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
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
Ryan
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.
Thanks
Christopher
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
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
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
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. .
Everyhody provided the really helpful answers.
Great work by all
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
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