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How to know the lowest MAC address for STP Root Election

Answered Question
Sep 11th, 2013
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I'm getting ready to take the CCNP switch exam, and I have what may be an elementary question.  In all the guides I've read, and lab manuals that I've used, anytime there are multiple switches that can be the possible root, the MAC address's are shown in a very simple format....such as aaaa:aaaa:aaaa or bbbb:bbbb:bbbb or some other such simple address thus making it obvious.  How about in the real world, or on Cisco exams?  Do you need to convert the entire address to be sure or just a portion? 


Thanks in advance

Correct Answer by John Blakley about 3 years 11 months ago

MAC addresses are in hexadecimal format ranging from 0-9 and a-f. It's pretty much sequential from there.


A mac address of 0020.01cc.de23 is lower than 0021.01cc.de23. Another example would be 0000.c0a7.ffc2 is lower than 000.c0a7.ffc3



HTH,
John

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Correct Answer
John Blakley Wed, 09/11/2013 - 12:34
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MAC addresses are in hexadecimal format ranging from 0-9 and a-f. It's pretty much sequential from there.


A mac address of 0020.01cc.de23 is lower than 0021.01cc.de23. Another example would be 0000.c0a7.ffc2 is lower than 000.c0a7.ffc3



HTH,
John

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madbeader Wed, 09/11/2013 - 12:46
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So starting at the left, as soon as one group is lower you know you have the lowest MAC?
So as an example 0020:01cc:de23 is lower than 0020:01dd:de23 because the middle group of hex is lower?  And

0120.01cc.de23 is higher than 0020.01cc.de23 because the fist group is higher.



Am I getting it correctly?

John Blakley Wed, 09/11/2013 - 13:06
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It looks like you got it. Alain responded with another very good way to determine which is lower too.



HTH,
John

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cadet alain Wed, 09/11/2013 - 12:44
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Hi,

To add to john's answer,

compare each MAC like this: look at each hexadecimal digit from left to right until you find a mismatch and the lowest digit will win( going from zero to F).


Regards


Alain



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