STP - Port not in ALTN/BLK state

[Chris Thomas]

Hello!

I was wondering if someone could help me out here!

In the attached diagram (from packet tracer), why isn't Fa0/2 on Switch4 (marked with the blue arrow) not in ALTN/BLK state?

I have tested the same on real switches in the same configuration.

[Jon Marshall]

You don't say whether it is blocking on the real switches or not.

Difficult to say much from your brief description and diagram but the loop only has to be broken in one place so it could be any of the interconnections that is blocking.

Are you saying all of them are forwarding or are you querying why that specific port is not blocking ?

Jon

[Chris Thomas]

Hi Jon,

Thanks for having a look at this and sorry for the incomplete information.

It is not blocking on the real switches as well.

Interconnections are all forwarding on Packet Tracer. I believe they are all forwarding on the real switches as well. I will have a look when I get back to work and let you know. But I am sure they are all forwarding.

I have got this config on the ports on the root bridge:

udld port aggressive
spanning-tree guard root

I have got this config on the ports on the other switches:

spanning-tree guard loop
udld port aggressive

[Jon Marshall · Accepted solution]

Apologies for delay in getting back.

So any of the interconnecting ports could be blocking ie. as I said it only needs to block one port to break the loop. However by the sounds of it you are saying that all links are forwarding which shouldn't be happening.

What version of STP are you running and are there multiple vlans on the switches ?

Jon

[Chris Thomas]

Hi Jon,

You are right. One of the interconnecting ports is blocking. Should have checked that first. There are multiple VLANs on the switches (some not part of the trunk - not being used). Got confused with packet tracer showing all ports green rather than amber.  

Thanks for your help, Jon.

[Jon Marshall]

Chris

No problem with the help and thanks for letting me know it is all working.

Jon