[c-nsp] Maximum spannig tree instances

[Matt Buford]

On Wed, Jul 15, 2009 at 8:30 AM, Tomas Daniska <tomas at soitron.com> wrote:

> > On Tue, Jul 14, 2009 at 3:45 AM, <A.L.M.Buxey at lboro.ac.uk> wrote:
> > > things go wonky when you have more than 1800 virtualports per slot
> > > (which you didnt quite reach) (1200 on older eg 100mbit blades)
> > > with 13,000 in total (PVST+), 10,000 in total (RPVST+)
> > >
>
> As a matter of coincidence, I've been in talks recently with our local
> Cisco SEs for some 6k5/3750E design, mostly discussing RSTP vs MST. I
> have asked about the 1800 virtual ports per blade limit and they say
> this only applies to 61xx and 63xx cards - the 65xx and 67xx have no
> such limit. There is a ddts that a message errorneously warning of
> exceeding 1800 virtual ports on a 67xx is removed since SXI (or SXI1 it
> was).
>

Strange, as I have had a number of discussions with my SEs about this issue
and they have never mentioned the limit not applying to certain cards.  This
is the number 1 hardware limitation that affects my design and hardware
purchasing.  The docs seem to clearly state that the limits are per-slot and
do not mention model numbers. However, I can confirm that I have greatly
exceeded this specification for years now without serious wonkyness.  I
have WS-X6516A-GBIC cards running as high as 6,400 virtual port instances.
 I do notice RSTP isn't quite as rapid as it used to be though.  If all STP
instances reconverge at the same time, it might take a second or two.  If
only one VLAN reconverges, it is still sub-second.

Of course, I don't want to just let it grow because I certainly don't want
unexpected future wonkyness to show up.  So, I have capped the size.

These days, my network is chopped up into small VLAN regions where any VLAN
within the region is present on all switches within the region, but is not
available outside of the region.  My data center people would prefer to be
able to physically put servers anywhere in the data center and have all
VLANs available throughout the data center (only requiring them to worry
about putting it in the correct building).  I could do this for them, if not
for the virtual port slot limitations.  Instead, they have to live with
VLANs only being available within the "region" that is capped to a max of a
certain number of racks (usually we attempt to map VLAN regions to physical
rooms).

My Cisco team suggested the Nexus as a potential way to alleviate my 6500
virtual port limitation pain.  I asked for specifics on the STP limits of
the Nexus, and they are significantly lower than the 6500.  Oops.