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MST root path cost of zero

Hello,

 

I am just wondering how a switch participating in a single region MST architecture can have a root path cost of zero.

The switch itself is an 2960 connected to an SG500-X through a dual 1gbps port channel which is again connected through an 10gbps link to

an 6509 (which is the root).

 

[2960] -------- port channel --------- [SG500-X] --------- 10gbps ---------[6509 (root)]

 

 

The SG500-X shows a root path cost of 2000 (i am using long cost values) which is fine. The  connected 2960 shows the correct root with its

correct priority. The cost of the affected port channel on the 2960 is correctly shown as 10000. Just the root path cost is set to zero which is something that I do not understand.I would expect a value of 12000 here.

 

Thats what the 2960 shows:

 MST0 is executing the mstp compatible Spanning Tree protocol
  Bridge Identifier has priority 32768, sysid 0, address 1caa.070f.7f00
  Configured hello time 2, max age 20, forward delay 15, transmit hold-count 6
  Current root has priority 24576, address 0022.be66.ac00
  Root port is 256 (Port-channel5), cost of root path is 0
  Topology change flag not set, detected flag not set
  Number of topology changes 23 last change occurred 03:24:16 ago
          from Port-channel5
  Times:  hold 1, topology change 35, notification 2
          hello 2, max age 20, forward delay 15
  Timers: hello 0, topology change 0, notification 0

 

 

Can anyone of you guys push my limited knowledge on MST and tell me how a root path cost of zero is possible on a switch where the incoming port channel has a cost of 10000 and the preceeding switch shows a root path cost of 2000?

 

 

Thanks alot for your help,

Heri

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