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 My position is that with today's layer 2/3 switches in the core, the distribution layer is outdated for a network of this size.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

Re:It can be done.

[Kaamoss]

That's the real key, if the network can't be scalable then you're not setting your self up to do further work for the company. When you give someone a solution it should have the ability to grow with them. In the end it's almost allways cheaper to go with the more complete solution than the simple one.

Spoken like a true CCNA

[Schezar]

The very concept was never spoken of at university (Rochester Institute of Technology), nor has it ever come up in work (IBM).

Those three "layers" are abstractions, nothing more. The "distribution" layer is simply a term for traffic shaping and optimization. It's very useful in eliminating excess resource use on beleagured routers. Eliminating the layer is nothing more than simplifying your backbone architecture. There is no "layer" to eliminate except the theoretical one.

It always amazes me how Cisco-certified (not making any acusations here) network techs speak an entirely different language from university-educated ones. They talk about Cisco-specific concepts like they're set in stone universally, and use Cisco jargon for common and/or basic concepts.

There are other options besides Cisco, and not every network fits within the nomenclature of Cisco Jargon. You'd do yourself an immense favour to lean more about generic architecture concepts.

I don't want to sound mean, but a Cisco cert is about as useful as an MSCE.

It's usually not needed in a network of that size

[jsailor]

You didn't state the size of your network other than to say small-to-mid size, but most small to mid-size networks can run fine without a distribution layer. You're also correct that it is an artifact of 1996-1999 switching technology limitations and large vendor propaganda that sells ports. You need to be careful about:

1. how you link your merged core/distribution switches: if your access uplinks are layer 2, you then have to span VLAN across core/distribution switches. If you plan on having your access switches perform layer 3 routing look into the costs your vendor may charge for that functionality. Some charge as much as $10,000 for the license.

2. Be careful you grow your VLANs and spanning trees. Definitely use per-VLAN spanning trees. Also seriously consider rapid spanning tree or vendor specific hacks (uplinkfast, backbone fast, etc.)

3. Use server access switches. Seriously consider redundant control processors in these.

4. Seriously consider redundant control and switch fabrics for the the core/distribution switches. In the three-layer model, this was not as much of a requirement. Also seriously consider the failover time associated with the redundancy you bought. Times ranges from stateful/1 second failover to 90 second reboots to the redundant processor.

5. If you do layer 3 routing and the access layer be very careful with your routing protocol design and avoid black-holes. Run through all failure scenarios and make sure you're covered.

6. Consider where you want to perform filtering for security, QoS, etc. By eliminating the distribution layer, you're forcing this the access layer. (arguably it belongs there, but think about how many places you'll be configuring and monitoring)

7. Most importantly, consider the costs after you've considered the above. You may find out that you're not saving much. Most of my clients do save, but some find out that after they've added redundancy and possibly upgraded switch models they are close the same cost.

8. Consider your support group. What are they used to? Can they adapt? Can they handle the added functionality that's been pushed to the core or access switches.

Again, I have clients with 1500 nodes running fine with a combined core/distribution. I also have a clients with 200 nodes that mandated three layers. IMHO the break point is somewhere around 1000-1500. As always every place is different, be careful, plan and you'll be fine.