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1014 Views 3 Replies Latest reply: May 31, 2011 8:34 PM by Keith Barker - CCIE RS/Security, CISSP RSS

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How come my cost to the root bridge isn't 38?

May 31, 2011 5:32 PM

PJ 84 posts since
Jun 5, 2010

In the Module 2 L2 video, I recreated a 4 switch network in PT. 

 

map.GIF

 

My root bridge, which is SW4 is 2 segments away from SW2 but is only showing a cost of 19 rather than 38.  I thought if I have 2 100 Mb connections it would add up to 38.  Here the output from SW2:

 

SW2#sh spanning-tree

VLAN0001

  Spanning tree enabled protocol ieee

  Root ID    Priority    32769

             Address     0001.4398.C737

             Cost        19

             Port        23(FastEthernet0/23)

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

 

 

  Bridge ID  Priority    32769  (priority 32768 sys-id-ext 1)

             Address     00E0.8F54.1B22

             Hello Time  2 sec  Max Age 20 sec  Forward Delay 15 sec

             Aging Time  20

 

 

Interface        Role Sts Cost      Prio.Nbr Type

---------------- ---- --- --------- -------- --------------------------------

Fa0/21           Altn BLK 19        128.21   P2p

Fa0/22           Altn BLK 19        128.22   P2p

Fa0/23           Root FWD 19        128.23   P2p

Fa0/24           Altn BLK 19        128.24   P2p

 

As you can see, as fa0/21 and fa0/22 are blocking, my trunk goes through SW1 on fa0/23.  So I'm a little unsure if I'm missing something or is this a limitation of PT?

 

Thanks

PJ

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