Spanning Tree not Blocking ports.

[Phobos1821]

I verified STP toplogy on my network. All switches know that the root is the designated core/distribution switch I selected. Here is the problem.

If I have a switch plug in two crossover cables to the distribution switch, it will not put one or the other in blocking state. Example.

Core/Distribution Switch
| |
Access Switch

Both links will be active. It will not block one of them, and loop the network. Now, If I connect two links together that is not the root. It puts in it blocking mode. Ex

Core/Distribution Switch
| |
Access -X- Access

How do I configure the switches to not put a port in blocking mode for redundant links that lead to the root switch?

 

[baddos]

Are you sure it's not blocking? Remember, that spanning tree will block in two ways (sending and receiving).

It might be using one link to send and the other to receive.

 

[gunthnp]

and it takes time to setup the blocks it has to know a block is needed 1st

gunthnp from

http://wwww.advancedigitalsolutions.com

 

[inetworku]

Consider configuring the ports as a port group and enable trunking if you have multiple vlans.

 

[lui3]

can you post the configuration of the first example for both switches noting interfaces involved. you might also want to try logging and using the

debug spanning-tree events
debug spanning-tree packet

do you have spanning-tree portfast turned on either of these ports?

 

[NetEng631]

Only one port will be put on blocking. Did you check both switches?

 

[Phobos1821]

Yes. I checked both switches. Neither are blocking, and a broadcast storm ensues afterwards.

One bit of information that may be important. One switch is the root, the other is the secondary root. I can connect any switch to any other switch and have it go into blocking. But when I connect the root switch & the secondary root switch, they both go active, and neither blocks.

 

[baddos]

Is portfast turned on those ports?

 

[Phobos1821]

Negative. The port goes through the 30 second listening/learning/forwarding state. It just does not turn amber on either end.

 

[lui3]

have you tried setting the priority of one of the ports that should be blocking to something higher than default like 1000?

Also when you setup the configuration that is causing these problems are these two switches independent of other switches. Ie are there any other switches/devices plugged in to them that would cause them to think there was a loop free network in place.

Is spanning tree enabled on both switches?

Are the spanning tree timers set at the default. It should be approximately 50 secs from interface up to forwarding without spanning tree enabled. Incorrect timers on each end can cause wierd problems.

Is spanning tree uplink fast enabled.

finally, why do you have one of the switches configured as a secondary root in an environment with only two switches?