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ISDN and Dial-On Demand-Routing

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ISDN and Dial-On Demand-Routing

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Cisco CCNA ICND Study Guide

By Cbrzana

Table Of Contents

ISDN and Dial-On Demand-Routing

Typical Uses of ISDN

  • Occasional access (periodic need to connect)
  • Backup link (when frame relay/primary leased line fails)

ISDN Channels

  • Both BRI and PRI have digital bearer channels (B channels)
  • B channels transport data, operate up to 64 kbps per channel
  • Signals new data calls with D, or signaling, channel (16 kbps on BRI, 64 kbps otherwise)

Type of Interface of B channels of D channels Descriptive Term
BRI 2 1 (16 kbps) 2B+D
PRI (T1) 23 1 (64 kbps) 23B+D
PRI (E1) 30 1 (64 kbps) 30B+D

ISND Protocols

Issue Protocol Examples
Telephone network/ ISDN E-series E.163 – International telephone numbering plan

E.164 - International ISDN addressing
ISND concepts, aspects, and interfaces I-series I.100 series – Concepts, structures, and terminology

I.400 series – User-Network interface
Switching and signaling Q-series Q.921 – Link Access Procedure on the D channel (LAPD)

Q.931 – ISDN network layer

OSI Layer Comparisons

OSI Layer I-Series Q-Series Description
1 ITU-T I.430

ITU-T I.431
N/A Connectors, encoding, framing, reference points
2 ITU-T I.440

ITU-T I.441
ITU-T Q.920

ITU-T Q.921
Defines LAPD protocol, encapsulate signal requests
3 ITU-T I.450

ITU-T I.451
ITU-T Q.930

ITU-T Q.931
Defines signaling messages (call setup/teardown)

Tips To Remember

2nd 'digit in Q-series = OSI layer.
2nd '
digit in I-series = 2 more than OSI layer

Between local switch and router = Q.931

Between two switches = Signaling System 7 (SS7)

Out-of-band Signaling: D channel signals on different band than data transmission

ISND Switch Authentication: Service Profiler Identifier (SPID), uses free-form decimal value

ISDN BRI Function Groups and Reference Points

Function Group: A set of functions implemented by a device and software
Reference Point: The interface between two function groups, including cabling details

ISDN Interfaces
U = no other device required
S/T = cabled to function group NT1

Serial Interfaces
Function group Terminal Equipment 2 (TE2) and connect to Terminal Adapter (TA)

Function Group/Reference Point Summary

Function Groups Reference Points Type of Interface in Router
TE1, NT1 U ISDN Card, U interface
TE1 S/T ISDN Card, S/T interface
TE2 R Serial interface
TE1 S ISDN card, S/T interface

Function Group Definitions

Function Group Meaning
TE1 Terminal Equipment 1
TE2 Terminal Equipment 2
TA Terminal Adapter
NT1 Network Termination Type 1
NT2 Network Termination Type 2

Reference Points

Reference Point What it Connects Between
R TE2 and TA
S TE1 or (TA and NT2)
T NT2 and NT1
U NT1 and the telco
S/T TE1 or TA  NT1 (no NT2 used)


TE1 / TA  NT1/NT2

** Note **

Home-based ISDN modems include TA and NT1, serial port connects to TA

PRI Function Groups/Reference Points: Designed for businesses, so no function groups or reference points were defined (thank God!)

PRI Encoding: Alternate Mark Inversion (AMI) or Binary 8 with Zero Substitution (B8ZS).  Match what the telco is using.
E1 only choice is High-Density Bipolar 3 (HDB3)

PRI Framing: Distinguishes between the D channel and the B channels.
* Extended Super Frame (ESF)  newer, used by most T1’s today
* Super Frame (SF)

E1s use CRC-4 (tell router whether to enable it or not)

T1 = 24th 'channel is the D channel
E1 =
15th '
channel is the D channel (0-14, and 16-30 are B channel)

ISDN Configuration and Dial-on-demand Routing

Legacy DDR associates dial configuration with physical interface.

