Spanning tree - what would happen if ?

Unanswered Question
Jun 26th, 2009

Hi there

If I had 2 distribution switches, then an access layer switch dual homed to both dist switches, running STP, But if I effectivley turned off STP on the link between the dist switches, what would happen on the access switch ? would it still work and block as per normal ?

I have this problem too.
0 votes
  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Giuseppe Larosa Fri, 06/26/2009 - 06:03

Hello Carl,

>> But if I effectivley turned off STP on the link between the dist switches,

do it only if between the two distribution switches there is a L3 path otherwise you are in trouble.

Hope to help


glen.grant Fri, 06/26/2009 - 06:09

You would then likely not have anything blocking on the access layer as you have eliminated the inherent loop in the network.No different than say having 2 uplinks and then a routed crosslink .

carl_townshend Fri, 06/26/2009 - 06:34

hi there

thanks for that, one more question, can you still use HSRP on the dist routers for the access layer clients if there is a layer 3 link between them ?

carl_townshend Mon, 06/29/2009 - 03:02

Hi Glen

Does this mean that all the hsrp traffic will be flowing through the access to dist uplinks?

cbeswick Mon, 06/29/2009 - 05:00

Correct - you need a layer 2 link between the two SVIs participating in HSRP, so the HSRP hellos would be flowing through the access switch to maintain adjacency.

I have never deployed this type of design, we tend to stick with the traditional looped topology between access and distribution, using Rapid-PVST+ to maintain a loop free topology.

If you have many access switches using the same Vlan you may start to complicate design / troubleshooting by removing the layer 2 link between the distribution switches.


This Discussion