--> 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.
--> 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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