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New Member

STP advice/clarification

Hi all

Ive got a question re STP, ive attached a diagram of the current setup and along with a text file which shows the STP output for VLAN 40 from SW3 and SW4

My questions are:

1) for all vlans in the netwrk SW1 is the root, however only for VLAN 40 SW3 is set to be the root. WHy? and how can i change this so only SW1 is root for all

2) looking at the diagram, there are many links from SW1 to SW2 and one trunk(which allows all vlans on it) so if i were to shutdown interfaces G0/8, G0/12, G0/20,on SW2 would this make it more effecient and prevent loops?

3) looking at the output in the text file and the diagram, is there a chance of a loop here? if say host 3 or 4 were to send data to host 1 or 2. as both the trunk ports for switch 4 are set to FWD. Shouldnt one be in BLK mode?

The reason this question has arose is recently im noticing alot of events (flagged up by Whats Up Pro) saying that the device/port is down, then within a min its back up again, this is not only for servers but switches as wel. The funny thing is that although the status is down (as per Whats up) you can still telnet into the device/port in question.

Thanks for your help.



Re: STP advice/clarification

On SW1 give the following command:

spanning-tree vlan 40 root primary

This would make SW1 the root switch for vlan 40.

Here is an URL to understand STP.

New Member

Re: STP advice/clarification

hi thanks for that.

having revisited the configs on all switches, i can now also understand why SW3 is the root for vlan 40, this is because VLAN 40 is only configured on SW3 & SW4. SW1 & 2 dont know about that vlan. I appreciate your help.

Q 2 & 3 still need help with.

Re: STP advice/clarification

Wouldnt be able to tell you until the attachments are posted. Or am i the only one who is not seeing the attachments ?

New Member

Re: STP advice/clarification

sorry.. here are the attachments

New Member

Re: STP advice/clarification

Anybody got any thoughts on this???

Hall of Fame Super Blue

Re: STP advice/clarification

2) Not sure why you have this many links. If you want redundancy you could use some or all of the ports gi0/8, gi0/12, gi0/22 and add them to port gi0/17 as an etherchannel trunk link. This is what i would do.

Then STP will see on logical link.

Even if you did do this or you shut down the ports you still have a loop because of your topology.

3) Both ports on SW4 may be forwarding but somewhere within your topology one of the ports will be blocking - it has to be.

Actually with your current setup more than one port will be blocking because of all the additional interconnections between SW1 & SW2. If you implement suggestions in 2) then it will be a lot easier to see and one port in your square will still be blocked.