Configuring ISDN

For the purpose of your exams, you will need to be familiar with the configuration of ISDN. While many Cisco router models include an integrated ISDN BRI interface, many do not. In cases where an ISDN BRI port is not provided, you have two options. First, you can add a modular BRI port to your router using a WAN interface card. If one is not available (or your router’s chassis isn’t modular), you also have the option of using one of your existing synchronous serial interfaces in conjunction with an ISDN Terminal Adapter (TA).

If you’re located in North America, it’s generally a good idea to purchase a router whose BRI interface already includes an NT1 (this is usually marked next to the interface). If your model includes only an S/T interface, remember that you will require an external NT1 device. In Europe, this is obviously not an issue – you’ll simply need an S/T interface, as NT equipment is located at the service provider’s office.

BRI ports are referenced in the Cisco IOS using the same numbering scheme as other interfaces – the first BRI port will be bri0, the second bri1, and so forth. Configuring a router for ISDN is not terribly difficult, but does require that you have information provided by your service provider. For example, you’ll need to know the ISDN switch type that you’re connecting to, and potentially any Service Profile Identifier (SPID) numbers assigned to you. I’ll discuss both in detail shortly.

To begin, I’m going to assume that your ISDN connection will be permanent. In other words, you are not using demand-dial routing (DDR) to connect locations. We’ll look at DDR connections a little later in this section.

The first step in configuring ISDN on a Cisco router involves specifying the ISDN switch type in use at the service provider’s facilities. A variety of different ISDN switch types exist, and you certainly don’t need to memorize the keywords associated with them all. However, you do need to contact your service provider to find out which switch type they are using in order for your router to connect to their facilities properly. If a switch type is configured from global configuration mode, it will apply to all ISDN interfaces on a router. If configured from interface configuration mode, it will apply to that particular interface only. In this example, you’ll configure our router to use a National ISDN-1 switch, identified by the keyword basic-ni1 (note that the last character in this command is the number “1” rather than a lowercase letter “l”). Common ISDN switch types are listed below. The router used in this example is a Cisco 1604, a model with a built-in BRI interface that includes an NT1 – in other words, it has a “U” interface.

Common ISDN switch types:

AT&T Basic Rate Switch    basic-5ess

Nortel DMS-100 Basic Rate Switch    basic-dms100

NET3 ISDN and Euro ISDN Switch (BRI)    basic-net3

National ISDN-1 Switch (BRI)    basic-ni1

AT&T 4ESS (PRI)    primary-4ess

AT&T 5ESS (PRI)    primary-5ess

Nortel DMS-100 (PRI)    primary-dms100

Cisco1604#config t
Enter configuration commands, one per line.  End with CNTL/Z.
Cisco1604(config)#isdn switch-type basic-ni1

After configuring the ISDN switch type, the next step involves configuring the BRI interface. To access interface configuration mode, use the command int bri0.

Cisco1604(config)#int bri0

The default encapsulation type used on an ISDN BRI interface is HDLC. PPP encapsulation can also be used on ISDN interfaces by issuing the command encapsulation ppp.

Cisco1604(config-if)#encapsulation ppp

Many service providers also require that Service Profile Identifier (SPID – literally pronounced “spid”) numbers be specified in order to establish an ISDN connection with their switching equipment. A SPID is not a phone number, but is usually derived from the associated local dial number of the ISDN line. The purpose of a SPID is not only to identify you to the service provider’s ISDN switch, but is also used to define any additional services that you may have subscribed to (such as call waiting, etc). Not all service providers use SPIDs, so you’ll need to find out if yours does. If a SPID is not configured when required, you won’t be able to connect to the provider’s network. As a general rule, a SPID will be provided for each ISDN B channel, if necessary.

SPID numbers are added from interface configuration mode, using the commands isdn spid1 and isdn spid2 respectively. Many service providers also require that a local directory number (LDN) be provided with the SPID in order to use both B channels. If required, the LDN directly follows the SPID number.

Cisco1604(config-if)#isdn spid1 41655512120101 5551212
Cisco1604(config-if)#isdn spid2 41655512130101 5551213

Having configured your SPIDs, there are only two steps left – assigning the interface an IP address, and issuing the no shutdown command.

Cisco1604(config-if)#ip address
Cisco1604(config-if)#no shutdown

Author: Dan DiNicolo

Dan DiNicolo is a freelance author, consultant, trainer, and the managing editor of He is the author of the CCNA Study Guide found on this site, as well as many books including the PC Magazine titles Windows XP Security Solutions and Windows Vista Security Solutions. Click here to contact Dan.