To achieve a faster failover in scenarios with redundant active/standby routers performing Layer 2 multicast snooping, it is possible to synchronize the snooping state from the active router to the standby router, so that if a failure occurs the standby router has the Layer 2 multicast snooped states and is able to forward the multicast traffic immediately. Without this capability, there would be a longer delay in re-establishing the multicast traffic path because it would wait for the Layer 2 states to be snooped.
Multi-chassis synchronization (MCS) is enabled per peer router and uses a sync-tag, which is configured on the objects requiring synchronization on both of the routers. This allows MCS to map the state of a set of objects on one router to a set of objects on the other router. Specifically, objects relating to a sync-tag on one router are backed up by, or are backing up, the objects using the same sync-tag on the other router (the state is synchronized from the active object on one router to its backup objects on the standby router).
The object type must be the same on both routers; otherwise, a mismatch error is reported. The same sync-tag value can be reused for multiple peer/object combinations, where each combination represents a different set of synchronized objects; however, a sync-tag cannot be configured on the same object to more than one peer.
The sync-tag is configured per port and can relate to a specific set of dot1q or QinQ VLANs on that port, as follows.
CLI syntax:
configure
redundancy
multi-chassis
peer ip-address [create]
sync
port port-id [sync-tag sync-tag] [create]
range encap-range sync-tag sync-tag
For IGMP snooping and PIM snooping for IPv4 to work correctly with MCS on QinQ ports using x.* SAPs, one of the following must be true:
MCS is configured with a sync-tag for the entire port.
The IGMP snooping SAP and the MCS sync-tag must be provisioned with the same Q-tag values when using the range parameter.