10 Replies Latest reply: Jun 22, 2010 12:16 AM by remek gorecki RSS

    Bridge ID Clarification

    Mike.B

      Hi Guys,

       

      I'm confused on something so please bear with me as I'm not entirely sure I can explain this well enough. =D

       

      It's my understanding that a Bridge ID consists of two parts: 1) A priority value  2) A MAC Address.

       

      What specific MAC address is used here?  How does the switch decide what MAC address to use here?  Is there only one available for this specific purpose per switch? Or is this the specific hardware address of the switchport itself?

       

      Sorry if this doesn't make sense.  I guess I'm wondering if the hardware addresses for the individual switch ports are uses in the bridge ID or not... for example, if I run a show interfaces, I'll see stuff like:

       

      FastEthernet0/4 is down, line protocol is down (disabled)
        Hardware is Lance, address is 0002.16c5.0d04 (bia 0002.16c5.0d04)
        MTU 1500 bytes, BW 100000 Kbit, DLY 1000 usec,
           reliability 255/255, txload 1/255, rxload 1/255

      If this port was used to connect to another switch (which it obviously isn't), would its hardware address be used in the bridge ID?

       

      Thanks!

       

      -M

        • 1. Re: Bridge ID Clarification
          Chris

          Hi Mike,

           

          The MAC address used for the BID is the switch's Base MAC address.  You can see this address when rebooting an switch.

           

          HTH

          • 2. Re: Bridge ID Clarification
            Mike.B

            Ahhhhhhh! That's EXACTLY what I wanted to know.  Thanks vlan. Brilliant stuff.

             

            -M

            • 3. Re: Bridge ID Clarification
              Scott Morris - CCDE/4xCCIE/2xJNCIE

              Use the "show spanning-tree bridge id" command to see what BID is being used.  Remember that this is 4 bits priority + 12 bits VLAN ID + 48 bits MAC address.

               

               

              Rack1SW1#sh span bridge id
              VLAN0001         8001.0019.060c.9080
              VLAN0003         8003.0019.060c.9080
              VLAN0005         8005.0019.060c.9080
              VLAN0006         8006.0019.060c.9080
              VLAN0007         8007.0019.060c.9080
              VLAN0017         8011.0019.060c.9080
              VLAN0022         8016.0019.060c.9080
              VLAN0036         8024.0019.060c.9080
              VLAN0042         802A.0019.060c.9080
              VLAN0052         8034.0019.060c.9080
              VLAN0100         8064.0019.060c.9080
              VLAN0127         807F.0019.060c.9080
              VLAN0473         81D9.0019.060c.9080
              Rack1SW1#

               

              The MAC used is your system MAC.  This will be one bit value lower than your lowest port MAC.

               

              Rack1SW1#sh int | in bia
                Hardware is EtherSVI, address is 0019.060c.90c0 (bia 0019.060c.90c0)
                Hardware is EtherSVI, address is 0019.060c.90c1 (bia 0019.060c.90c1)
                Hardware is EtherSVI, address is 0019.060c.90c2 (bia 0019.060c.90c2)
                Hardware is Fast Ethernet, address is 0019.060c.9083 (bia 0019.060c.9083)
                Hardware is Fast Ethernet, address is 0019.060c.9084 (bia 0019.060c.9084)
                Hardware is Fast Ethernet, address is 0019.060c.9085 (bia 0019.060c.9085)
                Hardware is Fast Ethernet, address is 0019.060c.9086 (bia 0019.060c.9086)
                Hardware is Fast Ethernet, address is 0019.060c.9087 (bia 0019.060c.9087)
                Hardware is Fast Ethernet, address is 0019.060c.9088 (bia 0019.060c.9088)
                Hardware is Fast Ethernet, address is 0019.060c.9089 (bia 0019.060c.9089)
                Hardware is Fast Ethernet, address is 0019.060c.908a (bia 0019.060c.908a)
                Hardware is Fast Ethernet, address is 0019.060c.908b (bia 0019.060c.908b)
                Hardware is Fast Ethernet, address is 0019.060c.908c (bia 0019.060c.908c)
                Hardware is Fast Ethernet, address is 0019.060c.908d (bia 0019.060c.908d)
                Hardware is Fast Ethernet, address is 0019.060c.908e (bia 0019.060c.908e)
                Hardware is Fast Ethernet, address is 0019.060c.908f (bia 0019.060c.908f)
                Hardware is Fast Ethernet, address is 0019.060c.9090 (bia 0019.060c.9090)
                Hardware is Fast Ethernet, address is 0019.060c.9091 (bia 0019.060c.9091)
                Hardware is Fast Ethernet, address is 0019.060c.9092 (bia 0019.060c.9092)
                Hardware is Fast Ethernet, address is 0019.060c.9093 (bia 0019.060c.9093)
                Hardware is Fast Ethernet, address is 0019.060c.9094 (bia 0019.060c.9094)
                Hardware is Fast Ethernet, address is 0019.060c.9095 (bia 0019.060c.9095)
                Hardware is Fast Ethernet, address is 0019.060c.9096 (bia 0019.060c.9096)
                Hardware is Fast Ethernet, address is 0019.060c.9097 (bia 0019.060c.9097)
                Hardware is Fast Ethernet, address is 0019.060c.9098 (bia 0019.060c.9098)
                Hardware is Fast Ethernet, address is 0019.060c.9099 (bia 0019.060c.9099)
                Hardware is Fast Ethernet, address is 0019.060c.909a (bia 0019.060c.909a)
                Hardware is Gigabit Ethernet, address is 0019.060c.9081 (bia 0019.060c.9081)
                Hardware is Gigabit Ethernet, address is 0019.060c.9082 (bia 0019.060c.9082)
              Rack1SW1#

