I've got the following set up at home and have today noticed something I didn't before, possibly after the recent firmware updates to the airport range.
I have a cable modem, connected to the gigabut ethernet port on my primary airport extreme base station. I then have a category 5e patch cable running from this, to the internet port on the second airport extreme base station.
When looking at my airpoty utility, the second base station shows an internet connection of good and that this is running at 100mb. Why doesn't it shows 1gig, as this is the supposedly gigabit connection between base station A and B?
AirPort Extreme 802.11n (5th Gen), Both base station running 7.6.3
To help make a comparision, I have a 4G Time Capsule as my "main" router in my network with a 4G AirPort Extreme configured in a roaming network. The connection between the modem and the TC is 1000 Mb/s, as well as, the connection between the TC & the AEBS. Both routers are also at firmware v7.6.3. The AirPort Utility is version 6.2.
What is the connection value in your case between your primary AEBS and the modem? Is it also 1000 Mb/s or is it 100 Mb/s?