BBB/BBG ethernet phy sometimes don't works, refer this issue
This patch used to fix the problem through a image update for convenience, the patch source is here.
# Internet connection is needed, bash environment, please copy only to avoid typo
# Method 1
# single command line
sudo sh -c "wget -q -O - https://github.com/turmary/bbg-phy-fix/raw/master/phy-fix.sh | bash"
# Method 2
# the equal mutliple commands
sudo wget https://github.com/turmary/bbg-phy-fix/raw/master/phy-fix.sh
sudo chmod a+x phy-fix.sh
sudo ./phy-fix.sh
refer this