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Beginner

Command to block a port in Vlan

I have 3 switches interconnecting to each other. A PC in switch A wishes to ping to PC on Switch C. There are 2 paths leading to it, what command can i use to block one of the port so that there is only one path?

Is it setting one of the switch as root? Or is it something to do with portfast?

Note:Pulling out the physical link is not an option.

5 REPLIES 5
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Collaborator

Hi,

By default, you have Spanning tree enabled on the switches and it should automatically block the redundent port. Use the command " show spanning-tree brief " command to check it is enable on the switch.

Based on the lowest bridge ID( Lowest MAC + Priority ) the switches will automatically calculate the root and once agreed upon that the root switch blocks the redundent path based on some calculation. If you want you can force a particular switch to be root switch by lowering dwon the priority which is by default 32768 for the switches. You can use command " spanning-tree priority in the config mode.

Portfast puts the port into forwarding state directly from blocking and thus skipping the intermediate states. It is generally used on host ports. Never enable portfast on the ports connecting to switches as it might results in a loop.

HTH,

-amit singh

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Beginner

Hi,

Believe STP is active on all the three switches. If yes, STP will take care of blocking one of the paths by default.

You need to think about tuning the STP parameters only if you want to manipulate the path selection.

Regards,

Shijo George.

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Yes i wish to manipulate the path selection. How do i do that? Any links on cisco? i tried looking but i can't find anything useful.

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Thanks, Shijo George, think this will solve my problem.