Anatomy of BRI

When configuring an ISDN device for BRI service, you must have the proper information for communication on the service provider's network. First and foremost, you need to know what type of switch you are communicating with. The switch type is important because not all vendors have implemented ISDN in the same way. For this reason, if you tell your equipment that it is communicating with a Lucent 5ESS, but you are actually connected to a Nortel DMS-100, you might not be able to access the ISDN network. Refer to Cisco documentation for all the supported ISDN switch types.

Switch vendors began to diverge on compatibility in the United States in the early to mid-90's, so a set of standards was developed to help alleviate this problem. Contrary to what a lot of people say, NI-1 is not an actual switch, but a set of programming that allows many vendors to communicate with one another over the ISDN network. The National ISDN standards were developed as a three-stage process: NI-1, NI-2, and NI-3 to be deployed over a period of about 10 years. These standards allow customer ISDN equipment to internetwork with a larger number of vendors. Thus, Lucent doesn't have to figure out how to directly communicate with a Nortel switch in all aspects of ISDN. They merely have to program their switches to adhere to the National ISDN standards. The caveat with this is that if you are using a private ISDN configuration, you can use whatever switch type you want.

It is perfectly conceivable to use the primary-ni switch type on E1 circuits as long as they are consistent on both sides of the connection. On back-to-back configurations, it is also necessary to specify one of the two sides as the network. Cisco ISDN devices default to user, and two user devices on the same link do not work. If you have both sides set to user, Layer 2 never comes up.

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