3 Replies Latest reply: Apr 16, 2012 10:23 AM by DelVonte RSS

    STP - MAC Address Priority


      Hi All,


      Greetings from Perth, Australia! I'm currently studying for an upcoming CCNP Switch exam and have got myself confused over something seemingly very easy.


      Here's the scenario. I've four switches setup in a lab environment to run STP (PVST) on VLAN 1:


      1. ALS1 (2900XL  MAC address = 00d0.9765.36c0)
      2. ALS2 (2950-12 -MAC address = 000f.2400.e340)
      3. DLS1 (2950-24  MAC address = 0012.0000.aec0)
      4. DLS2 (3550 -24 MAC address = 000c.ce03.af80)


      As far as I can see all switches have default settings (i.e. no bridge priorities configured). For some reason the switch I would assume to become the root bridge isn't taking on this role. My assumption is that switch with the "lowest" MAC address would assume the root bridge role in the election process.


      The switch that is the root bridge is 1. ALS1 with the MAC address = 00d0.9765.36c0

      The switch I assume would become the root bridge is 4. DLS2 MAC address = 000c.ce03.af80


      Have I gone mad and got my hexidecmial confused or is there something I'm missing here? Doesn't the order of Hex digits go 0 - 9, A- F?


      I would assume (again) the order of MAC address from lowest to highest is:


      1. 000c.ce03.af80
      2. 000f.2400.e340
      3. 0012.0000.aec0
      4. 00d0.9765.36c0


      Any help would be greatly appreciated!


      As a first time poster, apologies if I've posted this in the wrong section, or have inadvertently posted in an unconventional way.





        • 1. Re: STP - MAC Address Priority

          check to see if the 2900xl is running the same version of STP. You may have to manually set the priority in order for it to work as intended. Not being able to see everything, I'm going to go out on a limb and say its because of the extended-system-id.


          The 2950s and 3550 may be using the longer ID that includes prirority and mac address, while the 2900XL is not. If so, the 2900 would look like the smaller value, as the bits that contain the priority would be all zeros.


          Like I said, I'm going out on a limb here, and could, and most likely am, totally wrong.




          • 2. Re: STP - MAC Address Priority

            I'm completely humbled. You DelVonte, my good sir, are 100% correct! Exactly as you've suggested/implied, the 2900XL's IOS appears to pre-date the sys-id-ext causing the priority to remain as 32768 as shows below:


            ALS1#show spanning-tree vlan 1

            Spanning tree 1 is executing the IEEE compatible Spanning Tree protocol

            Bridge Identifier has priority 32768, address 00d0.9765.36c0

            Configured hello time 2, max age 20, forward delay 15

            We are the root of the spanning tree

            Topology change flag not set, detected flag not set, changes 5

            Times:  hold 1, topology change 35, notification 2

                       hello 2, max age 20, forward delay 15

            Timers: hello 1, topology change 0, notification 0


            ·          All other switch priority's were set to 32769 - that being the default of 32768 + 1 for the sys-id-ext (VLAN1)

            A quick update of the priority to 32769 causes a re-election and 4. DLS2 MAC address = 000c.ce03.af80 with the lowest MAC address assumes the illustrious root bridge status based on lowest MAC address as shown below:


            DLS2#show spanning-tree vlan 1



            Spanning tree enabled protocol ieee

            Root ID    Priority    32769

            Address     000c.ce03.af80

            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     000c.ce03.af80

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

            Aging Time 300


            Many thanks DelVonte! Not to beat myself up here, but to remember for future reference; In hindsight this problem should have been initially identified by myself studying the output of the Bridge/Root ID priority, by running a show spanning-tree vlan 1




            • 3. Re: STP - MAC Address Priority

              I'm glad it worked out.