Spanning tree

Unanswered Question
May 19th, 2007

if my two switchs connected with each other using two link... now one switch A is root bridge and other switch B is the non-root and had one port in blocking state and other is in non blocking.... now i want to change the ports status.... which paramter will be consider first... .port priority or cost?

I have this problem too.
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Paolo Bevilacqua Sat, 05/19/2007 - 23:31

Cost would be the parameter to change.

You may want to send further similar questions to "Getting started with LAN" forum.

royalblues Sun, 05/20/2007 - 10:29

When creating a loop-free logical topology, Spanning Tree always uses the same

four-step decision sequence:

Step 1. Lowest Root BID

Step 2. Lowest Path Cost to Root Bridge

Step 3. Lowest Sender BID

Step 4. Lowest Port ID

In your scenario, the first three parameters will be same and hence you need to manipulate the port ID.

Port ID is a combination of port priority and port number. Since port number cannot be chnaged, you need to manipulate priority

HTH, rate if it does


ankbhasi Sun, 05/20/2007 - 10:36

Hi Friend,

You can play around with both PORT COST as well as PORT PRIORITY.

If you manipulate port cost do it on the upstream switchport as port cost is carried in BPDU from root switch to the downstream switch which build the path cost from that switch to the root switch so change it on upstream switch.

If you are playing around with PORT Priority you have to do it on the downstream switch port as it takes into affect on the switch where it is changed.

So in your case when you have 2 parallel connections between switch A and switch B and switch A is working as a root switch and you want to play with POR COST change it on Switch A and if you wnat to play with port priority do it on the switch B.



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