Just recently I can across this issue TWICE!
Thats enough to warrant another glass of…never mind, a post on the blog will have to suffice 😉

Federation was failing and I got the following error message in the event log on the Edge Server I was building…

Running a S4 and SIPStack trace on the Edge also reported a 504 error between the two environments but most interestingly the highlighted error.

This alluded to the real issue, the new Edge server was unable to resolve the federation partners Edge Server discovered address.

Ahhh…gotta love DNS issues