How to bridge ADSL to Ethernet on a Cisco 800 series router?

Short and sweet. To convert a fully-functioned Cisco ADSL router into a ATM/Ethernet converter (bridge) do the following:

interface ATM0
 no ip address
 no atm ilmi-keepalive
 pvc 8/35
  encapsulation aal5snap
 !
 dsl operating-mode auto
 bridge-group 1
!
!
interface Vlan1
 no ip address
 bridge-group 1
!
bridge 1 protocol ieee

Make sure the ATM interface does not have any sub-interfaces/Dialer interfaces associated and remove any firewalls on the router. Having no IP addresses on interfaces is important as is retaining the PVC settings for the internet provider on the ATM interface.

This is useful for when you need to have a non-ATM device behind the Cisco router do the ISP authentication or hold the IP address that the ISP provides (without doing double-nat).

By doing this you are taking your shiny Cisco router and making it do a job I'd prefer to see a D-Link/Netgear do. Such a waste of a good router. Hope it helps...