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Root bridge assignment

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  • Root bridge assignment

    Hi everybody!

    I would like to seek your advise on my setup.

    I have two core switches[6500 series] running HSRP. Both switches are Servers.
    Switch 1 is of course the primary switch making Switch 2 the standby switch.
    All vlans are using Switch 1 as its primary.

    I enabled RSTP for both switches... after a while I discovered that Switch 2 is the root bridge. Is this ok? I haven't done any manual configuration for STP.

    should I manually configure switch 1 as my primary root?
    or leave it alone?

    thanks all.


  • #2
    Re: Root bridge assignment

    Did you change the priority for switch 1?
    From memory I think they all come as the same so it uses the lowest (or highest) MAC address? Could be wrong on that but check the priority first.

    Please read this before you post:

    Quis custodiet ipsos custodes?


    • #3
      Re: Root bridge assignment

      You have to manually set your root bridge if you want to specify which switch will be the root. By default, the factory STP settings for the Bridge ID is 16384 plus the MAC address, so the device with the lowest (oldest) MAC address becomes the root bridge. Do you have multiple links connecting these two switches or just a single link? If you have just a single link then there's no point in setting up STP as there's only one path to any host connected to the switches.

      IMHO there's really no point in connecting multiple links between two switches and configuring STP, as a port failure that renders the link inoperable would be a rare and coincidental event. I only enable STP if I have more than two switches connected in a loop or mesh topology and there are multiple paths to any one host. STP is a loop avoidance mechanism and if your switches aren't connected in a loop or mesh topology then there's no point in configuring STP as there's only one path to any host and a configuring STP won't accomplish anything.


      • #4
        Re: Root bridge assignment


        thanks for your response. I am aware of stp as a loop avoidance. it's a broad subject..

        yes, I have more than one path. I have around 25 switches connected to the coreswitches. i have checked the priority. Switch 2 has won the election... which i prefer Switch 1 should be elected.

        so changing and manually setting switch 1 as primary is ok?

        thanks for the confirmation!

        sometimes we need to just make sure



        • #5
          Re: Root bridge assignment

          Yes, setting switch one as the root bridge is OK. With multiple switches I prefer to set the Brifge ID manually on all switches according to the path I want traffic to take if the designated root bridge fails.


          • #6
            Re: Root bridge assignment


            Kindly confirm if the below command is ok.
            I only have like 7 vlans.

            in the coreswitch
            --> spanning-tree vlan 1 root primary
            --> spanning-tree vlan 2 root primary



            • #7
              Re: Root bridge assignment

              Or you could use: spanning-tree vlan 1-7 root primary

              BTW: Since all your access layer switches are dual homed, have you considered configuring both 6500's to route packets instead of configuring active/standby? Thats one expensive switch to have configured in just standby mode.

              Unless you have overridden PVST mode, consider something like:

              # Core switch 1
              spanning-tree vlan 1,3,5,7 root primary

              # Core Switch 2
              spanning-tree vlan 2,4,6,8 root primary

              But in addition to the above, you would also need to reconfigure the HSRP active interface for each vlan to match the STP root.


              • #8
                Re: Root bridge assignment

                Hi. thanks for reminding of setting the vlan range.

                You are right about routing the packets instead of the HSRP.. however this is what the client specified .

                All my active interface are set to Core Switch 1. This is why I prefer Switch 1 to be the primary root.

                I have enabled stp mode rapid-pvst.

                So in summary:

                All active Vlan interface is Switch 1.

                Next step:

                Manually configure all vlans to use Switch 1 as the root bridge. [primary]

                Should I configure coreswitch 2 as secondary or just leave it alone?

                Thanks for all the help!

                Last edited by Dipp; 19th March 2009, 05:11.


                • #9
                  Re: Root bridge assignment

                  Considering the fact that switch 2 was already selected as Root, it's safe to assume that if you manually set switch 1 to be the root and it fails then switch 2 will be selected as root.

                  A problem might occur if you add another switch to your network, and in that case you might ending up with one of your access layer switches becoming the root if switch 1 fails, and we don't want that.

                  You can simply use the spanning-tree vlan 1 root Secondary command or simply configure the priority manually.
                  Another good thing to do is to set root guard on the ports to your distro layer switches... to make sure that even if some one will manually enter a lower priority on one of the distro/access layer switches they won't become the root bridge.


                  • #10
                    Re: Root bridge assignment

                    Hi Idos,

                    Thanks for that information.

                    I have set switch 2 as secondary now.

                    I will soon be trying your advise on placing the set root guard on the ports to the distro layer switches. This will be my access switches..

                    i am on stack setup. per stack there are 4 switches.. . for these switches, i have the first and last switch connected to both switch 1 and 2.. so i'll set the root guard on these 2 switches right? and to the other stacks as well..

                    thanks so much for the help!



                    • #11
                      Re: Root bridge assignment

                      Root guard configured per interface, not per switch.
                      The idea of spanning-tree is that if a bridge with a lower priority comes, it will preempt the current root bridge and will become root.
                      by configuring root guard on a port, you tell the switch that even if it recieves a BPDU with lower priority it will not "give up" and will stay root.