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  • From: Mark J. Scheller
  • Date: Tue Jan 21 17:27:48 2003

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

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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