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Cisco 877w with External Modem (RFC 1493 / IPoATM)

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
!

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