Impact of CEF accounting

Jozef Janitor wrote a highly relevant comment to my post on CEF accounting: enabling it on a Catalyst switch drastically reduces its performance. The impact of CEF accounting (or other forwarding plane features) depends on switching implementation:

• There is almost no impact on single-CPU software platforms; the router has to perform CEF lookup anyway and increases the CEF accounting counters on-the-fly;
• Distributed software platforms are more complex, as the central CPU has to (at the very least) collect the switching statistics.
• The impact on hardware platforms is dependent on the layer 3 lookup implementation

In the case of a Catalyst 3550 switch, the L3 TCAM it uses to support IP routing, PBR and access-lists is so different from the CEF data structures that IOS has to fall back to software L3 switching to collect CEF accounting data.