The ID (inside diameter) was 2-1/2″, supposedly. That black, ribbed insert is meant to prevent water leaks, but it also made installing the 2″ bushing I wanted to use fairly difficult. And yes, I reduced down the size at the drain itself, installing a 2-1/2″ OD (outside diameter) pipe bushing that reduced down to 1-1/2″ pipe, which would allow me to use a shorter radius elbow and thus save me vital inches on the height of the base, which didn’t actually work out in the end…