ded aging time for all vlans is at least three times the Installing VSS activity interval and global aging time will be changed automatically if required VSS dual-active detection Access switches with the proper IOS level can support enhanced PAgP messages that allow the detection of a dual-active scenario. Dual-active, also called split-brain by many people, describes the situation where both physical switches become active. This is a problem because both switches will own the same IP address. That's a bad thing, and we should work hard to defend against the condition. There are multiple ways to do this. If you've got all Cisco switches, Cisco recommends using enhanced PAgP. If you've got other vendor's switches, then you can use another method: VSS#sho switch virtual dual-active ? Dual-active bfd summary fast-hello Dual-active fast-hello summary Dual-active pagp summary summary Dual-active config summary Cisco has enhanced PAgP to sense a dual-active situation when a third switch is con- nected to the VSS cluster. To use this feature, you must have the minimum IOS version installed on the third switch as outlined in Table 17-1. Table 17-1. Minimum IOS requirements Device series Minimum IOS required Catalyst 3750 IOS 12.2(46)SE Catalyst 4500 IOS 12.2(44)SE Catalyst 6500 IOS 12.2(33)SXH Catalyst 6500 VSS IOS 12.2(33)SXH1 By default, PAgP dual-active detection is enabled on the VSS pair. However, the en- hanced messages are sent only on port channels with trust mode enabled. To enable trust mode, you must shut down the port channel being trusted, if it already exists. Once the port channel interface is shut down, configure the switch virtual domain. Here, I'm configuring po30, the MEC connecting the third switch to the VSS pair as shown in Figure 17-9: S(config)#int po30 (config-if)# shut /SS(config)#switch virtual domain 100 VsS(config-vs-domain)#dual-active detection pagp VsS(config-vs-domain)#dual-active detection pagp trust channel group 30 (config)#int po30 VSS (config-if)#no shut To see the status of dual-ac