Spanning Tree help

Unanswered Question
Sep 12th, 2008


I have a Cisco 2950 switch and I wondered how I can create a loopback issues then setup a spanning tree and prove that it fixes this issue?


I have this problem too.
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Giuseppe Larosa Sat, 09/13/2008 - 02:19

Hello James,

STP should be enabled by default in PVST+ mode on the 2950.

If you want to see why is so uesful you can:

a) disable the stp instance for a vlan X

no spanning-tree vlan X

b) use a cross-over cable to connect two switch ports in vlan X

c) have a pc connected to a third port in vlan X that tries to make an ARP request = L2 broadcast

I suggest X <> 1.

Then you issue again

spanning-tree vlan X

one of the two ports connected with the cross-over cable should go in blocking for vlan X.

You may need to shut one of the two ports cross-connected, if the switch becomes unmanageble you need to unplug the cross-over cable.

You can do this only if the 2950 is part of a lab setup, don't do this in a production network.

Hope to help


glen.grant Sun, 09/14/2008 - 02:17

If you have 3 spare (lab) switches just hook them up in the classic triangle where there are all hooked togther with a single patch, this is a classic redundancy setup that people use , with it is a built in loop that STP will prevent the loop on , somewhere within that setup you will see a blocked port thus eliminating the loop.

Istvan_Rabai Sun, 09/14/2008 - 03:12

Hi James,

If you use the "spanning-tree bpdufilter enable" interface command on the trunk ports connecting the 2950 switches you will effectively disable spanning-tree on those ports.

As result of this, your switches will not be protected against the loop. Just put any broadcast frame into an access port and you will see the trunk ports blinking continuously, indicating the looped traffic.

If you issue the "show spanning-tree" command, you will not see blocked ports.

Probably you will have a difficulty of access to the console port as well, but you can break the loop by disconnecting a trunk cable.

Now if you issue the "spanning-tree bpdufilter disable" on the trunk ports, you will see that the spanning-tree algorithm works again and you will have one or more blocked ports.




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