[j-nsp] Stackable switches, looping stacking ports

[Chuck Anderson]

On Tue, Apr 09, 2013 at 11:48:36AM -0700, joel jaeggli wrote:
> On 4/9/13 11:15 AM, Tom Storey wrote:
> >Hey all.
> >
> >A colleague of mine tells me that, if you have a single stackable switch
> >(not in a stack obviously) and do not loop the two stacking ports on the
> >back using the stacking cable that comes in the box, then you reduce the
> >effective throughput of the switch.
>
> The ex4200's asic has a capacity of 136Gb/s from the front panel
> ports which is 100% of line rate across all ports. I don't imagine
> connecting the asic back to itself is that useful or usable
> topology.

It does make a positive difference to loop back the stack cable on a
standalone unit.  See this diagram:

http://blog.cochard.me/2010/08/juniper-ex-4200-internal-pfe-routing-in.html

Connecting from ports 0-23 to 24-47 has to normally cross 3 internal
PFEs.  If you connect VCP-0 to VCP-1, then it only has to cross 2
PFEs.