Here is a tutorial on Cisco dialup

Hello All,

Once again, I am writing a tutorial to help the simpler Cisco kids! This tutorial is on Dialing an ISP using Cisco and AUX port.

Please could any of you scan your eyes over this very short tutorial and give me your comments, tips, changes or additions ...

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This tutorial is not quite complete as I am having problems completing NAT, please could anyone show me how NAT should be implemented in this dialup case? I have tried putting "ip nat inside" on the LAN side and "ip nat outside" on the Dialer and/or Async interfaces (not sure which should use?) and "overloading" etc. I can access the internet from the router, but want to provide it to the LAN, do I need to do it with a loopback address?

It is nearly there, but your help (as it has been in the past) would be greatly appreciaed, and of course you will get credit where due!



========= SNIP ================= Cisco Configs - Using AUX port and analog modem to dialup ISP

For some reason it seems that configuring a dialup connection from a Cisco router to an ISP is a great mystery, the confusion is mainly due to the many commands which are needed to setup a simple modem call and the various different ways in which it can be achieved.

This tutorial attempts to describe, one of the simpler method of connecting an analog modem to the AUX port on any Cisco router and configuring a Async interface which will utilize the modem to dial any standard ISP like Wannado, BT Openworld or AOL with a dialup account.

Equipment Setup

Following the same steps as found in the AUX Port Communications tutorial. Connect the modem to the router?s AUX port using a Rollover Cable.

When using cables which terminate in an RJ-45 from the AUX port, the 25-pin modem adapter is required.

Find TTY number

You will need to find out which TTY/Async line the AUX port corresponds to, this is done by issuing the ?show line? command:

router1>sh line

Tty Typ Tx/Rx A Modem Roty AccO AccI Uses Noise Overruns Int 0 CTY - - - - - 0 0 0/0 - 65 AUX????????????? - ???- - - - 1 0 0/0 - ? 66 VTY - - - - - ?? 0 0 0/0 - 67 VTY - - - - - ?? 0 0 0/0 - 68 VTY - - - - - ? 0 0 0/0 - 69 VTY - - - - - ? 0 0 0/0 - 70 VTY - - - - - ? 0 0 0/0 -

Line(s) not in async mode -or- with no hardware support:


You will notice in the ?Typ? field (second column) there is one AUX port, take note of the corresponding ?Tty? number (first column) in this example it is 65 on a 2611 router, but this may differ between router models.

The Configuration

The following configuration is the complete solution

On this occasion we will preview the whole configuration and then break it down later in the tutorial describing what each section does:

! version 12.3 service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname router ! no aaa new-model ip subnet-zero ! ip name-server ! ! chat-script DIALOUT ABORT ERROR ABORT BUSY "" "ATZ" OK "ATDT \\T" TIMEOUT 120 CONNECT \\c modemcap entry MY_USR_MODEM:MSC=&F0X2L1M0S0=1 ! ! interface Ethernet0/0 ?ip address ?no shutdown ! interface Async65 ?no ip address ?encapsulation ppp ?dialer in-band ?dialer pool-member 1 ?dialer-group 1 ?async mode interactive ! interface Dialer0 ?ip address negotiated ?encapsulation ppp ?dialer pool 1 ?dialer remote-name ?dialer idle-timeout 300 ?dialer string ?dialer hold-queue 10 ?dialer-group 1 ?no peer default ip address ?ppp authentication pap chap callin ?ppp chap hostname ?ppp chap password 0 ! ! ip route Dialer0 ! ! access-list 101 permit ip any any dialer-list 1 protocol ip list 101 ! ! line con 0 ?password cisco ! line aux 0 ?exec-timeout 0 0 ?script dialer DIALOUT ?modem InOut ?modem autoconfigure type MY_USR_MODEM ?no exec ?transport input all ?autoselect ppp ?stopbits 1 ?speed 115200 ?flowcontrol hardware ?password cisco ?login ! line vty 0 ?password cisco ?login line vty 1 4 ?password cisco ?login ! end

That is it! You may see other (over complicated) configs which attempt to do the same thing, but this one works and is fairly self explanatory. Next we will break it down to gain a greater insight as to what each command is doing.

Name Server

ip name-server

It is always a good idea to specify a name server, to enable the resolution of domain names, if only for testing purposes. Use the configuration line above, replacing the relevant part with the address of your ISP?s DNS server. You can have multiple ?ip name-server? lines with alternate backup DNS server addresses.

Chat and Modem Scripts


The two configuration lines above are used to communicate with the modem giving it instructions about what and how to dial. The ?chat-script? is used to issue the ?AT? commands which get the modem off the hook and dialing, the ?chat-script? also specifies the timeout after which the modem will hangup if an unsuccessful call is made.

