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RE: Stumper

  • From: Ray Burkholder
  • Date: Tue Jan 21 17:39:47 2003

This might be an MTU setting issue.  If pppoe, then on my Cisco stuff,
an MTU of 1492 (I think that is the right value) seemed to clear things

Ray Burkholder

> -----Original Message-----
> From: Mark J. Scheller [] 
> Sent: January 21, 2003 18:26
> To:
> Subject: Stumper
> I have run into a problem that has me completely stumped, so 
> I'm tossing it out to NANOG for some help.
> Before I lay out the specifics, I'm not trying to point 
> fingers at any particular ISP or vendor here, but this 
> problem only exhibits itself in very specific configurations. 
>  Unfortunately, the configuration is common enough as to get 
> unwanted attention from the higher-ups.
> Here's the particulars:
> Users that have Verizon DSL and a Linksys cable/DSL router 
> have difficulties accessing sites on my network -- whether 
> they are trying with http, https, smtp, pop3, ssh, ftp, etc., 
> etc.  Oh, but pings seem to be fine.  Low latency, no loss.  
> This is true even for access to a server brought up in the 
> DMZ, to keep the firewalls out of the equation.
> Doing some packet sniffing on the ethernet side of my router, 
> I could see specific http requests never showed up (and the 
> user saw the broken image icon).  This was for an mrtg graph 
> page with +/- 30 images.  I saw the request for almost all 
> the image files, save for one and the user reported the 
> broken image icon for the one.  So this looks and smells like 
> a packet loss issue..... but who/where/how?
> Taking the Linksys out of the pictures (connecting their PC 
> directly to the Verizon DSL modem) makes the problem go away.
> These same users report no trouble whatsoever accessing many 
> other common sites across the internet.
> Here's another interesting data point:  when one user runs 
> Morpheus (on any machine in his home network) he then has 
> absolutely no problems accessing servers/services on my network.
> Other users with Linksys routers and, say cable modem, do not 
> have this problem!
> So I'm looking for some pointers.  What could I have done to 
> my edge router (a Cisco 3640 if that helps any) that would 
> make it drop packets from Verizon DSL customers with Linksys 
> routers so long as they aren't running Morpheus?
> Mark J. Scheller (

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