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which mac-address is big?
15 years 8 months ago #30019
by Redpix
which mac-address is big? was created by Redpix
Hi ,
When i was studying STP (spanning tree) i got a doubt as to how switches break the tie to get elected as a root bridge when the bridge id is the same. It is by comparing the mac -addresses but how can one decide out of two mac-addresses which one is big to break the tie for root bridge election.
I hope i have asked it correctly.Can anyone elaborate on this.How to calculate this?
Thanks in Advance
Redpix,
Working with an ISP(Network Admin)
When i was studying STP (spanning tree) i got a doubt as to how switches break the tie to get elected as a root bridge when the bridge id is the same. It is by comparing the mac -addresses but how can one decide out of two mac-addresses which one is big to break the tie for root bridge election.
I hope i have asked it correctly.Can anyone elaborate on this.How to calculate this?
Thanks in Advance
Redpix,
Working with an ISP(Network Admin)
15 years 8 months ago #30020
by skylimit
"...you are never too old to learn" anon
Replied by skylimit on topic Re: which mac-address is big?
I believe by mere reading the various switch MAC addresses (from left-right), you can determine which one is numerically lower (i.e. the root bridge).
example 1:
Switch A with bridge-id: 30768-04-11-cc-12-77
Switch B with bridge-id: 32768-04-88-88-cc-12-36
Switch C with bridge-id: 34768-04-00-00-cc-12-77
In the above example, the root bridge will be Switch A since it has the lowest bridge id (the lowest bridge priority of 30768).
Example 2 (your question)
Assuming all the switches had same default bridge-id, then the MAC is used to determine the root bridge:
Switch A with bridge-id: 32768-04-11-cc-12-77
Switch B with bridge-id: 32768-04-88-88-cc-12-36
Switch C with bridge-id: 32768-04-00-00-cc-12-77
The root bridge will now be Switch C since it has the lowest MAC address.
How?
Reading each MAC from right to left we have:
04-1...
04-8..
04-0... (lowest)
i.e. 04 is common in all the addresses, 0 is lower than 1 and 8.
Please give credit to Keith Bogart from whose whitepaper on STP calculation this was learnt.
You can download the pdf here:
cisco.hosted.jivesoftware.com/docs/DOC-4052
Hope this helps
example 1:
Switch A with bridge-id: 30768-04-11-cc-12-77
Switch B with bridge-id: 32768-04-88-88-cc-12-36
Switch C with bridge-id: 34768-04-00-00-cc-12-77
In the above example, the root bridge will be Switch A since it has the lowest bridge id (the lowest bridge priority of 30768).
Example 2 (your question)
Assuming all the switches had same default bridge-id, then the MAC is used to determine the root bridge:
Switch A with bridge-id: 32768-04-11-cc-12-77
Switch B with bridge-id: 32768-04-88-88-cc-12-36
Switch C with bridge-id: 32768-04-00-00-cc-12-77
The root bridge will now be Switch C since it has the lowest MAC address.
How?
Reading each MAC from right to left we have:
04-1...
04-8..
04-0... (lowest)
i.e. 04 is common in all the addresses, 0 is lower than 1 and 8.
Please give credit to Keith Bogart from whose whitepaper on STP calculation this was learnt.
You can download the pdf here:
cisco.hosted.jivesoftware.com/docs/DOC-4052
Hope this helps
"...you are never too old to learn" anon
15 years 8 months ago #30021
by S0lo
Studying CCNP...
Ammar Muqaddas
Forum Moderator
www.firewall.cx
Replied by S0lo on topic Re: which mac-address is big?
I agree with skylimit on how to detect the lower Bridge IDs.
Ties in the bridge ID cannot happen. The bridge ID is composed of both the bridge priority and the MAC address. The MAC address is designed by vendors in a way that NO two devices will have the same MAC address.
Check here,
books.google.com/books?id=jNN7kBkScm8C&a...=result&resnum=2
books.google.com/books?id=TFdo0H8OKVMC&a...=result&resnum=1
Hope this helps
i got a doubt as to how switches break the tie to get elected as a root bridge when the bridge id is the same.
Ties in the bridge ID cannot happen. The bridge ID is composed of both the bridge priority and the MAC address. The MAC address is designed by vendors in a way that NO two devices will have the same MAC address.
Check here,
books.google.com/books?id=jNN7kBkScm8C&a...=result&resnum=2
books.google.com/books?id=TFdo0H8OKVMC&a...=result&resnum=1
Hope this helps
Studying CCNP...
Ammar Muqaddas
Forum Moderator
www.firewall.cx
15 years 8 months ago #30062
by Redpix
Replied by Redpix on topic Re: which mac-address is big?
Hi Guys,
Sorry for the delay in checking this stuff posted by you..
Thanks very much for a speedy response
Got it and your replies are really helpful and the pdf is really
useful...
Thanks again.
Redpix..
Sorry for the delay in checking this stuff posted by you..
Thanks very much for a speedy response
Got it and your replies are really helpful and the pdf is really
useful...
Thanks again.
Redpix..
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