I'm using rapid-pvstp in my fiber ring and here is problem: i have to main sites, A and B, and a chain of 30
switches from site A to site B, for the main site A i have the vlan 11 and for the main site B the vlan 12, the
switch closer to site A is configured to be the root of vlan 11 and the switch closer to site B is configured
to be the root of vlan 12, the problem that i have is that the network is very unstable, the root bridge is
changing from the right one to other one very often (sometimes every second) and that is causing me that some
ports sometimes are in blocked state and a second later in forwarding state and then blocked again.
It looks that the switches that are far away from the root of each vlan are not
receiving the bpdu or they receive them with bad values so they start to make them self roots or choosing
another root, doing some researching i found this: "When a BPDU propagates from the root bridge toward the
leaves of the tree, the age field increments each time the BPDU goes though a bridge. Eventually, the bridge
discards the BPDU when the age field goes beyond maximum age. If the root is too far away from some bridges of
the network, this issue can occur. This issue affects convergence of the spanning tree."
I was thinking is to modify the hello, forward and max age timers on the root switches (root for vlan 11
and root for vlan 12) but i'm not sure what value should i use in each timer.
Any other ideas??
Here i attach the diagram for in order to have a better idea of the scenario.
Thanks in advance for your help!
your calculations are good.
Anyway I would simply use
hello = 1 sec
max-age= 40 sec
This should give margin also for the chain scenario where one link is broken
Hope to help