Cisco Support Community
cancel
Showing results for 
Search instead for 
Did you mean: 
New Member

STP question

Hi,

my question is in order to clarify the standard STP functionality.

Scenario:

three switches connected in a

ring and so forming a loop;

only one of the six ports linking the switches has STP enabled (other ports have STP disabled).

Question: is the switch able to detect the loop and so blocking the STP enabled port ?

If no ...in this scenario how many of the ports must be STP enabled ?

Thanks in advance.

Pierluigi

1 ACCEPTED SOLUTION

Accepted Solutions

Re: STP question

I agree, it all comes down to what you mean by disabling STP. If the switches that have STP disabled are flooding BPDUs like user traffic (dumb switches that act like hubs do that, PVST+ switches with "no spanning-tree vlan X" configured attempt to do that also), then the unique bridge running STP should be able to see its own BPDUs coming back at itself around the ring and prevent the loop. Note that you could still have temporary loops when a failed link is recovered, and that convergence will be slow (based on timers).

Regards,

Francois

4 REPLIES

Re: STP question

Hi,

Only ports partecipating in spanning tree move between different states:blocking, listening, learning and forwarding.

You have to enable spanning tree on 6 uplink ports, in order to detect loop, and then have a topology loop-free.

For more information look at: http://www.cisco.com/en/US/tech/tk389/tk621/technologies_configuration_example09186a008009467c.shtml

and

http://www.cisco.com/en/US/docs/switches/lan/catalyst2950/software/release/12.1_22_ea5/configuration/guide/swstp.html

I hope this helps.

Best regards.

Massimiliano.

Re: STP question

Yes STP is active & thats the reason why some ports are blocked. However, I'm not sure of the statement, only 1 port is active out of six ports. You may check this by "sh spanning-tree".

Re: STP question

If you have disactivated STP on the ports by using bdpufilter, then you will have a loop. The BPDU will not make it round the loop, so the loop will not be detected.

Kevin Dorrell

Luxembourg

Re: STP question

I agree, it all comes down to what you mean by disabling STP. If the switches that have STP disabled are flooding BPDUs like user traffic (dumb switches that act like hubs do that, PVST+ switches with "no spanning-tree vlan X" configured attempt to do that also), then the unique bridge running STP should be able to see its own BPDUs coming back at itself around the ring and prevent the loop. Note that you could still have temporary loops when a failed link is recovered, and that convergence will be slow (based on timers).

Regards,

Francois

258
Views
15
Helpful
4
Replies
CreatePlease to create content