| Engineering Journal 5.5.4.2: The LDN argument The ldn argument is optional and indicates the local directory number (LDN), as delivered by the service provider in the incoming setup message. This is a seven-digit number also assigned by the service provider. This command is required for DMS-100 and National ISDN-1 (NI-1) switches only. You must define the LDN if you want to receive any incoming calls on the B1 channel. The ISDN switch checks for the LDN to determine whether both channels can be used to transmit and receive data. If the LDN is not present, then only the B2 channel can be used for full-duplex communication. However, the other channel can still be used for making outgoing calls. The following example defines, on the router, a SPID and an LDN for the B2 channel: isdn spid2 415555121202 5551214 Each SPID points to line setup and configuration information. When a device attempts to connect to the ISDN network, it performs a D channel Layer 2 initialization process that causes a TEI to be assigned to the device. The device then attempts D channel Layer 3 initialization. If SPIDs are necessary but not configured or are configured incorrectly on the device, the Layer 3 initialization fails, and the ISDN services cannot be used. There is no standard format for SPIDs. As a result, SPID numbers vary depending on the switch vendor and the carrier. A typical Cisco IOS SPID configuration is as follows: interface bri 0 isdn spid1 0835866201 8358662 isdn spid2 0835866401 8358664 These commands also specify the LDN, which is the seven-digit number assigned by the service provider and used for call routing. The LDN is not necessary for establishing ISDN-based connections, but it must be specified if you want to receive incoming calls on a B2 channel. The LDN is required only when two SPIDs are configured (for example, when you're connecting to a DMS or NI1 switch). Each SPID is associated with an LDN. Configuring the LDN causes incoming calls to the B2 channel to be answered properly. If the LDN is not configured, incoming calls to the B2 channel might fail.