04-18-2012 07:50 AM
A warning to anyone using PVST+. Be extremely careful in implementing AOS >22.214.171.124 with PVST+. I encountered a serious spanning-tree loop (are any not serious? :-}) upon upgrading from 126.96.36.199 to 188.8.131.52, but I traced the issue back to 184.108.40.206. Interestingly enough, there was a PVST+ bug fixed in 220.127.116.11 which probably has something to do with this.
In environments where you are running rapid-pvst on a controller which is connected to another switch (Cisco in my case) running either PVST or rapid-PVST, the controller will always think it's the root of spanning-tree regardless of its priority. In my case, one 3200 controller is connected to two Cisco 6500 switches, which are in turn connected to each other. The 6500s are running PVST and one of the 6500s is the true root. Regardless, the 3200 thinks he's root and an STP loop is created as all ports on all switches go into forwarding. The non-root 6500 normally puts its connection to the 3200 into blocking when the 3200 is running 18.104.22.168 or earlier. With 22.214.171.124 or higher, that port is forwarding.
If I convert the 6500s to rapid-PVST, the 3200 still thinks he's the root, but the non-root 6500 puts his port to the 3200 into Dispute and blocks. So no loop here, but thre 3200 is still in an error condition believing he is root.
Bottom line, be EXTREMELY careful in using rapid-pvst past AOS 126.96.36.199. Test throughouly in your lab before going into production.
TAC has replicated the issue and I'm awaiting a bug ID and a response from engineering.