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New Member

spanning tree

If I want to make spanning tree block a port, Do I do it by upping the cost of the port, i.e i saw most of my ports saying cost of 19, so I set the one I wanted to block to say 25, is that correct ?

  • Other Network Infrastructure Subjects
4 REPLIES
Silver

Re: spanning tree

If you have 2 ports that will create a loop, setting the cost of one port higher than the other should cause STP to dynamically select the lower cost port for forwarding.

Hope this helps.

Steve

New Member

Re: spanning tree

Is there a priority on a port, or is the priority set for the whole switch, and would I just use the spanning-tree cost command ?

thanks

Silver

Re: spanning tree

Use the spanning-tree cost command on the port itself.

Hope this helps.

Steve

Re: spanning tree

There is a bridge priority, but it's use to influence the root election, which will have no obvious impact on which port is selected.

There is a priority on a port, but it can only be used to select a port from redundant links between two switches.

http://www.cisco.com/warp/public/473/15.html

The simplest is to indeed use the cost. But be careful that it's not only the cost of the port that is to be taken into account. Just increasing the cost of a port will not necessarily make it blocking. You have to compare in fact the bpdu the port is going to send and the bpdu it receives. Comparing the cost of these bpdus will probably be enough in a simple topology. Even if this document is not directly related to what you are looking for, it explains the port roles: http://www.cisco.com/warp/public/473/146.html#roles

What you are looking for is tuning the port cost so that it becomes alternate.

Regards,

Francois

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