Refer to the exhibit.
The VLAN-to-MST mapping is shown. (Assume SW1 acts as root for all possible MST instances.)
spanning-tree mst configuration name MST?
revision 2?
instance 0 vlan 1-200,301-4094 instance 1 vlan 201-300
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If this topology is deployed, which action is required for traffic to flow on VLAN 200 and 300?
A.
Map VLAN 300 to instance 0.
B.
Map VLAN 200 to instance 2.
C.
Move instance 0 root to SW2.
D.
Move instance 1 root to SW2.
E.
Map both VLANs to instance 2.
B is correct
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For those who don’t have access to Cisco switches. I’ve just simulated this scenarion using two 3750E and can confirm that option B is the right one.
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Can someone please tell me why B is the correct option and not A?? I think A is the correct one!
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I don’t even fully understand the question. Can someone please explain why B s correct?
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Look at Caveats arising from VLAN/STP decoupling item in http://blog.internetworkexpert.com/2008/07/27/mstp-tutorial-part-i-inside-a-region/
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The IST is active on all links, so IST has alternative bloking port on link gig0/24
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MST is not a “per-vlan” STP, it is a “per-instance” STP. Instance 0 is the Internal Spanning Tree instance or IST. The IST BPDU is sent on ALL ports. The switches see vlan 200 on all ports since vlan 200 is mapped to instance 0. Therefore it blocks it on one of the ports. If you move vlan 200 to instance 2 then it no longer is seen on all ports so it no longer gets blocked.
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It’s wrong wording in the question – both VLANs have to be in same instance 0, then B is correct. If they in different instances I think we don’t have the issue.
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