Like every other blogger, I get occasional e-mails from people fishing for free consulting or second opinion (note: asking a serious technical question is a totally different story; as many people know, I always try to reply and help) and as I’m totally overloaded with OpenFlow symposium and Net Field Day these days, I decided to share one of the better ones.
It all started pretty innocently:
I am happy to find you, because I have a question about number of VLANs in a small Data center. We have about 300 PCs and about 100 servers connected to 2 Cisco Catalyst 4507R and we decided to design our infrastructure as a collapsed core (no distribution layer). How many VLANs do you recommend for us? Is more VLANs good or no?
Trying to be at least marginally helpful, I replied with some generic recipes:
No specific recommendations. Use a different VLAN for every security zone, use firewalls or L3 switches with packet filters between them; don't have more than ~100 hosts/subnet.
However, that was not what he was looking for:
Thanks for your attention, but I’m confused because we want to implement our new network with about 15 vlans, but another guy gave us a design with about 70 VLANs. Which design is better? Is 70 VLAN design very complicated or not? Which one do you prefer?
I could use “it looks like a donut to me” answer that Jeremy Stretch once used (replacing donuts with Mikado Sticks), but still tried to tell him that it’s impossible to make a recommendation based on no input data. 70 VLANs for 100 servers does sound like an overkill, but maybe they’re running a virtualized environment with 1000+ virtual machines and there’s a good reason for numerous VLANs.
It’s totally impossible to tell you which design is better without having a detailed look into what your requirements are and the review of both designs, which would require a proper consultancy engagement.
... but all he needed was a simple answer:
As I said this is a general question about VLAN planning. If we can setup a network for example with 15 VLANs and can also design the same network with 70 VLANs, which one is better?
What shall I reply?
A) Small is beautiful, go with 15 VLANs.
B) Bigger is better, use 70 VLANs.
C) More VLANs will definitely increase your job security.
D) It depends.
F) All of the above.