Resolution
In the Cisco 2821, 2851 and 3800 Integrated Service Router platforms, two copper Gigabit Ethernet ports are built-in. On the Cisco 3800 Series platform, an SFP Gigabit Ethernet port can be used instead of one of the copper Gigabit Ethernet ports. The feature set of the on-board Gigabit Ethernet ports and the HWIC-1GE-SFP is the same, except that Cisco EtherChannel is not supported on the on-board Gigabit Ethernet interfaces.
In other words, port channeling does not work on the Fast Ethernet ports on the previously mentioned routers.
The only way to set up this is to add an EtherSwitch module into the router and configure a Layer 2 EtherChannel on both sides. Port Channeling can be set on FE interfaces on 7000 series routers and later.
Refer to Configuring EtherChannel Between Catalyst Switches Running Cisco IOS System Software and a Cisco Router for more information.
