There are two methods of Dual-Active detection inherent to VSS:
If you are planning on using PAGP you have to have one or more switches connected to each VSS shelf via a multi-chassis Etherchannel. In your case this would be your distribution layer switch(es). The switches in question also have to be running an IOS image that supports the new Enhanced PAGP 'tags' used by the VSS system to alert each shelf of a dual-active event.
BFD (Bidirectional Forwarding Detection)
This method requires a heartbeat connection between the two VSS switches. It will quickly detect dual-active events and cause the same recovery scenario as PAGP: the active switch goes into recovery mode and the standby goes active.
For more information go to this URL and search for - Complete VSL Failure (Dual Active) - :
We are pleased to announce availability of Beta software for 16.6.3. 16.6.3 will be the second rebuild on the 16.6 release train targeted towards Catalyst 9500/9400/9300/3850/3650 switching platforms. We are looking for early feedback from custome...