The Trouble with CEF and Layer 3 Switching

The Trouble with CEF and Layer 3 Switching
It may seem wrong to refer to CEF as layer 3 switching, but layer 3 switching is so poorly
defined that it is easy to see how someone could become confused. Remember, though, that the
CEF process has much in common with the way we have previously described layer 3 switching.
The routing decision is taken for the first packet at the route processor, and the frame address
is rewritten to allow the packet to be properly forwarded. This is true in both CEF and layer 3
switching. It is also true to say that subsequent frames are forwarded (and the MAC address
rewritten) according to cached information, and that they never get to the route processor.
I guess that, arguably, the story of layer 3 switching began a long time ago with the introduction
of fast switching, and it has just progressed to caches further away from the route
processor. Sometimes the cache is moved all the way to a separate box, namely the switch.
But once modern switches incorporate IOS and the associated routing capability, the cache
would naturally move back into the same housing.
The point here is that layer 3 switching really is vendor-speak, and in an ideal world we would
not even have a chapter with this title—we would be calling it something like “How to Speed Up the
Routing Process.” The problem is that Cisco is under pressure from other vendors who call their
offerings layer 3 switching, and so the myth continues to be propagated. And as long as Cisco exams
are going to have questions on this topic, we have to use the same language. The term means little
enough, but once you start building boxes with IOS that have the capability to perform both switching
and routing, all this business of switch-router intercommunications disappears inside the proprietary
architecture, and you can’t see it anymore. So the early switches didn’t do layer 3 switching
at all, the (dying) range of CatOS-based switches do it in a complex fashion (as in the first part of
this chapter), and the new IOS-based switches do it wonderfully, but it’s a secret!
578 Chapter 18  Multilayer Switching (MLS)
CEF, then, is not a first-generation attempt to speed up the forwarding process, but is the
most recent mechanism to be tested. I think that it would assist us in placing CEF in the proper
context if we looked at how we got here, so I propose to first consider the actual forwarding
mechanisms that have traditionally been used by Cisco routers.