DDR Dialer Profiles dissociates dial configuration from physical interface.

-> Provides for great flexibility

Concepts on how Legacy DDR Works

1. Route packets out the interface to be dialed.
a. Static routes must be configured (ip route command)

2. Determine the subset of packets that trigger signaling process.
a. Interesting packets cause the dial to occur, all other are "boring"
i. Interesting by either L3 protocol, or if ACL allows them

To assign by L3 Protocol
interface [interface]
dialer-list 1 protocol ip permit

To assign by permission
interface [interface]
access-list 101 permit top any host eq 80
dialer-list 2 protocol ip list 101

3. Dial (signal).
a. configure the number to call (single connection):
dialer string [phone number]

b. For multiple routes, need a mapping (IP and their phone numbers)
Add username/password for CHAP support:
username [username] password [password]
dialer map ip [IP address] broadcast name [username] [phone #]

4. Determine when connection terminates.
a. Although interesting and boring packets can be sent, only interesting count towards idle time.
dialer idle-timeout [seconds]
- or -
dialer fast-idle [seconds] (Used to bring down more quickly)

ISDN BRI Configuration

Use global command to tell the IOS what type of ISDN switch the router is connected to:

isdn switch-type [type]

The types are as follows:

Type of Switch Where it is Found
basic-net3 Australia,Europe,UK
vn3 France
ntt Japan
basic-5ess North America
basic-dms100 North America
basic-ni1 North America

To configure Service Profile Identifier:

isdn spid [SPID #]

ISDN Show/Debug Commands
show interfaces bri 0:1

show dialer interface bri 0
* "Dial Reason"
* Time Until Disconnect
* B channel information

show isdn active
* Called
* Active second (duration)
* Remote name

show isdn status
* L1-L3 status
* of active calls
* of available B channels
* Switch status

debug isdn q931
* Show signaling to setup the called
* disable with no debug all command

debug dialer [events/packets]
show interesting packets that caused the call to occur

ISDN PRI Configuration
* Configure the type of switch to which the router is connected
* Configure the T1 or E1 encoding and framing options (controller config mode)
* Configure the T1/E1 channel range for the DS0 channels
* Configure interface settings (encapsulation, address)

Controller Configuration Mode allows you to configure physical layer parameters such as encoding, framing, and channels that are in use.

controller t1 1/0  Specifies which controller to configure
framing esf  Type of framing used
linecode b8zs  Type of encoding used
pri-group timeslots 1-24  What channels will be used (typically all of them)

ISND PRI Configuration

Full PRI Configuration: All previous settings that were on BRI, plus the ones above.


** Shaded lines are ones added for PRI configuration **

interface serial 1/0:23

  • Identifies the D channel that needs to be configured (23rd channel)
  • B channels are designated as 0-22, D channel is last channel (23)

* Note *

SPIDs are not configured, since PRIs do not use them.

DDR Configuration with Dialer Profilers

Legacy DDR doesn’t support a single set of remote sites through configuration using multiple BRIs or PRIs in a single router (only allows one set of sites per interface).

Dialer Profilers allow this by using Dialer profiler pools, that pool available B channels.
: Virtual Interface called Dialer Interface


interface dialer '''x 'Creates virtual dialer interface
dialer pool-member '''
x Groups the physical ISDN interfaces into a dialer pool
dialer pool '''
x ' Tells the dialer interface which dialer pool to use.

* Requires separate subnet for each dialer interface
* Uses the
dialer string [number] command (not the map command, no maps used)

MultiLink PPP

Allows multiple link between a router and some other device.

MLP can break larger packets into smaller segments, and send over multiple links (receive faster over multiple lines)  load balances them

* MLP treats multiple links as a single link, with one route in the routing table. *

ppp multilink  Enable MLP
dialer-load threshold load [inbound | outbound | either]
-> Distribute across if load is at this certain percentage (for example, 25)

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