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.
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