This is the 6 post knot that was included with Construx sets of the 70's and 80's. It's the primary connector used to join the long pieces together, and though they're pretty robust even after 35 years, they're still prone to breaking. Now you can print your own!

Simply print and insert the smaller part into the larger part. It takes some effort, but once they're joined they shouldn't be loose at all. It works just like the original Construx design, and the parts are interchangable (as seen in one of the images, you can combine them with the original parts)