Sudden ethernet port failure - unknown cause

Hi there - hoping to find some help from the community... I'm kinda unsure where to go on this one... I've done much of the troubleshooting that comes to mind, and nothing so far.

I have a Mid-2009 MacBook Pro 13" running El Capitan (10.11.6) and it's been working just fine - both ethernet and wifi work. I've been using Ethernet more lately as I've been doing some large file transfers and the wifi is a bit spotty in my basement.

Anyhow, though, I wake up one morning to find that nothing is resolving, and when the ethernet cable is plugged in that it cannot access any network resources (I have the service order set to Ethernet first).

What really is driving me nuts is that there is nothing coming to mind that changed on this system prior to the ethernet stopping working; no new hardware, no new software, etc...

Symptoms

• When ethernet is plugged in, the Network settings shows 'green' that it's connected, and it gets a DHCP-assigned IP address, DNS, etc...
• Service order is set to use ethernet, then wifi.
• Loading a webpage just sits there - even a long time it doesn't budge (so I don't think it's a non-responsive DNS, since even IP addresses fail to establish a connection).
• Wifi works fine without any issues whatsoever.

Where I'm at now is just plain stumped... here's all that I've tried... I'm at my wits end.

• Rebooted the system - that didn't resolve anything.
• Replaced the cable and tried a different port on my Airport Extreme - no dice.
• All other devices connected to the Airport Extreme work fine - even when I try different plugs and cables... so I'm confident it's with the MacBook itself.
• Reset the NVRAM~PRAM - no help.
• Reset the System Management Controller (SMC)... no help there, either.
• I see there was an older system update issue for El Capitan that had to do with a version of the "Incompatible Kernel Extension Configuration Data" and looked into that - it's not the issue here.
• This computer is my 'workhorse' and I don't keep anything on it that's important, so I even just formatted the drive and installed El Captian... mostly out of frustration since nothing is working. That didn't help...

I just don't know what to do next... :-/ Suggestions?