On Cisco switches, you can configure whether you mirror rx, tx or both;

For example:

monitor session 1 source interface gi0/1 rx

for receive

monitor session 1 source interface gi0/1 tx

for transmit

monitor session 1 source interface gi0/1 

If you don't specify, you mirror both tx and rx

On Cisco switches, you can configure whether you mirror rx, tx or both.

For example:

monitor session 1 source interface gi0/1 rx

For receive only:

monitor session 1 source interface gi0/1 tx

For transmit only:

monitor session 1 source interface gi0/1

If you don't specify, you mirror both tx and rx.