Hot Downloads

Welcome, Guest
Username: Password: Remember me

TOPIC: which mac-address is big?

which mac-address is big? 7 years 7 months ago #30019

  • Redpix
  • Redpix's Avatar
  • Offline
  • Frequent Member
  • Posts: 21
  • Karma: 0
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)
The administrator has disabled public write access.

Re: which mac-address is big? 7 years 7 months ago #30020

  • skylimit
  • skylimit's Avatar
  • Offline
  • Distinguished Member
  • Posts: 158
  • Thank you received: 1
  • Karma: 0
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:
https://cisco.hosted.jivesoftware.com/docs/DOC-4052

Hope this helps
"...you are never too old to learn" anon
The administrator has disabled public write access.

Re: which mac-address is big? 7 years 7 months ago #30021

  • S0lo
  • S0lo's Avatar
  • Offline
  • Moderator
  • Posts: 1577
  • Thank you received: 7
  • Karma: 3
I agree with skylimit on how to detect the lower Bridge IDs.
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
The administrator has disabled public write access.

Re: which mac-address is big? 7 years 7 months ago #30062

  • Redpix
  • Redpix's Avatar
  • Offline
  • Frequent Member
  • Posts: 21
  • Karma: 0
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..
The administrator has disabled public write access.
Time to create page: 0.085 seconds

CCENT/CCNA

Cisco Routers

  • SSL WebVPN
  • Securing Routers
  • Policy Based Routing
  • Router on-a-Stick

VPN Security

  • Understand DMVPN
  • GRE/IPSec Configuration
  • Site-to-Site IPSec VPN
  • IPSec Modes

Cisco Help

  • VPN Client Windows 8
  • VPN Client Windows 7
  • CCP Display Problem
  • Cisco Support App.

Windows 2012

  • New Features
  • Licensing
  • Hyper-V / VDI
  • Install Hyper-V

Linux

  • File Permissions
  • Webmin
  • Groups - Users
  • Samba Setup