If the link goes down on either port, both ports on the segment are disabled. This functionality propagates the link state across the device. In the case of Router A and Router B, if the link to router A goes down, then the ports both ports are disabled, resulting in the link to Router B going down, which Router B detects. With Link-Down Synchronization, ports respond according to the configured setting.

The setting include the following: 

Hub - When a port goes down, the system ensures the partner port remains up.

 

Breaker - When a port goes down, the system disables the partner port until both ports are manually restarted. The breaker option requires manually restarting both ports.

 

Wire - When a port goes down, the system disables the partner port, automatically restarting both ports when the link is re-established.

In addition to the ability to enable Link-Down Synchronization for each segment, you can change the amount of time after detecting a link is down before forcing both ports down on a segment. The default is one second. You can configure the setting to any number of seconds in the range of zero to 240. Once you enable Link-Down Synchronization for a segment, monitoring of that segment begins only after link up is detected on both ports. 

When Link-Down Synchronization disables the ports on a segment, two audit log messages are generated. The first message in the audit log corresponds to the port with the link down. The second message corresponds to the segment partner. Additionally, an error message is added to the system log indicating which port was detected with the link down, activating Link-Down Synchronization for that segment.