8 Replies Latest reply: Mar 5, 2014 4:34 PM by mark RSS

    STP Root Bridge selection MAC with letters




      Sometimes it is the easiest things.


      When a MAC address has letters in it how do you calculate the lowest value for Root Bridge selection ?

      I follow that letters A -F are hex values but do I plug the value into the number sequence or do I break it down to binary




        • 1. Re: STP Root Bridge selection MAC with letters
          Ryan Schuett

          Yeah, you can break it down to binary if you wish and then convert it to decimal to determine what value will be used for the root bridge election. An easier way, if you have access, is to use windows calculator. After some exposure to it though, you should just be able to glance at the MAC and determine the lowest MAC address quite easily.


          So which one is lower?






          Well, the MAC is the same until the fourth octect from the left (which would be where we start when determining the lowest address). In hex, 0 is lowest and F (15) is highest value. So E would be lower than F so the first choice would be the lower MAC address. This would be the determining winner if the bridge ID value is equal. Remember bridge ID is 64 bits long (4 bits or priority plus 12 bits of vlan ID plus 48 bits of MAC address). 802.1D (regular spanning tree) is 16 bits of priority and 48 bits of MAC address.





          • 2. Re: STP Root Bridge selection MAC with letters



            Thanks for this.


            So if I can restate


            When figuring lowest MAC address and a letter is included with the MAC giving the letter it's hex value 0 - 5 (simplified) will give me the value.


            Ok it becomes clearer and I will use the windows calculator to see if it becomes any more clear.





            • 3. Re: STP Root Bridge selection MAC with letters
              Chris Buscemi



              Just like Ryan said... When you're trying to figure out who has the lower MAC address, just compare each hex character from left to right. The only one that matters is the first character from the left that doesn't match, that's the one that decides who wins.


              It's easy when they're numbers as we all know 1 is lower than 3, but as long as you remember that A comes after 9, B comes after A (counting up) it will be easy for you.


              You could break everything down into binary, but since you really only have to concern yourself with one particular hex character, it would actually be a waste of time.


              The more you use Hex the easier it gets, but for MACs just remember... 0 < 1 < 2 < 3 < 4 < 5 < 6 < 7 < 8 < 9 < A < B < C < D < E < F




              Hope that helps,




              • 4. Re: STP Root Bridge selection MAC with letters
                Ryan Schuett

                Hi Christopher,


                So in hex, once you hit 10 you are starting with the letter A. Since hex is base(16), we can only have 16 numbers, hence 0 to F. However, once you want to goto decimal 16, the hex value will now be 10 (don't get this confused with decimal 10)


                Hex     /     Decimal

                0     =          0

                1     =          1

                2     =          2


                9     =          9

                A     =          10

                B     =          11

                C     =          12

                D     =          13

                E     =          14

                F     =          15


                10     =          16

                11     =          17


                1E     =     30

                1F     =     31


                Hopefully you can see the pattern for this. Just look at it by the octet as well. HTH,



                • 5. Re: STP Root Bridge selection MAC with letters
                  David Sessa

                  Which MAC address will be the Root for these two:






                  Since the first octet shows us that the lower MAC has a 4 in it and the upper MAC has an e it does this mean that the lower MAC would be the root bridge? Do we stop counting/comparing once we find a difference?



                  • 6. Re: STP Root Bridge selection MAC with letters

                    You can read nibble by nibble and stop when u see first difference. Switch reads/sees bits (binary) not Hex.  Looking at binary system for numbers 0 thru 16, you can see digits are lined up nicely: from lower values to higher.  But some people compare 1st octet. . 

                    • 7. Re: STP Root Bridge selection MAC with letters

                      Everyhody provided the really helpful answers.


                      Great work by all

                      • 8. Re: STP Root Bridge selection MAC with letters

                        I'm going for my ccna now and had some trouble with this also


                        The way that I worked it out is that since hex is base 16


                        lets say the number is 1a as part of the hex


                        to get the equivilant decimal number

                        1 * 16 =16

                        a = 10 in hex

                        so 16 + 10 =26


                        so for 2c for the digit

                        2 * 16 = 32

                        c = 12 in hex

                        32 + 12 = 44