The following configuration is for BE Broadband / O2 Broadband Specific and will work with an external modem working on RFC1493 / Bridge mode.
When it comes to setting up a Cisco Router with a modem under bridge mode, you only need a Layer 3 port to configure the given IP Address. In any Layer 3 capable device, it is rather straight forward. To configure, you only needs to add the IP address, subnet mask to the interface; along with the default route to the ISP given default-gateway and it should work.
When it comes to the 877 / 877w etc, they only come with 4 Layer 2 capable switch port, therefore one needs to configure a VLAN Interface and assign one Switchport to the given VLAN as per following configuration.
You do not need to create a Dialer Interface as PPPoE/PPPoA is not involved in this configuration.
First, we setup FastEthernet3 as part of VLAN 20, which will be our VLAN Interface for the IPoATM Bridge.
! interface FastEthernet3 description EXTERNAL DSL IPoATM switchport access vlan 20 !
Then we setup Interface Vlan20 and add the IP Address & Subnet Mask given by the ISP.
! interface Vlan20 description EXTERNAL DSL IPoATM VLAN INTERFACE ip address 87.194.x.x 255.255.x.x ip nat outside ip virtual-reassembly !
Then add the default route as follows. The Default-Gateway IP Address is given by the ISP.
! ip route 0.0.0.0 0.0.0.0 87.194.x.x !
And add the NAT Statements as follows. Interface VLAN 20 becomes the outside interface, as for Inside Interface, it could use be BVI/Dot11Radio etc
! ip nat inside source list WIRED interface Vlan20 overload ip nat inside source list WIRELESS interface Vlan20 overload !