Q: What is the client's network topology?
Network topology is really the core of your design. It's the shape of the network.
If we follow from the network jack to the deepest part of the network, typically you have a jack or multiple jacks in every office. That cord or bundle runs to a wiring closet known as the IDF, or intermediate distribution frame. The placement of your IDF is important because Ethernet has length limits, so usually every jack has to be within 100 meters of an IDF. For larger buildings you might need multiple IDFs, then you join the IDFs by linking them together, usually to an MDF -- a main distribution frame. Often you have one MDF per building. The MDF connects to the IDF over fibre because fibre has longer length limits. If you have multiple buildings, then you have to figure out how to connect the MDFs, and this all creates a large tree structure, which is the topology of your LAN.
That's the physical topology, how the wires and other pieces of physical equipment are connected. The other topology you have to consider is the logical topology. It's common to build an overlay network over the physical topology, and this overlay network might be segregated into VLANs or sometimes even different protocols, but usually you have different VLANs that overlay your physical protocols. If in a LAN your jacks might be assigned to a number of virtual LANs -- so you might have the engineering VLAN, the sales VLAN, the guest VLAN and a VLAN for your server -- they're all going over the same trunk lines, but logically the routers keep the packets separate. These VLANs have to be routed together, and this is your logical topology.
I can't stress enough to not go crazy by allocating a million little VLANs. I find fewer is better, simpler is better.
It's also important to keep your VLAN IDs consistent, so if VLAN 100 is your engineering VLAN, you should probably use that VLAN ID in every building so that your engineers don't get confused. While you'll often use the same VLAN identifier in multiple buildings, I've never seen a VLAN spanning multiple sites [where the company] didn't later regret it. Sometimes I wonder why vendors provide this feature.
A VLAN is commonly a broadcast domain, and broadcasts need to travel within a certain amount of latency, so having a VLAN that is bridged between New York and Los Angeles usually doesn't work very well because of latency problems. So while VLAN 500 might exist in New York and in Los Angeles, those should be considered separate networks and routed instead of switched.
This was first published in March 2008