To sniff or to analyse traffic on specific port or Vlan you can use SPAN. As it is shown in Figure 1, we want to have traffic of FastEthernet0/1 on port FastEthernet0/10. So, simple we can define monitor session for interface FastEthernet0/1 as a source and FastEthernet0/10 as a destination.
SW# conf t
SW(config)#monitor session 1 source int fa0/1
SW(config)#monitor session 1 destination int fa0/10
You can define a receive, transmit or both in monitor session:
SW(config)#monitor session 1 source int fa0/1 rx | tx | both
2- RSPAN(Remotely Switched Port Analyzer Network)
This is exactly the same scenario of SPAN, but here we remotely monitor a port in switch. First, you have to define separate Vlan between two switches. Then, you can specify source and destination.