How does CPU affect performance

[carl_townshend]

Hi All

Can anyone answer me a question around CPU on routers

If I have a 2811 for example running a number of ipsec tunnels with qos on them, and the router is running at 98% cpu.

what is the effect on this on traffic going through the router? will it drop? queue etc ?

cheers

[Mark Malone]

From my own experience router could eventually fall over if its constant at that , reboot itself or worse
I had a couple of 800s like that running for a while before I got chance to replace one the board failed most likely we were hammering the router too much its was always that high under specked from install, we also kept seeing dropped packets on the circuits due to excessive cpu was effecting performance on some apps that were sensitive , if its a bug though and its cosmetic it may be causing any issue but if your hardware is running that hot constantly its going to run into issues at some stage

[carl_townshend]

Hi

So, can you clarify if you think that the router would drop packets etc?

For example if the router is at 100% and a packet comes in that needs to have encryption and qos applied, would it fail to encrypt? would it queue? or would the router drop it ?

[Mark Malone · Accepted solution]

depending whats going on in the backplane of the router and what processing power if any it has left I would think any of those things could happen , from what I seen we were loosing at least 25% of our outbound traffic at the height of it , our router was so heavily impacted it struggled on the terminal to even respond at the time to show commands to see what was going on

[Joseph W. Doherty]

Transit traffic, with an overloaded CPU tends to either slow its transfer rate and/or packets are lost.

I.e. a maxed out CPU, on a software only based router, tends to be very adverse to your transit traffic.