The ETH-VSM Multicast Class 1 DA announcement includes the start of a grace period, the new remote timeout value of 90 s, and the completion of the grace process.
At the start of the maintenance operation, a burst of three packets is sent over a 3-second window to reduce the chance that a remote peer may miss the grace announcement. Following the initial burst, evenly-spaced ETH-VSM packets are sent at intervals of one third of the ETH-VSM grace window; this means that the ETH-VSM packet are sent every 30 seconds to all appropriate remote peers. Reception of an ETH-VSM grace packet refreshes the timeout calculation. The local node that is undergoing the maintenance operation alsos delay the CCM timeout of the local MEP during the grace window using the announced ETH-VSM interval. MEPs restart their timeout countdown when any ETH-CC PDU is received.
At the end of the maintenance operation, there is a burst of three ETH-VSM grace packets to signal that the maintenance operation has been completed. After the first of these packets has been received, the receiving peer transitions back to the ETH-CCM message and associated interval as the indication for the remote timeout (3.5 ✕ ccm-interval + hold (where applicable)).
CCM packets continue to be sent during this process, but loss of the CCM packets during the advertised grace window do not affect the peer timeout. The only change to the CCM processing is the timeout value used during the grace operation. During the operation, the value that is announced as part of the ETH-VSM packet is used. If the grace value is lower than the configured CCM interval standard timeout computation (3.5 ✕ ccm-interval + hold (where applicable)), the grace value is not installed as the new timeout metric.
This is a value-added function that is applicable only to nodes that implement support for Nokia’s approach for announcing grace using ETH-VSM. This pre-dates the introduction of the ITU-T Y.1371 Ethernet-Expected Defect (ETH-ED) standard. As specified in the standards, when a node does not support a specific optional function such as ETH-VSM, the message is ignored and no processing is performed.
The ETH-VSM function is enabled by default for reception and transmission. The per-MEP configuration statements under the grace>eth-vsm-grace context can affect the transmission, reception, and processing of the ETH-VSM grace function.