Cisco Cisco Catalyst 4500 Series Supervisor II-Plus-10GE Libro bianco
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In some circumstances, this configuration may not be possible. Cisco Virtual Switching System has different
mechanisms to address this dual-active scenario:
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Configuration of the VSL failure-detection feature
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Detection of a dual-active scenario
●
Action taken to resolve the situation
●
Recovery behavior upon restoring the VSL
Detection Mechanisms and Configuration
Because of the challenges to distinguish a remote chassis power failure and a VSL failure, each chassis attempts
to detect its peer chassis, in order to avoid the dual-active scenario. In a dual-active scenario, it must be assumed
that the VSL links cannot be used in any way to detect the failure. The only remaining options are to use alternative
paths that may or may not exist between the two chassis.
Currently, there are currently three mechanisms for detecting a dual-active scenario:
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Enhanced PAgP
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Layer 3 BFD
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Fast Hello
Enhanced PAgP
With the introduction of Cisco Virtual Switching System in the first software release, an enhancement to the PAgP
protocol (Enhanced PAgP or PAgP+) has been implemented to assist in the dual-active detection. A list of all
software releases for respective switch platforms supporting Enhanced PAgP is included in Table 1 (Figure 26).
Figure 26. Dual-Active Detection Mechanisms
The result of this detection is that the standby virtual switch (Switch 2) always transitions to become an active
virtual switch, and the active virtual switch (Switch 1) always enters into recovery mode.