LAN1 always connects at 100 mbps

I have a year old RT-N66U with Firmware:3.0.0.4.372.30_3 (Merlin build). The LAN1 port of my RT-N66U always connects at 100 mbps no matter what.

I tried,
1. with different Ethernet Adapters like, Intels, Marvells, Realteks
2. changing the cable and even made one myself.
3. Upgrading/Downgrading firmware
4. Stock ASUS firmware,
5. Multiple full resets (with nvram erase)

Could this be a hardware problem?

Here is a screenshot taken from Tools -> Sys Info section.

Attached Images

SysInfo.jpg (26.2 KB)