We will use a 3103 (1E/1BRI) router for our example, configured to route IP over a Dial-on-Demand Routing(DDR) link. Switch types refer to the ISDN software that runs on the switch, not the manufacturer or the model. You'll need a basic understanding of IP routing in order to understand this document. See the document "Beginning IP For New Users" for more information.
---------- _________ ----------
| |BRI 0 ( ) BRI 0| |
| REMOTE |---------( ISDN )--------| MAIN |
| | (_________) | |
---------- ----------
--------------------------------------------------------------------------
| Configuration of remote : Configuration of main |
|-----------------------------------:------------------------------------|
| : |
| hostname branch1 : hostname main |
| ! : ! |
| username main password secret1 : username branch1 password secret1 |
| ! : username branch2 password secret2 |
| isdn switch-type basic-dms100 : ! |
| ! : isdn switch-type basic-dms100 |
| interface Ethernet 0 : ! |
| ip address 131.108.64.190 : interface Ethernet 0 |
| 255.255.255.0 : |
| : ip address 131.108.38.1 |
| ! : 255.255.255.0 |
| interface BRI 0 : ! |
| encapsulation PPP : interface BRI 0 |
| ip address 131.108.157.1 : encapsulation PPP |
| 255.255.255.0 : |
| : ip address 131.108.157.2 |
| isdn spid1 415988488501 9884885 : 255.255.255.0 |
| isdn spid2 415988488602 9884886 : isdn spid1 415988488201 9884882 |
| ppp authenticate chap : isdn spid2 415988488302 9884883 |
| dialer idle-timeout 300 : ppp authenticate chap |
| dialer map IP 131.108.157.2 name : dialer idle-timeout 300 |
| main 4883 : |
| : dialer map IP 131.108.157.1 name |
| dialer-group 1 : branch1 4885 |
| ! : dialer-group 1 |
| ip route 131.108.0.0 255.255.0.0 : ! |
| 131.108.157.2 : |
| : ip route 131.108.64.0 |
| ip route 0.0.0.0 131.108.157.2 : 255.255.255.0 131.108.157.1 |
| ! : ! |
| dialer-list 1 protocol ip permit : dialer-list 1 protocol ip permit |
--------------------------------------------------------------------------
hostname name
The hostname of the router is used to identify the router to another router when sending CHAP messages.
username name password secret
The username command is required and is used to specify the CHAP secret to use when challenged by another router. The two routers that need to talk must share the same password.
isdn switch-type switch-type
isdn spid1 spid-number [ldn], isdn spid2 spid-number [ldn]
This router connects to a Northern Telecom DMS-100 switch. The Service Profile Identifiers (SPIDs) for the station are 415988488501 and 415988488602. These numbers were configured on the switch and will be given to a user by the local service provider. 9884885 and 9884886 are regular phone numbers for the two channels. They are normally called Local Directory Numbers (LDNs). The isdn spid1.. and isdn spid2.. commands are not required for AT&T 5ESS switch software configured for point-to-point operation, but they are required for National ISDN-1 (NI1) software. Remember, switch types in this document refer to the ISDN software that runs on the switches, not the manufacturer or the model.
These are the only ISDN-specific commands required. The rest of the configuration is really DDR configuration and applies to other kinds of DDR interfaces, like in-band and asynchronous.
dialer-group group number
dialer-list dialer-group protocol protocol-name
{permit | deny}
dialer-list dialer-group list access-list-number
These are required to define "interesting" packet types. An interesting packet is a packet that will activate the dialing sequence and will reset the "idle-timeout" timer. In this example, all IP packets are interesting on one side, and an access list is used on the other to demonstrate the two forms of the command. On branch1 all IP packets are interesting, but only non-IGRP packets are interesting on main.
ppp authentication chap
Specifies that CHAP athentication is to be used on this interface. See the explanation of dialer map for more details.
dialer idle-timeout seconds
This makes the router disconnect calls that haven't had any interesting traffic for the configured time. Only outbound packets reset the idle timer.
dialer map protocol name remote-name broadcast speed 56 phone-number
The dialer map is used to distinguish between different remote sites, which can be connected at various times. A map is required for each site and each protocol used. The remote name is the hostname of the other router. The broadcast flag specifies whether broadcast packets, such as routing updates, are sent for this protocol to this site, and it is off in our example. The speed parameter defaults to 64; setting it to 56 may be necessary allow the call to complete successfully over networks that are not end-to-end ISDN. The phone number is used when originating calls, or authenticating calls in networks with calling number delivery. The number can be left out to create a "receive-only" dialer map. Our example shows a four-digit interoffice extension number, but a valid phone number of any length will work.
Dialer maps provide the network-layer-to-link-layer mapping required by the router to successfully encapsulate, or send packets out an interface. In this example, it includes the phone number to call and the name of the other router. The name is essential for identifying calling routers, through the use of CHAP (Challenge Handshake Authentication Protocol). CHAP also provides some security, but is included in this basic configuration mainly because it allows identification of calling routers. In the general case, CHAP is required for identification of the calling router because calling number delivery is not always possible.
ip route network [mask] {address | interface} [distance]
Provides the static routing information necessary in order for the packets to route packets to the proper next hop. Routing of packets to a DDR interface is what creates the Demand, which as the name implies, is an essential part of Dial-on-Demand Routing.