Virtual LAN

Everybody is talking about virtualization today. Virtual servers, virtual classes, virtual anything. There is one more thing: The virtual LAN.

It must have been around the end of the 1990s when they introduced VLAN. It was a big topic, because many Ethernet devices could not deal with it; it is not 100 % backward compatible with the good old LAN. The core problem was that the packet got 4 bytes longer, and that could break the MTU, so that the last bytes did not make it any more.

Admins can just run a physical LAN cable and then split it up into different “virtual” LAN, which behave pretty much like physical LAN. The main difference is that that physically available bandwidth now needs to be shared with the different LANs. This raises the question which packets get routed first, in case that not everything fits into the LAN. With the introduction of VLAN, you can also say bye-bye to the idea that there is a fixed bandwidth like 100 MB/sec. That’s why they introduced priority bits, so that you can have up to eight different priorities. And that’s something that is useful when it comes to VoIP; because you want to make sure that voice packets don’t get dropped when there is a lot of traffic.

This actually works quite well. I remember someone called me up on the VoIP phone, screaming “the LAN is down!!!” After a short moment, I asked “how can you talk to me if the LAN is down”, and then I realized that we actually were using the VLAN. It turned out that there was a loop in the LAN, but fortunately on a lower priority than the voice VLAN, so that even under such a bad situation the voice packets made it. Voice is more important than data.

VLAN typically have 12 bits, and 3 bits for the priority. There is a lot of stuff around those bits, for example the question who is allowed to be in a VLAN, what quality is allowed in which VLAN, and so on. Switches have to work together with the devices, and if you want to have a waterproof system, there is a lot of work to do to have everything set up properly. What most people do is to pick just a VLAN number like 128, and just hope that nobody abused the VLAN for bad stuff. In most LAN that is okay, the setup is easy and the gain from that is good.

However, talking about the cloud, VLAN are also available in the cloud. When service providers offer metropolitan Ethernet, 4096 VLAN are not enough anymore. So they came up with another VLAN type, which can have millions of different VLAN, so that the service provider can offer a couple of VLAN to their customer. IMHO that is very cool. The virtual LAN extended into the Internet. Rent a DHCP server in the cloud! Fortunately, there is a backward compatibility into the on-premises VLAN, so that existing devices don’t have to worry about it. And the quality of service can also be guaranteed, so that voice packets don’t drop.

Of course everything works fine with the m9.