Here I come with my first quiz about a (rather common) MSTP misconfiguration that does not look very obvious on the first encounter.

This is my first quiz and I chose an MSTP misconfiguration scenario that could be easily overlooked on the first encounter.

You have a typical network with 2 COREs and 2 DIST switches running Multiple Spanning Tree Protocol. The server team requested that servers in vlans 100 to 199 will be considered privileged. You implemented MSTP in the following way:

  • created a dedicated instance in MSTP (vlans 100-199 mapped to MST 1) and made CORE-2 as ROOT
  • left all other vlans in MST 0 with CORE-1 as ROOT
  • first trunk between CORES (core-1:Gi0/13 <> core-2:Gi0/13) was dedicated only to vlans 100-199 (switchport trunk allowed vlan 100-199)
  • second trunk between CORES (core-1:Gi0/14 <> core-2:Gi0/14) allows all vlans

quiz-1_mstp

After your implementation, server team confirmed that all communications are ok (in both privileged vlans 100-199 and also in all other existing vlans).
What's wrong with this design ?

Give me a hint about the problem !

All good, everybody's happy ... until a cable got disconnected on DIST-2: the fiber between DIST-2:Gi0/13 and CORE-1:Gi0/19.
No problem, you say, DIST-2 still has one uplink to CORE-2 ... but very soon you receive complaints from admins that the 2 servers in vlan 11 are not able to communicate to each other anymore.

What do you think the problem is? What would be the best commands to troubleshoot this issue and on what device(s)?