2 Replies Latest reply: Jan 12, 2019 5:25 PM by ciscodaze1 RSS

    STP root cost question

    Learner

      Hello all,

       

      In the following paragraphs from ICND2 OCG, Point A gives cost values according interface bandwidth. But point C says "Cisco switches choose STP cost defaults based on the current speed, not the maximum speed"? So how to find out the "current speed" and the "maximum speed" of the interface?


      stp root port.jpg

        • 1. Re: STP root cost question
          ciscodaze1

          sh int status

          sh int x/x <<<< shows the hardware speed and current speed

           

          Switch#sh int g0/1

          GigabitEthernet0/1 is up, line protocol is up (connected)

            Hardware is Lance, address is 0001.6318.cd19 (bia 0001.6318.cd19)

          BW 1000000 Kbit, DLY 1000 usec,  <<< default, Max speed

            reliability 255/255, txload 1/255, rxload 1/255

            Encapsulation ARPA, loopback not set

            Keepalive set (10 sec)

          Full-duplex, 100Mb/s <<< current speed, because other end (Gig) has speed manually at 100Mb/s


          When you are using link aggregation, your cost numbers are different due to the combined speeds of links, so two switches connected with two 1 gig links in a port channel look like:

           

          Switch#sh spanning-tree

          VLAN0001

            Spanning tree enabled protocol ieee

            Root ID Priority 32769

            Address 0006.2A58.8176

            This bridge is the root

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

           

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

            Address 0006.2A58.8176

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

            Aging Time 20

           

          Interface  Role  Sts    Cost    Prio.Nbr    Type

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

          Po1          Desg FWD    3        128.27        Shr

          • 2. Re: STP root cost question
            Learner

            Thanks Kev, Well explained.