The other day I had a customer call and complain about high ping latency between our router and his server. I asked, what are you pinging? The default gateway he replied. Well, there’s your problem. Ping one of our servers, and it will look fine. Customer did not understand, and simply wouldn’t accept my answer that seeing spikes in ping latency on the ethernet handoff between his server and my router is normal.
Unfortunately, many people use ping to diagnose problems, but they dont understand exactly how to interpret the results. First, not all latency is bad. Some devices are slow to respond because there is an issue causing problem. But sometimes, a device is slow to respond because it doesn’t feel like responding right away. Huh? Its called priority queuing. When you ping one server from another server, that ping is treated is high priority by receiving server. The recipient server responds as fast as it can, just as it would for any other request. But when you ping a router, the router can care less about that ping. Routers are designed to treat pings as the lowest priority request, it will get around to it after it finishes the other more important stuff its doing. Two routers right next to other might show 3ms latency, with intermitent spikes to 20ms – perfectly normal.
Interpreting ping data is a balance of latency and packet loss. The two routers might show latency, but upon closer inspection, there is ZERO packet loss, even after 10,000 pings. Though you could have two routers with stable low latency between them, but 3 or 4 percent packetloss. So you have to look at all aspects of the ping result set and the overall environment.
