On a machine with multiple ethernet interfaces, the physical order is very often not as expected and even may be different between machines of the same production series.
Although there are different things that can be done about this, the easiest way is in most cases just to identify the physical port / ethernet interface relationship.
The solution: With ethtool, you may simply let an interface LED blink for a specified number of seconds, so this command does this for physical interface eth2 for 10 seconds (for example):
# ethtool -p eth2 10