Confused about a little concept on VLANS? :(

Llukman1

I know that a router is used to send data to different VLANS.

But lets say that we have a group of people in the engineering department and they span over multiple locations and they are in the engineering VLAN. How would they connect to each other I know they are in one broadcast domain and one subnet but I am confused when it says a router is needed to connect different VLANs and not a same VLAN that is spanned across multiple physical locations?


I'd really appreciated if someone can clear this up for me thanks.

clarson

read about trunking

carterw65

If I get what you are saying, there are several engineering offices that are not in the same location. They all use the same IP subnet; they all are on the same VLAN. Several switches TRUNKED together can pass that VLAN between sites.

As clarson said, read up on trunking.

If the sites need to traverse the WAN, they will probably need to go through routers. In this case, it is probably better for the various offices to be on different subnets.

GDaines

A router is required to route traffic between different physical locations, and also to route traffic between different VLANs locally.

It would be poor design (and probably hard to implement) having physically separated users sharing the same IP range/subnet/VLAN. Much better to have each location in it's own subnet (and possibly VLAN) and have your routers route between locations/subnets/VLANs.

OctalDump

What you are describing is end-to-end VLANs. This is typically in a large switched topology - like multiple buildings spread across a campus site -, and not using WAN links. The actual routing is generally done at the distribution layer in switches with layer 3 functionality.

Cisco recommends against end-to-end VLANs in most cases, and suggests Local VLANs, and then to use the better features of layer 3 to provide the features that end to end VLANs were typically intended for.

I'm guessing that maybe so far you've only done "router on a stick" set up for VLANs. This isn't so common, as you can get far better performance using layer 3 switches that basically are "routing" at switching speeds. So a switch might have multiple VLANs, and be able to send traffic between these VLANS (most of the time) as if they were all the same VLAN. This is usually seen as a function of the distribution layer (so a connection from one VLAN to another might still go through 2 devices, but faster than a pure router), but as the price of these layer 3 switches comes down, there is a trend to move the routing functions more to the access layer.

But this isn't one size fits all stuff. There can still be use cases for end-to-end VLANs. Maybe, for instance, in a distributed virtualised infrastructures where you might move a VM to a (physically) remote site (maybe for disaster recovery) and need it to have the same access to the same things on VLANs. But that's why we have network engineers and architects - to figure that stuff out.

There's a short article about this and more here.

Llukman1

So what is the the distance limit for VLANS? even for the same VLAN?

networker050184

There is not distance limit per se. It is recommended to keep your L2 domains as small as possible though.

james43026

I'd just like to clarify a couple of concepts here. And end to end VLAN, is not a VLAN that spans routers. An end to end VLAN is simply a VLAN that spans multiple switches VIA a trunk link. So, if you wanted your VLANS to span across routers, you would need to use integrated routing and bridging on your routers. I think everyone that posted before has done a wonderful job, I just wanted to throw that in there, to clarify the fact that you can have a VLAN span across a router. As others have mentioned, you generally wouldn't want a setup with the same VLANS spanning across WAN links, it just introduces lots of latency most of the time, and usually isn't needed.