Is the Distribution Layer Still Needed?

arnie_apesacrappin wonders: "I'm in the process of designing the network for a new building in what I would consider a small to medium sized company. It is on the scale of tens of access layer switches, not hundreds. There is a ongoing argument about the need for a distribution layer. My position is that with today's layer 2/3 switches in the core, the distribution layer is outdated for a network of this size. The layer 2/3 core can provide all the aggregation services of the old distribution layer and the routing/filtering functionality of the core with better price and performance. My opponents can only argue that having a distribution layer is the standard. So, are there good reasons for having a distribution layer in a small to medium network? If you were going to argue against the distribution layer, what points would you make?"

Comments

Depends on how you define "distribution layer"

[dtfinch]

We just have a stack of 24 port gigabit switches. 4 ports on each switch is set up as a trunk to connect them together, effectively turning them into one giant, fast, very cheap gigabit switch. Looking at the Cisco diagram, this might be considered our distribution layer.

We normally have one port on the switches for each system, with the exception that in some locations we have smaller switches to allow them to share a line, so that we don't have to rewire the building. We also use some smaller switches as repeaters to parts of the building too far away to connect directly to the central switches. Those small switches outside of the server room, along with all our servers and systems, might be considered our access layer.

Then we have a tiny linksys router, intended for home use, connecting the entire building to the internet. I know, it sounds scary, and unprofessional, but it seems plenty capable of filling the bandwidth of a T1 and tracking as many simultaneous tcp connections as we use. We'll consider replacing it at the first sign of trouble. I guess this is our core layer.

I suppose that whatever you use at the top level to connect your systems to create a single network can be called your distribution layer. The switches may get cheaper over the years, but it's the same thing. If you just have a chain of 8 port switches running around the building, then your distribution layer is a bunch of 8 port switches.

However you design your distribution and access layers, your main goals should probably be to minimize line problems (mostly due to distance) and avoid bottlenecks. You seem concerned about price, so if you decide to use 100mbit switches to keep the price down, I recommend that get the kind that have gigabit uplinks and plug them into a gigabit switch, and plug your servers into the gigabit switch as well. Otherwise, your effective bandwidth will be 100mbit total rather than 100mbit per user.