               

              Notice the gigabit ports have the lowest addresses.  9081 is the lowest.  So 9080 (as seen above) is the System MAC on the switch, which is used for BID values.

               

              HTH,

               

              Scott

              smorris@internetworkexpert.com

              • 4. Re: Bridge ID Clarification
                JohnB352

                I have a followup question to your answer.

                I'm studying for my ICND2, and I was trying to figure the same thing that the OP was asking; where does the MAC in the BID come from?

                 

                I'm using Packet Tracer, and on my configuration it looks like the MAC in the BID comes from VLAN 1.  But, where is that coming from?  Here's what I have from the Root Bridge:

                 

                Gig 1/1  0090.21b6.9101
                Gig 1/2 0090.21b6.9102
                VLAN 1  0090.2b47.8421

                Base ethernet MAC Address: 0090.2B47.8421   (shown on a reload)

                Show spanning-tree vlan 1
                Root ID    Priority    24577
                     Address     0090.2B47.8421
                     This bridge is the root


                The Base or System address on this switch isn't the lowest switch port (G 1/1) address, minus a value of 1.  It's VLAN 1, minus 1.

                Where is the MAC for VLAN 1 coming from?  And is the results here different than what you get on your switch, as far as how the BID was determined?

                Thanks

                • 5. Re: Bridge ID Clarification
                  Scott Morris - CCDE/4xCCIE/2xJNCIE

                  Yup.  As noted above, it's one below the lower port MAC.  Known as the system MAC.  In modular switches it's based on the supervisor card, but in fixed switches, it's one lower than lowest port MAC.

                   

                  HTH,

                   

                  Scott

                  • 6. Re: Bridge ID Clarification
                    JohnB352

                    swmorris wrote:

                     

                    Yup.  As noted above, it's one below the lower port MAC.  Known as the system MAC.  In modular switches it's based on the supervisor card, but in fixed switches, it's one lower than lowest port MAC.

                     

                    HTH,

                     

                    Scott

                     

                    There is a difference in what you're seeing and what I am seeing.

                     

                    You show one of your Gig interfaces with this MAC address:  0019.060c.9081

                    And you say the BID would be based on that:  9081-1 = 9080

                     

                    In my config the lowest MAC was Vlan 1, not the MAC of G1/1.

                    I'm trying to understand "why the difference".

                    The BID in my example is the Vlan 1 MAC minus 1.

                     

                    Is that because of the "supervisor card" or "fixed switch" that your'e referring to?  (I'm not familiar with those terms)

                    Does that explain why your BID is based on the Gig int and mine is based on the VLAN1 int?

                    • 7. Re: Bridge ID Clarification
                      Scott Morris - CCDE/4xCCIE/2xJNCIE

                      Actually, it depends more on which set of interfaces (chipset) powers up first.    In my switch, it happened to be the gig ports.  Depending on what hardware platform you are using, it may vary.  In modular chassis, the sup module powers first, therefore it's the base mac.

                       

                      HTH,

                       

                      Scott

                      • 8. Re: Bridge ID Clarification
                        JohnB352

                        Ok, thanks for clearing that up.

                        • 9. Re: Bridge ID Clarification
                          Scott Morris - CCDE/4xCCIE/2xJNCIE

                          No worries....

                          • 10. Re: Bridge ID Clarification
                            remek gorecki

                            Is it possible to change this MAC?

                             

                            in 3750 it is  Base Ethernet Mac Addres and its 1 bit lower than lowest MAC in switch  (in this case g1/0/1)

                             

                            RG