There was a latent misconfiguration, specifically a cable plugged someplace it shouldn’t have been, from a few months ago. Something called the spanning tree protocol kicked in and started trying to route all of our private network traffic to a public network over a link that was much too small and slow
via WP.com Downtime Summary — Blog — WordPress.com.
That’s rough. The spanning-tree design/config is the kind of thing that creeps up on you after years of organic growth and never really having a dedicated “network guy”. Its just bizarre enough that you spend a lot of time and effort digging into other obscure possibilities before you stumble on it. Its one of the ‘gotchas’ of doing stuff in-house rather than clouding it up, particularly if you mix switch vendors.
Here’s an old map of an L2 topology I inherited and caused several spanning-tree outages learning the hard way.