The ?modcap? line is used to tell the router further details about how to communicate with the modem, these may be manufacturer specific to the type of modem used (in this case relating to US Robotics modems). This line is not essential and does not matter too much but should work with nearly all modem manufacturers.

Async Interface

interface Async65 ?no ip address ?encapsulation ppp ?dialer in-band ?dialer pool-member 1 ?dialer-group 1 ?async mode interactive

Async is short for Asynchronous. Generally, commands entered in Async interface mode enable you to configure protocol-specific parameters for asynchronous interfaces. You need to configure an asynchronous interface in order to support the PPP connection which will be needed when dialing an ISP.

Earlier on in this tutorial we discovered the TTY line number that matched the AUX port, in this example it was noted that the TTY number was 65. This is of relevance because it also relates to the Async interface number, in the configuration lines above you can see that ?interface Async65? is used. This means that the Async65 interface directly relates to the AUX port on the front or back of the router, but this may differ between router models.

Dialer Interface

interface Dialer0 ?ip address negotiated ?encapsulation ppp ?dialer pool 1 ?dialer remote-name ?dialer idle-timeout 300 ?dialer string ?dialer hold-queue 10 ?dialer-group 1 ?no peer default ip address ?ppp authentication pap chap callin ?ppp chap hostname ?ppp chap password 0

The next stage is to configure the interface which will actually dial the ISP. The ?Dialer? interface is a logical interface which can be used for many purposes including ISDN and ADSL. In this example the Dialer interface is used to communicate with the analog modem, dial the ISP?s phone number, authenticate PPP information and receive an IP address.

?ip address negotiated? ? This command tells the router to gain its IP address automatically from the ISP during the PPP negotiations. This method of receiving an IP address is called IPCP (Internet Protocol Control Protocol) A protocol used to establish and configure the IP protocol over PPP. This is different to DHCP. ?encapsulation ppp? ? Sets the encapsulation type of the interface to PPP, this will enable the use of username and password authentication. ?dialer string? ?The phone number to dial the ISP. ?dialer-group 1? ? defines the ?interesting traffic? list to use. This is discussed later in this tutorial, it is used to only bring-up the phone line when a certain type of traffic (e.g IP) is sent through it. The number 1 relates to the ?dialer-list? seen further down in the configuration. ?ppp chap hostname? ? this is the username of the dialup account. ?ppp chap password? ? this is the password of the dialup account.

Default Route

ip route Dialer0

This command sets the default route (similar to default gateway) for ALL traffic, in this example all traffic is sent to the Dialer0 interface, setting the default route to the dialup connection.

Interesting Traffic

access-list 101 permit ip any any dialer-list 1 protocol ip list 101

Because phone calls (both analog and ISDN) are billed in time and not traffic usage, it is not a good idea to have the interface constantly up when not in use. Therefore an ?interesting traffic? list is used to let the Dialer know when it should bring the phone line up.

In this example ALL IP traffic to and from anywhere will activate the line, but this list can be homed just as you would any access list to only allow certain type of traffic to or from a certain place to activate the line.

The number of the dialer-list (in this example 1) relates to the ?dialer-group? command number seen in the Dialer0 interface above. The number at the end of the command (in this example 101) relates to the access-list which deny or permit the interesting traffic.

If there is no interesting traffic for a set period of time (dialer idle-timeout) the Dialer interface will bring the line down, saving money!

Configure Line

line aux 0 ?exec-timeout 0 0 ?script dialer DIALOUT ?modem InOut ?modem autoconfigure type MY_USR_MODEM ?no exec ?transport input all ?autoselect ppp ?stopbits 1 ?speed 115200 ?flowcontrol hardware ?password cisco ?login

The final step is to configure the AUX line which will communicate with the modem. Commands entered in interface configuration mode relate to protocol specific aspects, whereas commands entered in line configuration mode let you configure the physical aspects (e.g. Speed, Flow Control, Stop Bits). Notice the ?MY_USR_MODEM? command which relates to the ?moemcap? configuration line discussed earlier.

The maximum speed for an AUX port is 115200, this is sufficient to communicate with any modern modem. The ?line aux 0? could also be referred to as ?line tty 65?.


Debug ppp authentication

Debug dialer For More Information

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========= SNIP =================

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Hi Jim,

There is an online discussion going on right now until September 9th regarding Cisco Dialup Issues with Cisco expert Zulfiqar Ahmed, CCIE#


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Brad Reese BradReese.Com Cisco Repair Service Experts

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