Intermittent intra-LAN connectivity

From: Dan Horne (danDOESNOTLIKESPAM_at_taisweb.net)
Date: 05/25/04 

Date: Tue, 25 May 2004 05:39:51 -0700

Sorry for the long post, but I want to make sure I cover
everything:

2 weeks ago, a client began experiencing difficulties
with their local network. Pinging around inside to the
other desktops and their server produces strange
results. The first ping is almost always returned
immediately, followed by 3 timeouts. If you immediately
ping again, you get 100% loss. If you wait a few minutes
and try again, again you will get 1 followed by none.
This occurs for all computers EXCEPT MAIN SERVER and
except the gateway. There is a file server that also
experiences this problem. The clients cannot get files
off of the server due to connections dropping.

SETUP:
There are 2 servers (1 W2K3SBE, 1W2K), 5 XP Pro stations,
2 Mac desktops and 2 Mac PBooks all running OS X. The
W2K3 server and the gateway are in our server room, and
the rest is 15 floors down in their office, connected by
a single Cat5e cable (they never have a problem
contacting the server or the gateway, though). The
clients had a mixture of switches and hubs in their
office. The gateway is a Cisco PIX 515, and the clients
are connected to a dedicated interface on that firewall
and are port-address-translated to a dedicated public
address.

I HAVE TRIED:
1. Replaced all hubs with switches. No change.
2. Temporarily replaced all switches with hubs and ran a
packet sniffer. I saw NO unusual traffic, and not a
whole lot of it (about what would be expected from such a
small network).
3. Tested the long line for attenuation. Signal is good.
4. Only one of the switches is a "smart switch" (the one
connected to the server and gateway and the long cable),
but I am seeing no collisions or errors on any port on
that switch.
5. Turned off computers one-at-a-time (including servers)
to see if any single computer was the source. No change.
6. Disconnected switches one-at-a-time to see if any LAN
segment was responsible. No change on any segment.
7. Plugged into different interface on the firewall. No
change.

OBSERVATIONS:
1. The problem began when the firewall began refusing all
connections for all interfaces and was rebooted. The
problem does not show up on any other network connected
to the firewall.
2. The clients have a good, fast internet connection
through the firewall with no packet loss.
3. The Mac's seem to be "offline" more than the Windows
stations, but all of the comps are up-and-down.

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