Best way to troubleshoot intermittent network outages?

Solution 1:

Your first test should always be with a single computer directly connected to the modem and then ping your primary DNS for about an hour. Look for things like jitter, consistently high latency and packet loss. Comcast will ignore any results to IP destinations that are not on their network or go over networks that they do not control, so only ping your DNS.

Your second test should be something that monitors more than just ICMP, try SNMP monitoring. This will record data to prove that when it goes out that it is definitely not a utilization issue.

Third test is with a program called MTR which will give you real-time traceroute results, it will be able to run for a long time and when you see the problem you can see what is going on on the MTR. This will provide some insights to any possible routing problems (target your primary DNS).

Last, try to bypass any inside wiring you can, if you can get to where the cable connection comes in off the street, try plugging in there, this will rule out anything within the building and isolate it to only what comcast controls.

Solution 2:

First, I would recommend using some online monitoring tool. Not to advertise any products, but Pingdom offers free account for monitoring only one destination, so setup is straightforward. That may or may not provide some data, but anyway you are not using money or much time for it.

Second, if you have Linux server (or another computer running Linux), install smokeping and configure it to ping some well-known targets (www.google.com, ping.funet.fi, maybe Comcast website etc.). That may or may not give you something useful.

Third, running tracert/traceroute if there is long enough outage may be useful. If it shows traffic going out of your office and then dropping in some Comcast router, it's easier to prove that it's not caused by you.

Fourth, try changing switch too. It's possible (even though not likely) that's culprit.

Fifth, ask recommendations from Comcast (what's correct router/modem for their connection and what settings you should use). If there is nothing in your network (try to make it as simple as possible for testing it, maybe just computer connected directly to router with crossover cable), it's hard to blame you.

Diagnostics for short network outages (just dropping connections for example, without longer breaks) is very hard problem, as there is so many potential points of failure.