visitor68 wrote:
Hello - would like feedback from folks regarding their experience with total number of VLANs in an enterprise data center. Of course, this number varies tremendously, but I can say that in over 10 years of consulting, I have never run into an enterprise data center (and I have worked on very large environments in Fortune 500 type companies), with more than a couple hundred VLANs tops.
I raise this question because of a discussion recently had about VLAN limitations on switches. Most switches support 4K VLANs, which is an incredibly large number and a requirement I have never run into. So, this limitation really involves the size of a single L2 domain, regardless of how many there are in total in a data center.
The bottom line is this:
The limitation of the number of VLANs that a switch will support is only applicable within a L2 domain. Once the L3 boundary is reached, all bets are off - a new domain begins and VLAN numbers can be repeated. So, BARRING Service Provider and IaaS-provider networks, how big is a single L2 domain really going to be such that one would need thousands of VLANs? True, with TRILL, Fabric Path, etc, the L2 domain can and does span larger than it did before, but 4K VLANs is still huge for the average enterprise data center, even with virtualization.
For example, the IETF standard for TRILL does not recommend L2 environments larger than 1,000 physical servers anyway. So, if I did have such a large L2 domain, and if each one of those servers had a 30:1 VM-to-physical machine ratio, that would give 30,000 servers total. Assuming a 1:1 relationship between subnet and VLAN (which is typical), one thousand /27 VLANs would handle all those servers. Now, how many times have you seen a data center where all the subnets are that small??? The reality is that a few hundred VLANs would cover that.
THOUGHTS? EXPERIENCES?
I always try (and not always successfully) to keep server subnets having less than 100 devices (especially if they're Windows servers) simply to limit layer 3 broadcast traffic. So the layer 2 VLAN's are corrospondingly smaller (and more prevalent).
Saying that, I've *never* built a data center switch with more than 1000 VLAN's configured in it. I think the 4k limit in most Cisco switches is simply because it's a nice, neat, binary number boundary (4096) more than because anyone expects the switch to actually *have* that many VLAN's configured. I've gone close (I remember one which had about 900 VLAN's on it), but never over that.
Although, I have to admit I've never worked in a really big cloud environment (like Akamai or Amazon), so it's probably the requirements for those be vastly different from my experience - although I can't imagine a scenario like you postulated with 30000 VM's!
Cheers
