ISDN Basics and troubleshooting.

                ISDN- Integrated service digital network a technology which is used majorly for backup solution and on demand connectivity. From the scenarios i have worked on related to ISDN the design is either a backup solution where a floating static route is configured so that when the primary WAN link is down ISDN is triggered or  on demand for example when only a file transfer is needed.

 ISDN is a group of standards that define how Voice and Data traffic are setup dynamically. Supports two type of connections BRI(Basic rate interface) and PRI (Primary rate interface). Bandwidth guaranteed.

BRI has two bearer channels(B channel) and 1 signalling channel(D channel). B channel is used to tranfer voice,video and Data. D channel is used for signalling and syncronization. Note that each channel is 64 Kbps.

PRI can be used with E1 or T1. PRI E1 has 20 B channels and 1 channel each for signaling and framing/clocking.

ISDN uses three protocols:

1. Q.921 is the signaling protocol used at layer 2 for the switch and router to communicate with each other.
2.LAPD is the framing protocol used.
3.Q.931 is the layer 3 protocol used to setup and tear down phone connections between ISDN end point  devices


Dial on demand routing(DDR) isused in ISDN. Dialer profiles consists of two interfaces

1. Physical interface : BRI and PRI, contains physical properties (encapsulation,ISDN switch type etc) and to which pool this belongs to.

interface BRI0/0
no ip address
encapsulation PPP
dialer pool member 1 max-link 1         // Map the physical interface to a dialer pool.
isdn switch-type basic-net3                 //Mention the switch type supported both ends

2. Logical DDR interface : Where most of the configuration is done.(Dialer interface ,ISDN numbers,authentication etc).All configuration settings specific to the destination go in the dialer interface configuration. Each dialer interface uses a dialer pool, which is a pool of physical interfaces

interface dialer 1
ip addess 1.1.1.1 255.255.255.255          //IP address is configured on logical interface
encapsulation ppp
dialer pool 1                                             // Configure dialer pool. Physical interface will be mapped to this
dialer string 12345                                   //Number where this device needs to dial in
dialer group 1
ppp authentication chap                           // Authentication type

For PAP and CHAP authentication you will need to configure a username and password both ends.
username X password *****

When we say ISDN is on demand  solution we need to understand what triggers ISDN on demand. ISDN uses a concept of interesting traffic that triggers. This is nothing but what communication should bring the line up.

dialer-list 1 protocol ip permit

Above command would allow any traffic to trigger ISDN , If you requirement is only any backup communication or particular traffic (ex:SMTP,FTP ect) then you need to configure a access list and then map with dialer-list.

access-list 5 permit ip any host 1.1.1.1
dialer-list 1 protocol ip list 5.

Now the order of operations in ISDN
 1. Usually for backup solution a backup interface command is used on the WAN link so that the dialer comes up only when the primary link goes down.
2. Once the dialer is up,dialer profile looks for interesting traffic and if matched it will pull a physial   interface from dialer pool.
3. A dialer strings used to make a call.(if multiple strings are configured its processed the way its configured)
4. Authentication process started between the two devices.

ISDN TEST call a Savior
Yes, you can perform a test call to check if all is working. A test call will last for 120 seconds. During the test call show isdn status will help you identify if any issue.

isdn test call interface BRI x/x 12345

 It's important to know the proper show and debug commands for ISDN for several reasons.You should know the order of operation before troubleshooting ISDN. You might be debugging layer 3 when there is a isue at layer 2 or authentication. 
Debug isdn q921 :   used to debug layer 2 issue
Debug isdn q931 : Used to debug layer 3 issue
Debug ppp authentication : Used to debug PAP or CHAP authentication
Debug ppp negotiation : Used to debug LCP issue


Few  scenarios i used to get a call for.
ISDN test call not happening , check ISDN status.
ISDN call triggering dropping immediately , check authentication.
ISDN call triggering but dropping after 20 seconds , debug layer 3.
ISDN test call happening , but not triggering when leased line goes down. Check access list for interesting traffic

Further Reading:

ISDN DDR configuration

ISDN status that helps determining the cause

ISDN Layer -3 debug explained

ISDN BRI troubleshoot flow chart ( very helpful)