要用Sniffer抓包先要配置这个

interface FastEthernet0/1
port monitor FastEthernet0/2
port monitor FastEthernet0/5
port monitor VLAN1
!
interface FastEthernet0/2
!
interface FastEthernet0/3
switchport access vlan 2
!
interface FastEthernet0/4
port monitor FastEthernet0/3
port monitor FastEthernet0/6
switchport access vlan 2
!
interface FastEthernet0/5
!
interface FastEthernet0/6
switchport access vlan 2
!!
interface VLAN1
ip address 10.200.8.136 255.255.252.0
no ip directed-broadcast
no ip route-cache
!

3550/2950用这个
C2950#conf t
C2950(config)#
C2950(config)#monitor session 1 source interface fastEthernet 0/2
!-- Interface fa 0/2 is configured as source port
C2950(config)#monitor session 1 destination interface fastEthernet 0/3
!-- Interface fa0/3 is configured as destination port
C2950(config)#

8500用这个
8500CSR# configure terminal
8500CSR(config)# interface fastethernet 12/0/15
8500CSR(config-if)# shutdown
8500CSR(config-if)# snoop interface fastethernet 0/0/1 direction both
8500CSR(config-if)# no shutdown