Question : PVST

When having a handful of switches connected to each other with single links (i.e. non-redundant) what happens when the root bridge for one VLAN fails?


A is the root for one VLAN and D is the root for the other. The two span all 4 switches. When 1 root fails, is there an interruption of traffic for that VLAN?

Answer : PVST

Sorry to disagree, but it doesn't matter if there is a loop or not. If spanning tree is running, there IS a root. Now if there is no loop, then spanning tree doesn't have to block any ports, but it's still running.

If the root should fail, then after the Max Age timer (20 seconds) expire on the remaining switches, all the switches will generate BPDU's announcing themselves as the root. After a couple seconds, the switches that have higher Bridge ID's will stop sending these BPDU's when they realize that there is another switch with a lower Bridge ID. During this election process, there can be a disruption in traffic flow as the new topology is determined.
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