North American Network Operators Group

Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical

Re: debugging packet loss

  • From: Daniel Roesen
  • Date: Tue Jul 23 16:08:00 2002

On Tue, Jul 23, 2002 at 11:53:29AM -0400, Ralph Doncaster wrote:
> I'm seeing 2-5% packet loss going through a Cisco 2621 with <10mbps of
> traffic running at ~50% CPU.  (packet loss based on ping results)
> 
> Pinging another box on the same catalyst 2900 switch gives no packet loss,
> so it seems the 2621 is the source of the packet loss.  I need help
> figuring out why it is dropping packets,

Because pings are processed by the CPU which has already lots of
better things to do than answering to pings. Answering ICMP is
quite low-prio for routing engines.

Trying pinging ERXes... you'll get "packet loss" (correct: "sometimes
no answer") even though the RE twiddles thumbs. There is big difference
between vendors in respect of ICMP echo reply priorization. Cisco
is usually quite "good" at it (except for when BGP Scanner is running
you'll see an increase of some dozends to hundreds of milliseconds
in reply latency).

> and how to stop it.

Get a faster router and/or just don't care that much about ping
replies from routing engines of routers. :-)
 
Packet loss while _forwarding_ packets counts, not when pinging
loopbacks (yes, I know, there are many customers who can't tell the
difference).


Regards
Daniel