CCNA Guide to Cisco
Networking
Chapter 11: PPP and ISDN
Objectives
• Describe PPP encapsulation
• Configure PPP encapsulation and its
options
• Describe and enable PPP multilink
• Explain how to implement ISDN BRI on
Cisco routers
• Configure an ISDN BRI connection
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PPP
•
•
•
•
Internet standard protocol
Considered a peer technology
Used over dial-up or leased lines
Support for multiple Network layer
protocols
• Most widely use WAN connection protocol
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PPP (continued)
• SLIP
• Supports
– Encryption
– Compression
– Error correction
• Works with synchronous and asynchronous connections
• Can be applied on many different physical interfaces
– Asynchronous serial
– ISDN synchronous
– High-Speed Serial Interface (HSSI)
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PPP In The Protocol Stack
• Link control protocol
– Data Link layer
• Establish
• Configure
• Test
• Network control protocols (NCPs)
• Allows multiple protocols to used at the same
time
– IP control protocol (IPCP)
– IPX control protocol (IPXCP)
– AppleTalk control protocol (ATCP)
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PPP In The Protocol Stack
(continued)
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Frame Format
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Frame Format (continued)
• Frame fields of PPP
– Flag: Binary sequence 01111110, which indicates the
beginning of the frame
– Address: Binary sequence 11111111; because PPP is
used to create a point-to- point connection, there is no
need for PPP to assign an individual address for each
host
– Control: Binary sequence 00000011, which indicates
that the transmission of user data will not be
sequenced and is to be delivered over a
connectionless link
– Protocol: Two bytes used to identify the protocol that
is encapsulated
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Frame Format (continued)
• Frame fields of PPP (continued)
– LCP or Data: The LCP field is also known as
the Data field, which is the location contains
the LCP information and the data that has
been encapsulated from the higher layers
– Frame Check Sequence (FCS): Cyclical
redundancy check (CRC) to verify the integrity
of the frame
– Flag: Binary sequence 01111110, which
identifies the end of the data frame
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LCP
• LCP field of the PPP packet may include
–
–
–
–
Asynchronous character map
Maximum receive unit size
Compression
Authentication
• PAP
• CHAP
– Magic number
– Link quality monitoring (LQM)
– Multilink
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LCP Link Configuration
• LCP link configuration process includes
– Link establishment
– Authentication (optional)
– Link-quality determination (optional)
– Network layer protocol configuration
negotiation
– Link termination
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Establishing PPP
Communications
• Link establishment phase
– First phase
• Testing and configuration of the data link
– Second phase
• Optional authentication
• PAP and CHAP
– Third phase
• Network layer protocol configuration negotiation
– Data packet transfer may begin
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Establishing PPP
Communications (continued)
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Configuring PPP
Authentication
• Configure PPP authentication on each PPP host
– Link flapping will occur if authentication is only on one
host
• PAP
– Two-way handshake
– Usernames and Passwords sent in clear text
• CHAP
– Three-way handshake
– Usernames and passwords are protected
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Configuring PPP
Authentication (continued)
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Configuring PPP
Authentication (continued)
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Configuring PPP
Authentication (continued)
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Confirming PPP
Communications
• “show interface” command
• Router# show interface serial 0/0
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ISDN
• Circuit-switched service
– Existing telephone service
– Data
– Voice
– Video
– Audio transmission
– Faster than traditional modem speeds
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ISDN (continued)
• Basic Rate Interface (BRI)
– 128 kbps total data transfer bandwidth
– 2 B-channels
• Bearer channel for data transfer
• 64 kbps
– 1 D-channel
•
•
•
•
Delta or data channel for signaling
Out of band signaling
Link Access Procedure-D
16 kbps
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ISDN (continued)
• Primary Rate Interface (PRI)
– 1.544 mbps total data transfer bandwidth
– 23 B-channels
• Bearer channel for data transfer
• 64 kbps
– 1 D-channel
•
•
•
•
Delta or data channel for signaling
Out of band signaling
Link Access Procedure-D
64 kbps
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ISDN (continued)
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ISDN Standards
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ISDN Operations
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ISDN Operations (continued)
• Functions and references
– Function groups
•
•
•
•
•
