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mini-HowTo: Cisco ISDN LAN to LAN

Introduction

This document describes Cisco IOS 12.0 sample configurations for an ISDN dialup connection between two Cisco routers using PPP. Most of the configuration is generic and should work for any ISDN dialup PPP connection.

This sample configuration is based on the following design assumptions:

In this sample configuration routers are named routera/routerb internally, you can change that, but please follow rules for hostnames (RFC1035). Make sure you change all names in both configuration files.

Changes needed before use

Some changes must be made before you can use this configuration consisting basically of replacing placeholders for usernames by real usernames, placeholders for passwords by real passwords of your choice and last but not least MSN dialer information. Placeholders that need to be changed are:

Password Security

After you enter passwords, they will be encrypted in router memory (that's what service password-encryption is for).

A Cisco configuration can only be viewed (eg. using ' write terminal' or ' show running-config') after giving the enable secret on your router so keep this enable secret password a real secret. All other passwords can be decrypted using certain tools (type 7 passwords can easily be decrypted, type 5 are tough to hack), so don't leave printouts of your configuration lying around. Secure all servers storing your configuration files. It's beyond the scope of this document to explain how to do this.

Channel Bundling

In this sample configuration design I assume you want to limit the configuration to use a single B-channel (hence 64000 bps max. bandwidth). If you want to use channel bundling (both B-channels resulting in twice the speed but usually also twice the cost, hence 128000 bps), add the following two lines from the configuration below in the interface Dialer1 section on both routers:

10 is the threshold at which the second channel kicks in. Possible values are from 1 to 255. The higher the value the heavier/longer traffic needs to be there before the second channel kicks in. 10 is a rather low value and is recommended only if you expect heavy use and want to prevent unnecessary delays.

Cisco Configuration Modes

The configuration sample is in a format compatible with the format your router stores its configuration on a TFTP server or what it would show when you do a 'write terminal'. It's not necessarily the order in which you have to type in things, so if you get any error message that you need to define something before you can do whatever you were just about to do, define that something (most likely listed a few lines below) and try again.

In order not to confuse you even more by giving you two versions of each configuration and since the scope of this document does not include a tutorial on Cisco CLI (Command Line Interface), which is nicely documented by Cisco, I at least want to give you a hint there is something about the bush. You need to be able to navigate around 'enable', 'configure terminal' and such things without the help of this document.

Cisco ISDN Dialup Configuration Sample

The following Cisco ISDN PPP dial on demand configuration sample has been created for IOS 12. It has last been tested on two Cisco 1003 Routers using IOS 12.0(21a) equipped with 2 MB Flash, 8 MB DRAM. Cisco asks for 4 MB Flash but the Cisco 1003 IOS 12.0 "IP Only" image fits into a 2 MB Flash card just fine. This configuration should work with any IOS >= 12.0.

Configuration routera

Configuration routerb

Saving Configuration Changes

Make sure to store your configuration in NVRAM every time you make changes:

copy running-config startup-config

If you fail to do this, your changes will be lost the next time your router reboots (eg. due to power failure).

Remarks

A Word on Passwords

As you can see above there are four passwords on each router. Only the PPP chap password must be the same on both routers, the other passwords may and should be different on each router, even if that may mean you have to make a list of passwords. Most people will be tempted to make local router passwords all the same but it's not a good idea to make enable secret and "normal" passwords the same. Cisco recommends not doing this. It is however acceptable if your console password (line con) and your telnet password (line vty 0 4) are the same if you're concerned about the number of different passwords used.

Optional Configuration

The following two lines are to prepare the routers for GMT+1 timezone and automatic switch for summer time, but this really makes only sense if you also have a time source (eg. an NTP server) from which the router gets it's time. I left those lines in this sample to make you aware of the fact there is such a thing as a clock in your router which you may want to set and configure. You can discard those lines without any impact on the connection.

Search and Replace

Before you can actually use this configuration you will have to replace the placeholders mentioned above. In order to that I suggest you do this:

• save the configuration samples to two textfiles using a simple editor such as "notepad"
• search and replace those place holders for each file, save changes

Make sure you have your MSN data right. Also make a note of the passwords used and store them in a secure place.

Copy and Paste

There are two issues when trying to "cut & paste" the configuration files from a text editor to a router terminal such as "HyperTerm":

• HyperTerm may feed the router too fast and hence overwhelm the router's CLI which means some lines get lost
• After each "interface" and "line" block there should be an "exit " line - you have to add them before "cut & paste"

In order to circumvent overwhelming the router I suggest you don't feed the router the whole configuration all at once but in two chunks. First chunk is from the top to "dialer pool 1", the second chunk covers the rest.

Editing MSN data

Even though this document is not a tutorial on Cisco IOS CLI, I want to give you a hint how to change your MSN data since this is the most likely change you may make. When people move their MSN data often changes as well, so here goes. I assume you're logged in to your router and switched to "enable mode".

The changes mentioned above must be done on both routers, of course. It should not be done over your WAN since you may cut the connection you're using to make the changes...