Vsl Link Loss And Recovery - Cisco Catalyst 6500 User Manual

VSL Link Loss and Recovery

If the VSL fails, the standby chassis cannot determine the state of the active chassis. In order to ensure that
switchover occurs without delay, the standby chassis assumes the active chassis has failed and initiates
switchover to take over the active role.
If the original active chassis is still operational, both chassis are now active. This situation is called a
dual−active scenario. A dual−active scenario can have adverse affects on network stability, because both
chassis use the same IP addresses, SSH keys, and STP bridge ID. The virtual switching system (VSS) must
detect a dual−active scenario and take recovery action.
The virtual switching system supports these three methods in order to detect a dual−active scenario:
•
Enhanced PAgP †Uses PAgP messaging over the MEC links in order to communicate between the
two chassis through a neighbor switch. Enhanced PAgP is faster than IP BFD, but requires a neighbor
switch that supports the PAgP enhancements.