Port mirroring is the network engineer's equivalent of using breakpoints in a debugger for a programmer. It allows you to inspect what happens on the wire to get a clue for what's wrong.
Maybe I'm being naive, but as someone who's spent a lot of time capturing and reading PCAP files, 40GB/s worth of mirror is gonzo nuts! You may already be looking for a needle in a haystack, but this time the haystack is 1/60th of the entire bandwidth of the internet in the year 2000!
