Configuring ISDN

1
Configuring ISDN
2
Integrated Services Digital Network (ISDN)

• Conceived as a means of replacing the PSTN with
  an all digital network
• Although more successful in other parts of the
  world, ISDN has never achieved a high level of
  adoption

3
Standards

• E-Series
• Telephone network standards (e.g. E.163
  International telephone numbering plan)
• I-Series
• Concepts, terminology and general methods (e.g.
  I.100 General ISDN concepts, structures, and
  terminology)
• Q-Series
• Switching and signaling operation (e.g. Q.921
  LAPD ISDN data link processes or Q.931 OSI
  Layer 3 functions of ISDN)

4
ISDN Channels

• D Channel
• Operate at 16 or 64 Kbps
• Used for network signaling
• B Channel
• Operate at 64 Kbps (DS-0)
• H Channel
• H0 384 Kbps, H1 1.536 Mbps, H2 1.92 Mbps

5
ISDN Services

• Basic Rate Interface (BRI)
• 2 B Channels 1 D Channel (2BD)
• Up to 128 Kbps (sometimes up to 144 Kbps)
• Primary Rate Interface (PRI)
• Usually 23 B Channels 1 (64 Kbps) D Channel
  (23BD)
• Up to 1.544 Mbps

6
Function Groups

• Terminal Equipment 1 (TE1)
• ISDN compatible DTE
• Router with a BRI interface
• Terminal Equipment 2 (TE2)
• Non-ISDN compatible DTE
• Router without a BRI interface
• Network Terminal Adapter (TA)
• ISDN Modem used to connected non-ISDN compatible
  device (TE2) to ISDN network
• Network Termination Type 1 and 2 (NT1 or NT2)
• Terminate ISDN local loop at customer premise

7
Reference Points

• R between non-ISDN equipment (TE2) and a TA
• Usually a physical layer connection like TIA/EIA
  232 or V.35
• S between router and an NT2 device
• T between NT1 and NT2 devices
• When no NT2 is present then the S and T reference
  points are combined into the S/T reference point
• U between NT1 device and carriers DCE
• Outside North America the NT1 is provided by the
  carrier so the U reference point is irrelevant

8
Reference Points and Function Groups
TA
NT2
NT1
S
T
U
R
TE2
NT2
NT1
TE1
S
T
U
The U reference point is only relevant in North
America
When no NT2 is used then the CPE-to-carrier
interface is called an S/T interface
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Service Profile Identifiers (SPIDs)

• SPIDs may or may not be required by the carrier
• Less common outside North America
• SPIDs identify the setup and configuration
  information of an ISDN BRI

SPID LDN
Interface BRI0 Isdn spid1 0835866201 8358662 Isdn
spid2 0835866401 8358664
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Encapsulation

• ISDN uses HDLC by default but can use PPP or LAPD
  
• PPP should be used for its superior
  authentication, compression and mutliprotocol
  support
• Multilink PPP allows combining D channels

11
Configuring Multilink PPP

• Dialer-map command associates remote IP addresses
  (next hop) with ISDN LDNs for making outgoing
  calls
• Dial load-threshold command indicates what
  percentage of bandwidth triggers bringing up an
  additional link

dialer map ltprotocolgt ltaddressgt name lthostnamegt
broadcast ltLDNgt
dialer load-threshold lt1-255gt ltinboundoutboundei
thergt
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Configuring Multilink PPP
Interface BRI0 ip address 172.19.1.6
255.255.255.252 encapsulation ppp dialer map
ip 172.19.1.5 name r2 broadcast 8358662 dialer
map ip 172.19.1.5 name r2 broadcast 8358664
dialer load-threshold 1 either isdn switch-type
basic-ni isdn spid1 0835866101 8358661 isdn
spid2 0835866301 8358663 ppp authentication
chap ppp multilink
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Dial-on-Demand Routing

• DDR allows a router to establish a connection on
  a switched network only when interesting traffic
  needs to be transmitted
• Connection remains open as long as there is
  interesting traffic or until a idle timer expires
• Uninteresting traffic is only transmitted if the
  connection is already established and there is no
  interesting traffic to transmit

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Outbound Packet
No
No
Currently Connected?
Drop Packet
Interesting Traffic?
Drop Packet
Yes
No
No
Currently Connected?
Dial String Configured?
Yes
Yes
Yes
Reset Idle Timer
Dial Number
Send Packet
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Dynamic or Static Routing

• Dynamic routing has the disadvantage that routing
  updates will bring up the connection
  periodically
• Static routing has the disadvantage that the
  router will not be able to determine when the
  connection is not available
• If using static routes to connect to a central
  cite that uses dynamic routing you must
  redistribute static routes

Router RIP network 172.168.1.0 redistribute
static
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Specifying Interesting Traffic

• Interesting traffic is specified with a dialer
  list

dialer list ltnumbergt protocol ltprotocolgt
ltpermitdenylist access-list-numbergt Example
allows all IP traffic dialer list 1 protocol IP
permit interface BRI0 dialer-group 1
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Controlling Connection

• A map command for each protocol and destination
  must be specified
• A threshold for establishing additional
  connections must be established if multilink PPP
  is used

dialer map ltprotocolgt ltaddressgt name lthostnamegt
broadcast ltLDNgt
dialer load-threshold lt1-255gt ltinboundoutboundei
thergt
18
Controlling Connection

• The idle timer must be set

dialer idle-timeout ltsecondsgt
19
Legacy DDR
Dialer-list 1 protocol ip permit Interface
BRI0 ip address 10.1.0.1 255.255.0.0
encapsulation ppp Isdn switch-type basic ni1
dialer map ip 10.1.0.2 name r2 broadcast 8358661
dialer map ip 10.1.0.2 name r2 broadcast
8358663 dialer-group 1 dialer load-threshold
40 either dialer idle-timeout 300 isdn spid1
0835866101 8358661 isdn spid2 0835866301
8358663 ppp multilink
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Dialer Profiles

• A dialer profile is a logical interface that is
  bound to the physical BRI interface when a call
  is made
• Dialer profiles dont use static dialer maps, the
  dialer map is created dynamically
• The physical interface is assigned to a dialer
  pool

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First Configure the Physical Interface
interface BRI0 no ip address encapsulation
ppp ppp authentication chap dialer
pool-member 1
dialer pool-member ltnumbergt priority priority
min-link minimum max-link maximum
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Next Configure the Dialer Interface
interface Dialer0 ip address 172.19.1.5
255.255.255.252 encapsulation ppp dialer
remote-name r2 dialer string 8358661 dialer
pool 1 dialer-group 2 ppp authentication
chap Dialer-list 2 protocol ip permit