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Question about RSTP.

Hi there,


I have a question.


in this case,


Why switch C and D is corret?


I think switch A and B are desired to be root.

Is there any reason that A and B need to be root for vlan 3 and vlan 4??


Thank you in advance.




Usually the bridge ID would

Usually the bridge ID would have to be altered on all four switches to obtain the desired result.

For example for VLAN 3 the bridge priority on SW A could be 0 making this the root bridge for VLAN 3. 

The bridge priority on SW C could be 4096. Therefore traffic will flow directly down to SW A assuming SW A is assuming the Active role in your FHRP.

When using HSRP for example with STP you must make sure both protocols align to each other, so that the root bridge that is receiving traffic is also assuming the Active FHRP role. Otherwise in this example, traffic will flow the long way round leading to sub-optimal flow of traffic.

The answer is C and D due to the ALT and BLK port existing on these switches which provides the failover.

If you recall STP will block ports on a non-root switch (SW C or D), they are not blocked on the root bridge.

Try the example in a Packet Tracer lab and you will see that the ALT or BLK ports are positioned on a non-root switch, remember every non-root switch must either BLK or have an ALT port to the root bridge to prevent loops! This is what provides the almost seamless failover on rSTP.

All ports on the root bridge are designated ports.



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