Connect a Cisco 1801 POTS ADSL Router to BT Broadband

Introduction

This article will guide you through the process of setting up a Cisco 1801 ADSL router to a BT broadband circuit. It will probably work on other POTS ADSL suppliers as well.

Beginning the configuration

Starting with a fresh router lets begin by setting up our local inside interface. This is going to be FastEthernet0. We are going to assign this interface an address of 192.168.128.1. To do this from the routers command prompt enter the following commands:

Firstly if you are not already in configuration mode enter it by the following command:

These commands will tell the router you wish to configure interface FastEthernet0.
You will give the interface a brief description. And you will set the IP address and subnet mask. We also then tell it to autodetect the port duplex and speed. Finally we issue a "no shut" command to tell the router we want to bring the interface up as it may well be shut down.

If all has gone well you should see something like this:

Now we have configured the internal interface it is a good time to see if your router can see other internal devices (such as your PC). So try a ping command.

in my example I am pinging another box with an IP of 192.168.128.2

I get 5 ! exclamation marks which is a good sign that internal networking is up.

It is worth mentioning that the ping command might take a moment or two to respond depending on your switching set up. So you might have to run the command a couple of times before getting the response.

Next let's configure the ADSL broadband interface. Now assuming you have plugged it in with the correct microfilter into the broadband enabled POTS line this should be nearly as simple as the fastethernet0 interface you just setup.

Actually it is bit more complex because you actually end up dealing with the main ADSL physical port, a subinterface we create called ATM0.1 and a dialer interface called dialer0 which actually "dials" the ISP

Firstly lets make sure the ATM0 is not shut down

type

then set up the sub interface

Please note you might need to replace the pvc with your own ISP supplied one

and finally set up the dialer0 interface

Please note that you will have to replace the myuser@myISP.com username, mypassword and the ip address with your own supplied by your ISP . If you don't get given a static ip (dynamically assigned) then the commands for the dialer are slightly different they would look like this:

Now we have our interfaces configured we need to set our default route (or sometimes called the gateway of last resort) so all traffic destined for the internet goes via the dialer0 interface

And make sure the dialer0 will permit ip traffic to flow

Now the connectivity is in place to make it useful we need our internal computers to be able to go out onto the web. We use NAT hiding behind our dialer0 to achieve this.

So now we add the following commands to the fastethernet0 interface to enable nat

and on the dialer0 interface

So setup an access list which will represent your internal network. In my case it is 192.168.128.0 255.255.255.0 and create a nat command to nat inside traffic to hide behind the dialer0 interface ip address.

If all goes well that should be the lot and you can connect to the web via your Cisco 1801 series router.