Port Mirroring or Monitoring Cisco 857 Router
If you want to check what traffice is going on or you want to use Wireshark with your router cisco 857 so Clients are connected to port Fa0 and and your Wireshark Machine is connected to Fa1 then use below command.
monitor session 1 source interface Fa0
monitor session 1 destination interface Fa1
you can also use VLAN 1 as your source to capture everything