8/6/2019 How to Configure ISDN Internet Dialup on a Cisco Router

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How To Configure ISDN Internet Dialup On A Cisco Router

Some time ago, when broadband and ADSL wasn't available, analog and digital (ISDN)

modems were the only way to connect to the Internet. Even though today ISDN isn't usedmuch for Internet connectivity, it still remains an important alternative backup dialup

method.

Cisco routers are usually equipped with an ISDN BRI (Basic Rate) Interface. Thisinterface is capable of supporting up to two 64Kbps channels, providing a maximum of

128Kbps throughput.

You can learn all about the ISDN protocol and capabilities by reading our popularISDN

WAN Protocol article.

Here will show you how to configure step-by-step your Cisco router to perform basicISDN dialup to an Internet provider. We will also cover some basic authentication

options and ISDN parameters that will help you better control your connection.

Example Scenario

The diagram below is our example network. The router is the local network's gatewayand connects to our Internet provider via ISDN, when needed.

One first thing we need to do is configure the ISDN WAN interface with the necessary

parameters. While every BRI ISDN interface card on a Cisco router looks the same, the

actual ISDN parameters required to configure it (ISDN switch-type) changes depending

on the country you live in.

For Europe, Australia, Asia and the UK, the ISDN switch-type is usually set to 'ISDN

Basic Net3'. For North America, 'Basic 5ess' or 'Basic-dms100' depending on your

telecommunication provider. We will be using the 'Basic Net3' switch type.

For more information, please advise ourBasic ISDN configuration article. It contains

extensive information on the various switch-types used throughout the world.

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Further configuration of the BRI interface involves the encapsulation type, which is set to

'ppp' (Point-to-Point Protocol), common amongst all ISP's.

R1# configure terminalR1(config)# interface bri 0

R1(config-if)# description WAN InterfaceR1(config-if)# isdn switch-type basic-net3

R1(config-if)# encapsulation pppR1(config-if)# dialer pool-member 5

The 'dialer pool-member' will associate the physical ISDN interface (bri 0) with a dialer

interface (dialer 0) configured later on. In this example, any dialer interface assigned to

'dialer pool member 5' will use this BRI interface when dialing out.

Dialer interfaces carry the same role as a dialup connection in Windows, where you

create a new connection, inserting the phone number, username, password and other

parameters. This is a virtual interface that will use the BRI interface to complete thedialup and establish our Internet connection.

A number of parameters are necessary to make the connection work, these include IP

address, encapsulation type, authentication method (CHAP or PAP), ISP

username/password and of course, the number it needs to dial:

R1(config)# interface dialer 0R1(config-if)# description Internet-DialupR1(config-if)# ip address negotiated

R1(config-if)# encapsulation ppp

R1(config-if)# ppp authentication chap pap callinR1(config-if)# ppp chap hostname firewallcxR1(config-if)# ppp chap password ciscoR1(config-if)# ppp pap sent-username firewallcx password ciscoR1(config-if)# dialer string 2310559500R1(config-if)# dialer pool 5

R1(config-if)# dialer-group 1R1(config-if)# ip nat outsideR1(config-if)# exit

R1(config)# dialer-list 1 protocol ip permit

Again, the 'dialer pool 5' command associates this dialer with the physical BRI 0 interfaceas it too is a member of the same pool. The 'dialer-group 1' command is used to match

'Interesting Traffic', that is, traffic that would initiate the dialup to the ISP.

The 'ip nat outside' command is part of the NAT configuration for this router. While NATis covered in a different article, we will simply list the necessary commands to configure

NAT so that the internal network has access to the Internet. You can find more

information on NAT by reading through ourNAT Articles.

http://www.firewall.cx/nat-intro.phphttp://www.firewall.cx/nat-intro.phphttp://www.firewall.cx/nat-intro.php

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We now need to configure the 'ip nat inside' interface, global nat overload command and

associated access list. The access list is used to control which internal hosts or network(s)

will pass through the nat service so they can access the Internet. Lastly, we insert adefault route (0.0.0.0 0.0.0.0) instructing the router to send all traffic destined to other

networks, out through the dialer 0 interface:

R1(config)# interface fastethernet 0/0

R1(config-if)# description LAN interfaceR1(config-if)# ip address 192.168.0.1 255.255.255.0R1(config-if)# ip nat inside

R1(config-if)# exitR1(config)# ip nat inside source list 100 interface dialer0 overloadR1(config)# access-list 100 permit ip 192.168.0.0 0.0.0.255 anyR1(config)# ip route 0.0.0.0 0.0.0.0 dialer0

Here we've enabled the NAT service for the 192.168.0.1/24 network via access list 100.

Generally, if you want to restrict the NAT service to a few hosts, you would simply needto modify access-list 100 to include these hosts only.

The above configuration commands are enough to successfully make your router connect

to your ISP and provide full Internet access to your internal network.

Let's now take a quick look at a few additional commands you can use to help tweak your

connection.

Additional ISDN Tweaks

Because ISDN dialups are usually charged by telecommunication providers based ontheir time usage, having the connection active without any data passing through, will

eventually have you receiving a pretty large phone bill. For this reason, there is a

command that can be used to instruct the router to tear-down the dialup if no data

triverses the connection for an certain amount of time.

The 'dialer idle-timeout' command does exactly that. If no data passes through the

Internet link within the specified time, the dialup connection is teared down. In our

example, this timeout is 300 seconds:

R1(config)# interface dialer 0R1(config-if)# dialer idle-timeout 300

Up to this point, the configuration of our 'dialer 0' interface will only use one ISDN B

channel. This means you'll have a maximum throughput of 64Kbps. Because ISDN BRI

can handle up to 128Kbps, we can force the router to bring up the second 64Kbpschannel if the first one reaches its full capacity:

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R1(config)# interface dialer 0R1(config-if)# dialer load-threshold 255 either

The threshold that triggeres the second channel can be configured for incoming, outgoingor either traffic. In our example, if the incoming or outgoing traffic reaches 64Kbps, the

second ISDN channel will autmomatically come up.

Article Summary