high latency ds3 issue on unloaded line

Ben Plimpton bplimpton at sopris.net
Fri Sep 26 14:35:16 CDT 2008

We've had a similar issue with a few of our Qwest DS3's.  The solution  
has been 1 of the following....

1) 	Qwest has over-provisioned the transit links on their atm network  
that the DS3 is riding and the during peak times of the day, the  
transit link becomes congested causing high latency not related to our  
traffic levels.  So the congestion could be appearing beyond your  
local loop.

2) 	We also had an instance where qwest had an issue with the PVC on  
the atm switch that we connected into that was causing > 500ms of  
latency.  Like you, we are in a small town served by older ATM  
switches, so you might just see if they can rebuild both sides to see  
if that clears it up.  Sounds quacky, but after 12 hours of  
troubleshooting, that was the fix.


On Sep 26, 2008, at 12:59 PM, John Lee wrote:

> Mike,
> Your latencies which suddenly appear for several hours and then go  
> away and do this on a regular basis  sounds like a layer 2, facility  
> switching issue. As you indicated " the problem comes on during the  
> day and then lets up late in the evening" sounds like the under  
> lying facility is being switched back around the "long side" of the  
> SONET ring or other facility. Some carrier facilities are scheduled  
> for "one path or direction" say during the day that are supposed to  
> be for lower latency time periods for interactive work and then  
> switch for a lower cost, higher latency path in the evening when  
> computer to computer backups do not care. If you can plot the times  
> the issues start and end and that these occur daily during the week  
> and not on weekends etc that would be a strong indicator.
> John (ISDN) Lee
> ________________________________________
> From: mike [mike-nanog at tiedyenetworks.com]
> Sent: Friday, September 26, 2008 12:04 PM
> To: nanog at nanog.org
> Subject: high latency ds3 issue on unloaded line
> Hello,
>    I have a ds3 from qwest which has daily issues with insane
> point-to-point latencies sometimes exceeding 1000ms for hours on end,
> and which suddenly disappear, and does not appear to correspond with
> actual measured link utilization (less than 20mbps most days).
>    To make a long investigation short, the problem comes on during the
> day and then lets up late in the evening. I have tested and examined
> everything at the ip layer and no it's not high utilization, an ACL,
> router cpu or bad hardware, no line errors or other issues visible  
> from
> interface or controller stats. yes I have flushed all hardware, and I
> have a 7204vxr/npe-400 with this single ds3. The only clue seems to be
> millions of 'output drops' from qwest's side. And at night I can hit
> popular ftp mirrors from a directly attached server and observe my
> interface reporting about %100 utilization combined with my users and
> customers, so yeah it really is a full line rate ds3. And historically
> Mrtg always shows around 20mbps or less utilization and it's only
> smokeping that goes off, usually in the afternoon when the point to
> point latencies between my router and qwest start heading north, and
> consistently at that. I also have another in house tool that takes 30
> second snapshots of my ds3 interface in order to catch short bursts  
> that
> would be smoothed out with mrtg's 5 minute average, but during these
> high latency times there aren't any spikes noted. And for added
> confusion (or fun!), the latency can start at any utilization level -
> I've observed it while we were pulling just 12mbps, and I have not had
> it while we were doing 34mbps, only the time of day seems to be the
> common factor.
>    Qwest has not been able to identify the issue, only note that -
> yeah, this really is happening when there is otherwise no real load on
> the line - and I am certain we have done everything to rule out the ip
> layer. They have put in a 'request' to move me to another router,  
> but I
> am not hopeful of a resolution that way as the router we're  
> currently on
> doesn't appear otherwise to have the problem with any other  
> subscriber.
>    What I want to know, is it possible that the underlaying atm/sonet
> that carries my ds3 from my facility is somehow oversubscribed or
> misconfigured? We have an OC12 fiber entrance and this is the only
> circuit provisioned on it, and in our small tiny town the only other
> user on the ring with us is comcast (according to the att network
> engineer who installed this). I don't know enough about atm/sonet to
> imagine conditions that would cause the issues I am seeing here , but
> every ip layer tool I have only ever tells me there isn't an ip issue
> here. I can issue ping from my router directly to the attached qwest
> router and get > 1000ms and then other times (out of the problem
> window), I am getting 4ms.
>    If anyone has laughs or beers to offer me, send 'em on cuz I could
> use both right about now....
> Mike-

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