[c-nsp] Cisco IOS ping utility reports lower RTT than possible

[adamv0025 at netconsultings.com]

> Martin T
> Sent: Thursday, May 2, 2019 4:12 PM
> 
> On Thu, May 2, 2019 at 11:07 AM James Bensley <jwbensley at gmail.com>
> wrote:
> >
> > On Mon, 29 Apr 2019 at 11:14, Martin T <m4rtntns at gmail.com> wrote:
> > >
> > > Hi,
> >
> > Hi Martin,
> >
> > > I have a following very simple network topology:
> > >
> > > CISCO1921[Gi0/0] <-> [eno3]svr
> > >
> > > Gi0/0 in Cisco 1921 ISR has 10.66.66.2/24 configured and eno3 in
> > > Linux server has 10.66.66.1/24 configured. RTT on this link is 10ms:
> >
> > How do you know this to be 100% correct - have you OTDR/iOLM tested
> this link?
> >
> > Cheers,
> > James.
> 
> Hi James,
> 
> I can't OTDR it because this delay is made with Linux netem qdisc.
> However, I can compare it with for example using Juniper RPM(Cisco IP SLA
> analogue): https://i.imgur.com/i8jccwh.png ..or Linux ping
> utility: https://i.imgur.com/NeubqAV.png On both graphs I have plotted
> 21600 measurements. None of those are below 10ms.
> 
That's just 1921 taking it's time to reply -it's not it's priority to
respond to some ping packets even if it doesn't have anything better to do
which it almost always has.
Try this setup and the delay disappears:
svr [eno4] <-> [Gi0/1]CISCO1921[Gi0/0] <-> [eno3]svr

adam