Terminal adapter
Terminal equipment 1 (TE1)
Terminal equipment 2 (TE2)
Network termination 1 (NT1)
Network termination 2 (NT2)
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ISDN Operations (continued)
• Functions and references (continued)
– Reference points
• U: Demarc between client and telco
• R: Point between non-ISDN equipment (TE2) and
TA
• S: Point between customer’s TE1 or TA and the
network termination (NT1 or NT2)
• T: Point between a NT1 and a NT2
• S/T: Point between TA and NT1 in the absence of a
NT2
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SPID
• Connect to service provider switch
• Service Profile Identifier (SPIDs)
– Dial-in access
– ISDN phone numbers
– Provide a profile
• Service level agreement
– AutoSPID
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SPID (continued)
• Service provider switch types
– dms-100: Northern Telecom DMS-100 (as previously
described)
– ni1: National ISDN-1; used in North America
– net3: Net3 switch; used in Europe and the United
Kingdom
– ntt: Switch from NTT; used in Japan
– 1tr6: 1TR6 switch; used in Germany
– ts013:TS013 Australian switch
– none: Used when a switch has not been specified
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SPID (continued)
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Multilink PPP
• Combine the bandwidth of individual links or
channels
• Multilink provides
–
–
–
–
Load balancing
Packet fragmentation and reassembly
Sequencing for packets
Creates one logical connection
• Function over synchronous or asynchronous
connections
• Router(config-if)# ppp multilink
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DDR
• Dial-on-demand routing (DDR)
• Intermittent WAN access
– Saves money on metered lines
• Used with circuit switched access
• Define interesting traffic
• Common configuration commands
– “dialer-list” command
– “dialer-group” command
– “access-list” command
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DDR (continued)
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DDR (continued)
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ISDN BRI Configuration
Examples
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ISDN BRI Configuration
Examples (continued)
• Dialer profiles
• Scalable compared to legacy DDR
• Configuration of dialer profiles involves
– Dialer interface
• Logical configuration
• Dial string for each destination subnet
– Dialer map class
• Optional commands
• Define characteristics for each call
– Dialer pool
• Identify the physical interface that will be used by the dialer
interface
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ISDN BRI Configuration
Examples (continued)
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Monitoring ISDN
• ISDN problems are often associated with
– PPP configuration
• “debug ppp authentication”
• “debug ppp negotiation”
• “clear interface”
– Dialer configurations
• “show dialer”
• “show interface”
• “debug dialer”
• ISDN monitoring commands
– “show isdn status”
– “show interface bri 0”
– “debug q921”
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Digital Lines
•
•
•
•
•
•
•
•
T1: 24-channel, 1.544 mbps
T1C: 48-channel, 3.152 mbps
T2: 96-channel, 6.312 mbps
T3: 672-channel, 44.376 mbps
T4: 4032-channel, 274.176 mbps
E1: 30-channel, 2.048 mbps
Fractional T1 or E1
Digital Subscriber Line (DSL)
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Summary
• Many WAN connectivity options are available for
modern networks, including digital lines, Frame
Relay, and analog modems
• WAN technologies typically define Data Link and
Physical layer standards
• The Point-to-Point Protocol (PPP) is the most
widely used WAN protocol today
• On Cisco routers, PPP is used mainly as a Data
Link layer encapsulation method; however, it
does provide an interface to the Network layer
via specific Network Control Protocols (NCPs)
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Summary (continued)
• PPP provides link establishment, quality determination,
Network layer protocol encapsulation, and link
termination services
• PPP is often used over Integrated Services Digital
Network (ISDN) connections
• ISDN is a digital service provided by several
telecommunications companies worldwide
• ISDN was developed as a faster WAN connection to
replace analog modems, and as a cheaper alternative to
Frame Relay and full T1 connections
• ISDN service comes in Basic Rate Interface (BRI) and
Primary Rate Interface (PRI)
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Summary (continued)
• BRI offers connections of up to 128 Kbps for data
transfer, and PRI offers up to 24 channels in the United
States, each with the ability to transfer data at 64 Kbps
• Connections over ISDN can take advantage of dial-ondemand routing (DDR) and multilink services offered
through PPP connections
• DDR allows the router using an ISDN connection to dial
only when there is interesting traffic and to add more
channels as needed to support given traffic levels
• DDR can be implemented using dialer profiles, which
allow more flexibility regarding the calling parameters
• Multilink allows ISDN to use multiple channels evenly by
spreading the load across those channels
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