STP root bridge ?

Answered Question
Jan 23rd, 2008

Hi,

In order to set a switch as the root bridge for STP I can use one of the following two methods:

spanning-tree vlan 'vlan-id' priority 4096

or

spanning-tree vlan 'vlan-id' root {primary}

Is there any other method to configure a switch (3550) as the root bridge ?

I was thinking along the lines of changing the MAC address if the two priorities are tied.

Cheers,

Phil.

I have this problem too.
0 votes
Correct Answer by Jon Marshall about 8 years 12 months ago

Hi Phil

You cannot change the mac-addresses used for STP calculation on fixed chassis switches - see attached link for details

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml#span_mac

So as the mac-addresses will always be different on different switches manipulating the priority either up or down on switches is the only way to affect the root bridge election.

HTH

Jon

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Correct Answer
Jon Marshall Wed, 01/23/2008 - 03:49

Hi Phil

You cannot change the mac-addresses used for STP calculation on fixed chassis switches - see attached link for details

http://www.cisco.com/en/US/products/hw/switches/ps700/products_tech_note09186a00801c9b4e.shtml#span_mac

So as the mac-addresses will always be different on different switches manipulating the priority either up or down on switches is the only way to affect the root bridge election.

HTH

Jon

igor_kiselev Wed, 01/23/2008 - 07:41

Best way is to set STP priority to 0.

This puts switch unconditionally into STP root for VLAN so no other switch can ever beat it.

If you take a step further to make your LAN more deterministic then force another switch to become a secondary

Example: make Switch-1 to become ROOT Bridge for Vlan 100 and 101

Switch-1#

spanning-tree vlan 100,101 priority 0

Switch-2#

spanning-tree vlan 100,101 priority 4096

This makes you LAN solid.

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