IBM BladeCenter Installation Manual page 27

Ethernet Tx/Rx: This green activity LED is at the bottom of each external
10/100/1000 Mbps port. When this LED flashes on a port, it indicates that data
is being transmitted or received (that is, activity is occurring) between that port
and another device on the network link. The flash frequency is proportional to
the amount of traffic on the network link.
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Chapter 3. Information panel LEDs and external ports