Skip navigation
Login   |   Register
Cisco Learning Home > Certifications > Routing & Switching (CCNA) > Discussions

_Communities

This Question is Answered
10152 Views 8 Replies Latest reply: Mar 5, 2014 4:34 PM by mark RSS

Currently Being Moderated

STP Root Bridge selection MAC with letters

Feb 11, 2010 8:46 AM

syncros 8 posts since
Dec 22, 2008

Hello

 

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

 

Thanks

Christopher

  • Ryan Schuett 421 posts since
    Sep 29, 2008
    Currently Being Moderated
    1. Feb 11, 2010 9:33 AM (in response to syncros)
    Re: STP Root Bridge selection MAC with letters

    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?

     

    00:02:55:AE:66:01

    or

    00:02:55:AF:66:01

     

    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.

     

    HTH,

     

    Ryan

    Join this discussion now: Login / Register
  • Chris Buscemi 203 posts since
    Mar 16, 2009
    Currently Being Moderated
    3. Feb 11, 2010 10:58 AM (in response to syncros)
    Re: STP Root Bridge selection MAC with letters

    Christopher,

     

    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,

     

     

    Chris

    Join this discussion now: Login / Register
  • Ryan Schuett 421 posts since
    Sep 29, 2008
    Currently Being Moderated
    4. Feb 11, 2010 11:01 AM (in response to syncros)
    Re: STP Root Bridge selection MAC with letters

    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,

     

    Ryan

    Join this discussion now: Login / Register
  • David Sessa 95 posts since
    Sep 28, 2010
    Currently Being Moderated
    5. Oct 12, 2012 8:53 AM (in response to Ryan Schuett)
    Re: STP Root Bridge selection MAC with letters

    Which MAC address will be the Root for these two:

     

    000e.3843.00c0

              or

    0004.c070.8300

     

    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?

     

    Thanks

    Join this discussion now: Login / Register
  • Martin 13,892 posts since
    Jan 16, 2009
    Currently Being Moderated
    6. Oct 18, 2012 7:26 PM (in response to David Sessa)
    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. . 

    Join this discussion now: Login / Register
  • Aakil 69 posts since
    Apr 29, 2012
    Currently Being Moderated
    7. Oct 19, 2012 5:33 PM (in response to syncros)
    Re: STP Root Bridge selection MAC with letters

    Everyhody provided the really helpful answers.

     

    Great work by all

    Join this discussion now: Login / Register
  • mark 5 posts since
    Feb 27, 2013
    Currently Being Moderated
    8. Mar 5, 2014 4:34 PM (in response to syncros)
    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

    Join this discussion now: Login / Register

Actions

More Like This

  • Retrieving data ...

Bookmarked By (0)