CMS Replication Issue to Edge Server

Today I came across a CMS Database Replication issue that had me scratching my head for a while.

Scenario is 2 sites with SE and Edge each, the first site I setup a few months back and was expanding with the new site. For the sake of explanation I will call the existing site #1 and the new site #2.
All servers were replicating except for the #2 Edge Server.

I ran through the obvious checklist as follows:-

1. Edge Server has a valid Certificate
2. Edge Server can reach internal CA
3. Edge Server is able to resolve Lync FE
4. Can Telnet from FE to Edge on port 4443

No joy..So I ran the XDS Trace on the Logging Tool. It was at this time that I realized that the usual 3 XDS trace components were missing as below:-

So where are the other 2 trace components?
It was at this point that the lights went on...the Lync Server I was tracing on (#2) was not holding the CMS! Embarrassing. So I launched the logging tool on FE #1 and what do you know:-

So what did the trace reveal? It showed how Edge Server #2 was not responding. So I suspected that although I had tested Telnet access from the #2 FE to #2 Edge - I hadn't tested connectivity from FE #1 (where the CMS is) to #2 Edge.

My suspicions were confirmed, a quick chat with the networking boys and voila.