Manually selecting root bridge

Unanswered Question
Apr 14th, 2007

for exam purpose:

If i have to manually configure an stp root bridge and if the network has distribution switches but it also has access switch which one should i select if the access switch seems to be in the center of the network (access switch has a link to each one of the other switches)?

I have this problem too.
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desai.jaideep Sat, 04/14/2007 - 09:13


The default root bridge id is 32768. and the default spanning tree in cisco is pvst.

You can manually change that to a lower number by" spanning-tree vlan priority "

The root bridge in a network is the with lowest bridge id.

Hope that helps.



amermomani Sat, 04/14/2007 - 09:28

I know how to force it to be a root bridge. but what i need to know is which switch should i select to be the root if i need to manually force it. in the cisco press book it says it has to be in the center of the network, but it also said that distribution switch is better to be root than access switch ?

desai.jaideep Sat, 04/14/2007 - 09:37


Good that you know that.

Generally you deploy two dist. switches in the design which are conneted to the access switches. Say DistA and DistB. You connect the access switches Ac21 and acc2 such that one trunk from acc1 is connected to distA and second to dist2.

There is a trunk between distA and distB.

Hence you configure distA as bid 4096 and distB as 8192. So whenever, the distA fails , then distB takes over, not any other access switches.

Hope I have answered your query.



amermomani Sun, 04/15/2007 - 19:58


Thank for the reply. please consider this specific example:

A network consists of 4 switches (2 distribution switches and 2 access switches) connected as follows:

The 2 dist switches and one of the access switches are connected to each other to form a triangle. And then the 4th access switch is connectd to the access switch in the triangle.

What would be the best option for the root bridge?

Is it one of the distr switches because they are "distribution" ?

Or is it the one access switch that has direct connection to all the other switches because it is in the heart of the network ?


desai.jaideep Mon, 04/16/2007 - 10:09


Pls view the attachment. If possible give urs scenerio in drawing.

Its just not matter of access and dist. You manipulate just b'coz u want that switch to be main centre or secondary of the network.

Generally, these are dist. switches only.

Pls see the attachment. If I am understanding u then it will solve ur problem.




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