CCNA Guide to Cisco Networking Chapter 11: PPP and ISDN

CCNA Guide to Cisco Networking

Chapter 11: PPP and ISDN


2

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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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-on-demand 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

Documents

CCNA Guide to Cisco Networking Chapter 11: PPP and ISDN