Kevin Dorrell, CCIE #20765

19 Apr 2008

NMC 6.9 MST

Filed under: MST — dorreke @ 14:57

Following a question on the DISCUSiT forum, I loaded my old configs, and there seems to be something amiss with this lab. I could not get 172.16.64.30 to talk to 172.16.64.10. The problem seems to be the MST.

Consider the two lines between CAT3 and CAT4. The Fa0/15 is carrying VLAN 102 and the Po1 line is carrying VLAN 101. But 6.9.1. says that CAT4 must be root. I presumed that must apply to MST0 (which carries VLAN 102) as well as MST 1 (which carries VLAN 101). Thus, on CAT3, my spanning tree looks like this:

CAT3#show spanning-tree

MST00
  Spanning tree enabled protocol mstp
  Root ID    Priority    24576
             Address     0009.b703.7280
             Cost        0
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32768  (priority 32768 sys-id-ext 0)
             Address     0009.43d5.2000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Fa0/15           Altn BLK 200000    128.15   P2p
Po1              Root FWD 100000    128.65   P2p

MST01
  Spanning tree enabled protocol mstp
  Root ID    Priority    24577
             Address     0009.b703.7280
             Cost        100000
             Port        65 (Port-channel1)
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)
             Address     0009.43d5.2000
             Hello Time   2 sec  Max Age 20 sec  Forward Delay 15 sec

Interface        Role Sts Cost      Prio.Nbr Type
---------------- ---- --- --------- -------- --------------------------------
Po1              Root FWD 100000    128.65   P2p

Hey – look! For MST0, F0/15 is blocking. That is obviously because CAT4 is the root, and CAT3 has a best path to the root through Po1. But VLAN 102 is disallowed on link Po1. Clearly something is wrong.

Look at the SHOWiT. We find that CAT 4 is the root for MST1 only. For MST0, CAT3 is the root. So the question 6.9.1. must be questionable.

In fact, comparing the configurations of CAT3, I have spanning-tree mst 0-1 priority 24576, whereas the SHOWiT has spanning-tree mst 1 priority 24576. So I throw up my hands in horror and say “It’s not fair!” I have spent so much time working out what the problem was, that if it had been a real exam, I would have failed it.

So I made CAT3 the root of MsT 0, but I still couldn’t get CAT3 talking to CAT1.  Now Fa0/15 on CAT4 is blocking.  This all serves to illustrate some of the problems you can have if your pruning topology does not reflect you MST topology.

Maybe this time I am in the wrong.  Unfortunately I do not have a 3650 for my CAT4, so I was unable to do the dot1q tunnel between F0/15 and F0/22.  Maybe if I had that tunnel, CAT4 F0/15 would be forwarding.

I give up.  I have more cost-effective ways to spend my study time.

 

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