[c-nsp] Redirects / hair-pinning traffic vs. performance

[Peter Rathlev]

On Thu, 2009-06-18 at 00:01 +0000, Peter Rathlev wrote: 
> I have the need to introduce some PBR to solve a hopefully temporary
> problem. Some of the traffic being routed will leave the same interface
> as it arrives on.
> 
> My worry is if this would have any performance impact the traffic
> arrives on and leaves from the same interface. I could imagine that some
> forwarding implementations might penalize this scenario.

Follow up: We've tested this and it works fine. It seems to have some
CPU impact when the unit policy routes, but not much. When pushing 100
mbps traffic through the CPU rises to ~25-30% for a few seconds (spent
on interrupt switching) and then falls down ~5% again.

This might be PBR-specific and have nothing to do with the traffic
arriving on and exiting the same interface though. We will be doing some
more (production) testing soon, with more flows and more bandwidth. I
can't see why the number of flows should matter since the 3560 AFAIK
just pushes packets, but I also can't see why the start of a TCP session
should matter. The "ip route-cache" hasn't been disabled of course; I
assume this would have a detrimental effect on performance.

Regards,
Peter