["Hildebrand, Brian" <BrianHildebrand@xxxxxxxxxxxxxxxxxxxxxxxx>]

> It seems to me that an electrical problem with the dodgy switch was causing machines elsewhere on the network to keep ARP'ing repeatedly the same IP

> address but I have never seen this before and cannot understand why.

 

Since you say you had a dodgy switch, are you sure that it did not introduce a loop into your network (or something else that was done with a similar effect)? That could create a LOT of ARP traffic, because default switch behavior is to send a broadcast out every port but the one received. You get a loop in your network and the effect multiplies quickly until the network is pretty much unusable.