Force Spanning-Tree root

[denial]

I have 4x3750 stacked , one master and 3 members (2 are modulars switches ).

On the two modular switches are directetly connected 12 x 9200 switches and 4x 3560 switches .(star topology)

Is it a good idea to force spanning-tree root the master 3750 because right now the elected root is an old 3560 so i have issues everyday.

There are no vlans so only vlan1 right now.

The 9200 switches are with rapid pvst but the 3750 is with PVST from the old admin including the 3560.

So my plan is to force the master 3750 to be root and after change to RSTP so i need your opinion if this is wise to do it.

spanning-tree vlan 1 priority 4096  (on master 3750)

and after the 3750 is master

spanning-tree mode rapid-pvst

will work ?

(i am thinking about downtime if its going to be more than 15 minutes or so).

Thank you !

[David Ruess]

Hello,

Generally in networks it is best practice to manually specify the root as a switch you deem appropriate to do so. As far as downtime there will be some as the SPT will need to reconverge and process the interface states (block, listening, learning, forwarding, etc). I dont believe the outage will last more than a couple of minutes, if that. When you configure the switch you want as root with the command you provided I would also configure Root Guard. This is an interface level command to block superior BPDUs. Remember, just becasue you configure a lower priority doesnt mean another switch with the same priority and a lower MAC cant overtake it and become root giving you unwanted results. 

Use the command  on all interfaces connecting to other switches.:

interface g0/1

spanning-tree rootguard

Hope that helps

-David

[denial]

I will add root guard just in case

Thank you for your reply !

[MHM Cisco World]

are you sure it RSTP not Rapid-PVST 
MHM

[denial]

After checking each switch only the 9200 switches has indeed rapid-pvst.

So only the core 3750 stack which is now the root and the old 3560 switches has pvst.

[MHM Cisco World]

Yes friend I know that the Cisco SW not run standard stp except mst.

And rapid pvst is compatible with pvst/pvst+.

For root it depends one your topology' but since we clear all SW run PVST (rapid and non) so you need to config root for each vlan.

Goodluck 

MHM

[Joseph W. Doherty]

Generally, a STP root is selected to be the switch that's the center of a star topology.

You didn't described the particular 3750 variants in your stack.  Believe the 3750E and 3750-X series supported rapid-PVSTP with their initial release.  Don't recall whether the 3750/3750G did, although recall later IOS variants for those also support rapid-PVSTP.

You can mix and match STP and rapid-STP (ditto Cisco's PVSTPs), but when possible, all switches should run a rapid variant.  (BTW, Cisco has multiple STP optional improvement features, many incorporated, no longer optional, in the rapid-STP standard.)

A root convergence with a STP variant, I recall, can take in excess of 30 seconds, but a "rapid" variant, is called "rapid" because it usually re-converges in a much less time, often under 10 seconds.  (If you want to understand why STP and rapid-STP take the time they do to converge, you might read this.)