Spanning Tree

[londint]

Please find atached the spanning tree config.

I can not seem to see where it should be blocking. Please how can I check this or is there something wrong with the spanning tree on the network?

Thanks

[sourabhagarwal]

you have a stack of 3550 - 3 Nos switches with redundant connectivity to 4500 and 4006 switch.

depending on port cost or vlan cost set on the interface, root bridge, designated port have been elected.

you can check all the parameters using show spanning-tree command.

[amit-singh]

Hi ADETUTU,

In the above topology attached there is a redendant connection only in between the 4500 sereis switches. There is no other loop in the network topology. In this case one of the link between 4500 will go in blocking mode. This is how it will be decided:

1. The root path cost will be taken into account.If the links are equal then it will be same.

2. Neighbor with the lowest mac-address will be decided, this will be same I guess in this case.

3. The lowest port ID will be taken into account i.e 1/2 or 3/4. The port with the lowest poert id will be the forwarding in this case.

HTH,Please rate if it does.

-amit singh

[londint]

Thanks

But when I do a show spanning-tree, i dont see any of the interfaces in blocking mode?

Shouldn't blocking mode show up on one of these interfaces?

Thanks

[kamalnathsingh]

you should see blcoking port state on one of the 4500 switches that too if you span the vlan on both ports

[glen.grant · Accepted solution]

If the connection between the 4500's is an etherchannel (which it probably is or should be if it is not) there will be no blockedports because in spanning tree a etherchannel is considered a single attached port . On a IOS switch you can use the "show spanning-tree blockedports" command to look for any blocked ports on the switch . In your setup there is no inherent loop in the design so nothing would be in a blocked state . If you had a link between your 4006 and the 4500 you would then have a built in loop and you would find a blocked port for vlans traveling over that link .

[smurtaza81pk]

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