The process for establishing an ISDN connection is initiated when the channel D, which is always active, it sends the number you call the ISDN switch. Traffic on the D channel protocol uses the Link Access Procedure on D channel (LAPD). The LAPD is a link layer protocol of HDLC-based data. The local switch uses the protocols SS7 (Signaling System 7) to set the path to the terminating switch which ended the destination on the D channel.
The channel B mode is then connected end-to-end to send voice or data per channel, or both at once.
The devices must negotiate which protocol to use common data link (PPP, X.25 or Frame-Relay).
Enabling ISDN BRI
Once installed the ISDN service, the service provider give information about the switch type and SPID. The configuration of the isdn switch-type command in global configuration mode defines the same ISDN switch type for all ISDN interfaces.
After executing the global configuration command, each interface can be configured individually to reflect a different type of switch.
Router (config) # isdn switch-type switch-type
Router (config-if) # isdn switch-type switch-type
The SPID is used to define the services available to each individual subscriber of ISDN service. Depending on the switch, you may have to add this SPID to the configuration. To define a SPID, use the isdn spid command # in the interface configuration mode.
The following command is used to define the SPID numbers have been assigned to channel B.
Router (config-if) # isdn spid1 spid-number [ldn]
Router (config-if) # isdn spid2 spid-number [ldn]
ldn: The optional argument defines a number of local dial directory.