We're currently running a netwok of 800+ PCs and 20+ servers, the network infrastucture is along the lines of Core Switch 10Gb-> Area Switch 2GB-> Local Switch 1GB-> Desktop. All running 3Com equipment(1).
We have 3 Area switches for four areas (A, B ,C, D is merged with the core), each area switch will have between 10 and 20 local switches connected to these. There is also a backup core switch, lesser powered but connected as the main core switch is.
We also have an IP phone system. The Computers/servers and swicthes are on a 10.x ip range, the phones on a 192.168.x range. Computers generally don't have to talk to each other except in computer labs, but they do need to be able to talk to most of our servers (AD, DNS, Exchange, File storage etc.)
When we set up, it was decided that we were to have 3 VLANs, one for Switches & Computers, one for Phones and one for server replication (this was against the 3Com engineers advice). The network has been stable and working since this point(2), but we've now started to upgrade to SAN and Virtualisation environement. Now splitting this new infrastructure into seperate VLANs makes sense, and re visting how our VLANS are set up seems sensible.
It's now being proposed that VLANs should be set up in a room by room basis, ie a computer lab with 5+ PCs should be it's own VLAN, but if we follow this model we'll be looking at least at 25 "new" VLANS, plus the VLANS for SAN/Virtual servers. Which seems to me will add an excessive amount of administration, although I am quite happly to be proved wrong.
What would be best practice seem to suggest? Is there a certain number of PCs that are advisable not to go over/below in a VLAN.
(1) The 3Com switches (3870 & 8800) route between VLANs differently to how some others do it, it doesn't require a seperate router as they are layer3.
(2) We do sometimes get high discard rates, or STP changes, and at time 3Com Network director report that switches are underload and slow to respond to pings, or a failed switch managing to take down the network (all phone & computer VLANS!, once, no idea why)