5 Replies Latest reply: Feb 11, 2014 12:40 AM by Mrigendra RSS

    Difference Between Root ID & Bridge ID

    Muhammad Mataal Khan

      When I use a command Show spanning tree it shows me different Vlans. How do I know that which Bridge ID or Root ID belongs to switch A or B or C

        • 1. Re: Difference Between Root ID & Bridge ID
          Paul Stewart  -  CCIE Security

          From a spanning-tree perspective, each switch knows its own Bridge ID and the Bridge ID  of the Root (Root ID). So you would need to execute the command from all 3 switches and examine the output of Bridge ID on each.

          • 2. Re: Difference Between Root ID & Bridge ID
            Mrigendra

            Hi,

             

            When you configure STP, then by default Cisco IOS runs PVST+. That means your switch will run the STP instance for each VLANs. Now let say you have 3  switches A, B & C..and you have three vlans 10, 20 & 30 configured in each switch.

             

            Taking your first Que:

            All your 3 switches will run the STP instance for each VLAN, and that is the reason why you see different VLANS in the output.

             

            Second Que:

            Each vlan will have a Root switch/Bridge which will be common for individual STP instance running in your all three switches.

            What i mean by the above statement is: If let say Switch-A is a root bridge for Vlan-10 then Sw-B & Sw-C will see this switch as root switch for Vlan-10 only.

            And the same will be applied for other VLANs

             

            the question remains the same:

            How do I know that which Bridge ID or Root ID belongs to switch A or B or C

             

            Do the following step:

            1> Run the command:

             

            Switch#show spanning-tree

             

            In the output you will see two section. first one is Root ID and second one is Bridge ID...output will be like this...

             

            VLAN0001 <This is STP instance for VLAN-1>

              Spanning tree enabled protocol ieee

              Root ID   Priority    32769  <This is the information about the Root Switch for VLAN-1, you will have this same common view in all the switches for Root Bridge>

                         Address     0001.43A7.7310

                         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)  <This is the information about the Local Switch "Same switch where i executed this command", and it will be different for different switches for different VLANs>

                         Address     0001.43A7.7310

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

                         Aging Time  20

             

            Hope you know the selection method for Root Switch....So by that hope i cleared your doubts

             

            Please let us know if you need more explanation on this.

             

             

            Thanks!

            • 3. Re: Difference Between Root ID & Bridge ID
              Jonathan

              You can always use the command show spanning-tree vlan <vlan-number>

              If it says "This bridge is the root" then you can come to the conclusion that is the bridge. If it states that it is connected to a port ex Fe0/3 then if you have cdp enabled then you can type show cdp neighbors to verify which switch matches that port to determine the root bridge.

              • 4. Re: Difference Between Root ID & Bridge ID
                Muhammad Mataal Khan

                Thanks to all of you

                • 5. Re: Difference Between Root ID & Bridge ID
                  Mrigendra

                  You are welcome Muhammad!

                  Glad we were able to help you 

                   

                   

                  Thanks!