MEP in MA are CCM(Continuity Check Message. A type of CFM PDU is sent and received, and MEP in MA is learned. What the MEPs learn is used for the loopback functionality and the linktrace functionality.

If a switch on which the CC functionality is used does not receive CCMs or a port on the applicable switch in an MA cannot communicate, a failure is determined to have occurred. When this happens, a CCM with a failure detection flag is sent to notify MEPs in the MA of the failure.

The table below describes the failures detectable by the CC functionality. There are five such functionality levels. The Switch is initially configured to detect level 2 and higher failures.

Table 22-5 Failures detected by CC

Failure level	Problem	Initial state
5	A domain and the MA received different CCMs.	Detected
4	A CCM with an incorrect MEP ID or an incorrect sending interval was received.
3	CCMs are no longer received.
2	A port on the applicable switch has entered a state in which it is unable to communicate.
1	A CCM reporting failure detection was received. Remote Defect Indication	Not detected

When the failure recovery monitoring time after the failure recovery trigger point has elapsed, it is determined that recovery from the failure has succeeded.

Table 22-6: Failure recovery timing and failure recovery monitoring time

Failure level	Failure recovery trigger point	Failure recovery monitoring time
5	A domain and an MA no longer receive different CCMs.	Sending interval of the received CCMs x 3.5
4	A CCM with an incorrect MEP ID or an incorrect sending interval is no longer received.	Sending interval of the received CCMs x 3.5
3	A CCM is received again.	Immediately after reception of the CCM
2	A CCM indicating that the port on the applicable switch can now communicate.	Immediately after reception of the CCM
1	A CCM indicating no failure is detected is received.	Immediately after reception of the CCM

CC functionality behavior will be described using switch B in the following figures as an example.

Each MEP multicasts a CCM regularly inside the MA. Because CCMs are received from each MEP regularly, connectivity is always monitored.

Figure 22-18 Constant connectivity monitoring with CC

If a CCM from switch A cannot be delivered to switch B because of a switch failure or a network failure, switch B determines that the state is a network failure between switches A and B.

Figure 22-19 Failure detected in CC

When switch B detects a failure, switch B notifies all MEPs in the MA that a failure has been detected.

Figure 22-20: Notifying all MEP of an error

The MEPs that received the CCM indicating a detected failure acknowledge that a failure has occurred somewhere in the MA. If loopback and linktrace are performed on each switch, the switches can determine the route inside the MA on which the failure occurred.