The Cisco Catalyst 6500 series virtual switching system (VSS) is formed by combining two switches into a single, logical network entity from both network control-plane and management perspectives. The Cisco VSS appears as a single, logical switch, or router to the neighboring devices.

One chassis is designated as the active virtual switch and the other is designated as the standby virtual switch. All control-plane functions and software data path are centrally managed by the active supervisor engine of the active virtual switch chassis. The chassis containing the supervisor engine and acting as the single management point is referred to as the active virtual switch. The peer chassis is referred to as the standby virtual switch.

Special signaling and control information must be exchanged between the two chassis in a timely manner, if the two chassis need to be bound together into a single logical node. To facilitate this information exchange, you need a special link to transfer both data and control traffic between the peer chassis. This link is referred to as the virtual switch link (VSL). It is also used to determine which virtual switch becomes the active virtual switch and which becomes the standby virtual switch.

Whenever the virtual switch link (VSL) fails completely, the active supervisor engine discovers the failure of the VSL either through a link-down event or through the failure of the periodic virtual switch link protocol (VSLP) messages sent across the member links to check the VSL link status. From the perspective of the active virtual switch chassis, the standby virtual switch is lost. The standby virtual switch chassis also views the active virtual switch chassis as failed and transitions to active virtual switch state through a stateful switchover (SSO).

In this case, each virtual switch assumes the role as an active virtual switch and controls only its local ports. This scenario is known as a dual-active scenario. Duplication of this configuration can possibly have adverse effects to the network topology and traffic.

To avoid this disruptive scenario, configure the VSL as a multiple-link port channel and spread it across all the available supervisor engines and modules within the chassis. Also run the individual members of the VSL across separate physical paths when possible.

In some circumstances, this configuration may not be possible, and Cisco VSS has different mechanisms to address this dual-active scenario:

• Configuration of the VSL failure-detection feature.
• Detection of a dual-active scenario.
• Action taken to resolve the situation.
• Recovery behavior upon restoring the VSL.

The CISCO-VIRTUAL-SWITCH-MIB Enhancement for VSS dual active detection feature allows you to configure the Simple Network Management Protocol (SNMP) to receive messages when the state of the VSS changes to dual-active. The snmp-server enable traps vswitch dual-activecommand enables the dual-active state change notification. When the VSS changes state to dual-active, the SNMP sends out the cvsDualActiveDetectionNotif notification.