What is Dual Active Detection in VSS?

--> A Virtual Switching System is formed by using a VSL (virtual switch link) between both switches of 4500 or 6500 series.

--> In VSS, one switch acts as Active Switch and other as Standby Switch.

--> If the VSL Link between both the switches is broken, then standby switch assumes the active switch chassis has failed and becomes as an active switch in VSS.

--> But here, in this case, only the link between both the switches has been failed but the active switch is still working.

--> This Scenario results in both switches to work as active switches and it can cause the problem in the network.

--> Dual-Active Detection method can be configured in VSS to prevent this problem from happening.

--> It is recommended to configure Dual-Active Detection in VSS.

--> The dual-active detection can be configured by using the following methods,

i) Dual-Active Detection Using Enhanced PAgP

--> An enhanced version of PAgP is configured on the EtherChannel which provides the Dual-Active Detection.

--> Upstream Switch should support this functionality.

--> Not commonly used.

ii) Dual-Active Detection Using IP BFD

--> In this method we need to provide direct Ethernet connection between the two switches.

--> The VSS uses the Bidirectional Forwarding Detection (BFD) protocol to detect the connectivity between both the switches.

--> If the VSL between both the switches fails then both the switches try to form BFD neighbor relationship between them.

--> When the active switch receives a BFD message from neighbor switch then it understands VSL link between both the switches has been failed.


iii) Dual-Active Detection Using Dual-Active Fast Hello Packets

--> In this method also, we need to provide a direct Ethernet connection between the two VSS switches.

--> We can use up to four non-VSL links for this method.

--> The two switches periodically exchange special Layer 2 dual-active hello messages containing information about the switch state.

--> If the VSL link between both the switches fails and a dual-active scenario occurs, each switch recognizes from the peer’s messages that there is a dual-active scenario and starts recovery actions.


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