Investigating Network Loops

Looking Into Loops (by Tony Fortunato)
I’ve been involved in a few troubleshooting engagements where the good old loop reared its ugly head.
In this day and age of networking there is far more opportunity for a loop since we have load balancing redundancy, load sharing and devices with multiple interfaces.
Unfortunately as networks become more complex and devices become ‘smarter’ the risk increases. Back in a day, a loop would cause an outage, or major performance issues. Either way, you would have to deal with it to restore service.
In my many years troubleshooting, I’ve seen devices such as; printers, access points, computers and of course misconfigured switches or routers cause loops. Just recently a desktop with 2 Ethernet cards was configured to bridge the adapters causing a loop.  See it doesn’t take much.
Now with such resilient equipment, a loop can go largely undetected until you start poking around your logs, stats or packets.
In this video I show some of the telltale signs to determine if you have loop and if it’s a layer 2 or layer 3 loop.

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