With all the talk about Layer 2 Multipath (L2MP) designs going on, it might be worth highlighing a fundamental change in the way people approach high density network design. It’s possible that this point has been lost somewhere in the discussion of protocols.
The Spanning Tree Protocol blocks looped paths, and in a typical network this means that bandwidth is unevenly distributed. Of course, we might use PVST or MST to provide a rough sharing of load by splitting the spanning tree preferences for different VLANs, but the design still doesn’t change overall. I’ve talked about this in East West and North South networks designs. The basic point is that there is a LOT of bandwidth that is never evenly utilised – and that means wasted power, space and cooling (which costs more than the equipment itself).
For many years, Campus and Data Centre design has focused on the three layer hierarchy of Access/Distribution/Core. It looks something like this:
There are Core Switches at the top, Distribution in the middle and Access switches at the edge. Ok, it’s not perfect but you get the idea.
When you move to a Layer 2 MultiPath network, the purpose is to remove the unused switches in your network and increase the utilisation of all your assets. This can be most effectively done by getting rid of the distribution layer. Of course, you could use them as Access Switches like this:
In this rather simple (and overstated) example, you have moved a network of twelve switches with only FOUR usuable devices for connecting servers, desktops and printer into a network of twelve switches with EIGHT usable devices for connectivity.
That’s just one impact of L2MP in network design. Here some more:
- We don’t have to manually configure STP protocols for redundant paths, they are built into the protocol.
- We need less equipment to do the same job,
- We still get a lot more bandwidth at the end of the day.
- The newer protocols take advantage of faster hardware (which STP cannot really do) and can converge around a failure in subsecond times.
- better OAM support for features such as L2 Traceroute because they have been “baked in”
This all mean that there is significant momentum to move to these protocols, not only in the data center but also in the campus. It’s the end of the Distribution layer.