How to configure the WAN port to a LAN port?

[Neff27]

All ports on the Cisco router are basically bad. Trying to convert the WAN port to a LAN so i can connect a Comcast MODEM

[Mark Malone]

if its pure layer 3 you cant , you can go layer 2 to layer 3 alright with no switchport but its mostly on l3 capable switches , what model router do you have ?
what exactly is happening to the ports that there all bad ?

[DEENA VERAPPAN]

it's an 891 router, but I need to use the SFP ports, as trunk ports.

[Joseph W. Doherty]

If it's a routed port, you might be able to define subinterfaces to interoperate with a trunk port.