[c-nsp] Spanning Tree help sought

[Justin M. Streiner]

On Thu, 15 Nov 2012, Christopher Gray wrote:

> I have four switches (A, B, C & D) linked in a loop comprising 1Gbps fibre.
> Switch A is connected to a primary WAN router while switch C is connected to
> the secondary WAN router - the two routers working in a simple HSRP
> fail-over set.  I want to ensure that this loop will survive the failure of
> any one link (e.g. if the link between A & B goes down, B will still be able
> to connect to the primary router via C & D.
>
> I currently have the STP path costs set to A=4, B=5, C=6 and D=7
>
> Question 1: Does this make sense?  Should the "root bridge" (using Wikipedia
> terminology) always be the one connected to the primary WAN router?  Does
> STP work well when the WAN uplink fails over to the secondary or doesn't it
> matter.

It's generally a good idea to set one switch to be your root bridge (STP 
priority of 0).  In your topology, the switch that is connected to your 
primary WAN router would make the most sense.  You can also set a higher 
STP priority like 4096 on the switch that connects to your backup WAN 
router.

You can set a higher spanning-tree link cost on the link between C and D, 
and you really wouldn't need to set link costs on the others.  When the 
other switches run their STP calculations, they'll see two paths to the 
root bridge, and one will have a higher path cost, so that should go into 
a Blocked state and be listed as an alternate path.

You didn't say if you were running PVST/PVST+/rPVST/MST or if you have VTP 
domains, etc.

> Question 2: Should I set all non-uplink (interswitch) ports as "disabled"?

I think this might be worded somewhat backwards.  Your inter-switch links 
would be your uplinks.  You need to run spanning-tree there.  Cisco 
generally doesn't allow spanning-tree to be disabled on specific ports. 
You can set them as access ports in a specific VLAN if needed, and run 
portfast.  DO NOT run portfast on trunk ports.  I think newer versions of 
IOS will yell at you if you try to do this, but in older versions, it was 
a great way to create bridging loops :(

jms