[c-nsp] Spanning Tree help sought

[Christopher Gray]

Justin,

>> 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.

Justin M. Streiner <streiner at cluebyfour.org> responded

> 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.

RSTP is running on A & B but C&D are too old.  I'm about to upgrade C & D
and they will run RSTP.
No MST or VTP domains.  We are looking to set up VLANs and separate out such
as video (e.g. video conferencing) and low-priority back-up traffic in due
course.

>> 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 :(

I will run portfast on all non-fibre ports (Fibre is only used for the
inter-switch links).
> jms

Many thanks for the above.  Chris