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

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


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




      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


        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?