We have determined a SBS 2003 R2 server is causing a severe slowdown of our network. To rule out all other equipment, we connected only our router, a non-domain laptop, and the server to a 5-port 1Gbps switch (normally connected to a 24-port switch).
The laptop is configured statically to get DNS from the router, not the SBS server, and is using the router as the gateway. The SBS is not performing any router functions, it is just a server on the network with one NIC.
From the laptop, with the server NOT connected to the switch, our speeds are 3 Mbps down, 0.7 Mbps Up, ping time 55 ms (on speedtest). About 10 seconds after we connect the server to the network, speeds drop to 1 Mbps down, 0.05 Mbps Up, 3000 ms. Surfing is impossible, and accessing file share on the server is also very slow. If you watch the lights on the switch, the server does not blink anymore than the laptop did without the server connected.
Here is the odd part, this slowdown is much more evident in the afternoon. This made us think some scheduled task is causing the problem, so we checked and nothing is running. The server is also running Symantec AntiVirus Corporate Server, but we disabled all services related to Symantec, and slowdown still occurred.
We ran Wireshark to get an idea of the traffic, and the only thing noticeable was SMTP traffic, at no higher a rate than one of the other SBS servers we manage.
Any thoughts on what we can check next? We've swapped the network cables, ruled out the 24-port switch, as well as other devices on the network . The router is a standard Linksys sharing the internet, with no advanced routing enabled.