Monitor Link Fundamentals
After a Monitor Link group is configured, the uplink interface is monitored in real time. When the uplink interface fails (link failure, unidirectional OAM connectivity, or failure to establish OAM connections), all the downlink interfaces in Up state in the Monitor Link group are forcibly shut down. When the uplink recovers, the downlink interfaces are restored to the Up state.
In Figure 7-11, if the uplink of Interface1 fails, Monitor Link forcibly switches Interface2 to Error-Down state. Then SwitchC detects that a fault has occurred on the link between SwitchB and SwitchA. When the uplink of Interface1 recovers, Monitor Link unblocks Interface2.
When a Smart Link group is used as the uplink interface, the uplink interface is considered faulty only when both the master and slave interfaces of the Smart Link group are in Inactive or Down state. When downlink interfaces are in an aggregation group, all the interfaces in the aggregation group are forcibly shut down if the uplink interface fails. When the uplink interface recovers, all the downlink interfaces in the aggregation group recover.