Allied Telesis AT-S63 User Manual

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Chapter 29: Virtual Router Redundancy Protocol
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Section VII: Routing
Interface Monitoring
The virtual router can monitor certain interfaces to change the priority of 
switches if the master switch loses its connection to the outside world. 
This is known as interface monitoring. Interface monitoring reduces the 
priority of the switch when an important interface connection is lost. The 
reduction in priority causes a backup switch with a higher priority to take 
over as the master switch and restore connectivity.
If a master switch loses its connection to the outside world, the connection 
to the LAN is not affected. Advertisement packets are still sent by the 
master and received by the backup switches, but the master is unable to 
send data to other networks because its connection to the outside world 
has been lost