[Beowulf] Spanning Tree Protocol and latency: allowing loops in switching networks for minimizing switch hops

[Gerry Creager]

On 2/23/10 1:23 PM, Rahul Nabar wrote:
> Over the years I have scrupulously adhered to the conventional wisdom
> that "spanning tree" is turned off on HPC switches. So that protocols
> don't time out in the time STP needs to acquire its model of network
> topology. But that does assume that there are no loops in the switch
> connectivity that can cause broadcast storms etc. Thereby constraining
> the network design to a loopless configuration. Most cases this is
> fine but.....
>
> In the interest of latency minimum switch hops make sense and for that
> loops might sometimes provide the best solution. Just wondering what
> people think. Does STP enabled have other drawbacks aside from the
> initial lag on port activation? Or maybe all the latency advantage is
> always wiped out if the STP being on itself has some massive overhead.
>
> Do you always configure switches to not have loops? Or are loops ok
> and then I turn STP ON but just use PortFast to get away with the best
> of both worlds.

It's my firm opinion that loops and STP are evil for HPC installations. 
Period.

gerry