Chapter2 ppp

Point-to-Point Protocol
(PPP)

Accessing the WAN – Chapter 2
Glenn Avendano

ITE I Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 1

Describe the Fundamental Concepts of
Point-to-Point Serial Communication
 If parallel is faster than serial, is parallel more suitable
for connecting to a WAN? In reality, it is often the case
that serial links can be clocked considerably faster than
parallel links, and they achieve a higher data rate,
because of two factors that affect parallel
communications: clock skew and crosstalk interference.
 The more wires one uses and the farther the
connection reaches, compounds the problem and adds
delay. The need for clocking slows parallel transmission
well below theoretical expectations. Most serial links do
not need clocking.
 Since serial cables have fewer wires, there is less
crosstalk, and network devices transmit serial
communications at higher, more efficient frequencies.

ITE 1 Chapter 6 © 2006 Cisco Systems, Inc. All rights reserved. Cisco Public 2

 Serial Communication Standards
 RS-232 - Most serial ports on personal computers conform
to the RS-232C or newer RS-422 and RS-423 standards.
Both 9-pin and 25-pin connectors are used. Many network
devices use RJ-45 connectors that also conform to the RS-
232 standard.
 V.35 - Typically used for modem-to-multiplexer
communication, this ITU standard for high-speed,
synchronous data exchange combines the bandwidth of
several telephone circuits. In the U.S., V.35 is the interface
standard used by most routers and DSUs that connect to T1
carriers.
 HSSI - A High-Speed Serial Interface (HSSI) supports
transmission rates up to 52 Mbps. Engineers use HSSI to
connect routers on LANs with WANs over high-speed lines
such as T3 lines.


 Explain how two or more data streams are transported
across a single physical connection using TDM


 Define the location of the demarcation point relative to
customer and service provider networks


 Explain the terms DTE and DCE with relative to the
location of devices in a network


 Describe how high-level data link control (HDLC) uses
one of three frame types to encapsulate data


 Cisco HDLC is the default encapsulation method used
by Cisco devices on synchronous serial lines.
 One may use Cisco HDLC as a point-to-point protocol
on leased lines between two Cisco devices. If you are
connecting to a non-Cisco device, use synchronous
PPP.


 Click the status button on page 2.1.7.1 to identify the
following five possible problem states in the interface
status line: <from sh int serial command>


Activity


Describe Point-to-Point Concepts
 HDLC is the default encapsulation method between
Cisco routers. Use PPP to connect to a non-Cisco
router.
 The link quality management feature monitors the
quality of the link. If too many errors are detected, PPP
takes the link down.
 PPP supports PAP and CHAP authentication.
 PPP components: HDLC protocol for encapsulating
datagrams over point-to-point links; Extensible Link
Control Protocol (LCP) to establish, configure, and test
the data link connection; family of Network Control
Protocols (NCPs) for establishing and configuring
different network layer protocols.

 PPP Architecture: Physical Layer


The LCP provides automatic configuration of the
interfaces at each end, including:
 Handling varying limits on packet size
 Detecting common misconfiguration errors
 Terminating the link
 Determining when a link is functioning properly or when
it is failing
–PPP also uses the LCP to agree automatically on
encapsulation formats (authentication, compression, error
detection) as soon as the link is established.


 PPP permits multiple network layer protocols to operate
on the same communications link. For every network
layer protocol used, PPP uses a separate NCP.
 NCPs include functional fields containing standardized
codes to indicate the network layer protocol that PPP
encapsulates.


 Describe the purpose and format of each of the fields in
a PPP frame

Roll over the packet fields on page 2.2.3.1 to view descriptions.


 Define the three phases of PPP session establishment


 PPP can be configured to support various functions
including: Authentication using either PAP or CHAP,
Compression using either Stacker or Predictor, Multilink
which combines two or more channels to increase the WAN
bandwidth.


Activity 2 (page 2.2.6.2)


Activity 4 (page 2.2.6.2)


Configure PPP on a Serial Interface


 Explain the output of the show interfaces serial
command


 The no form of this command disables debugging
output.


 The figure shows the output of the debug ppp
negotiation command in a normal negotiation, where
both sides agree on network control program (NCP)
parameters. In this case, protocol type IP is proposed
and acknowledged.


Configuring PPP with Authentication
 PPP Authentication – RFC 1334


 PAP is not a strong authentication protocol. But, there
are times when using PAP can be justified.


 Unlike PAP, which only authenticates once, CHAP conducts
periodic challenges to make sure that the remote node still has a
valid password value.


 Configuring PAP

DTE 1 7 2 .2 5 .3 .0 /2 4 DCE
.2 /S 0 S e r ia l .1 /S 0
hostname SantaCruz hostname HQ
username HQ password HQpass username SantaCruz password SantaCruzpass

interface Serial0 interface Serial0
ip address 172.25.3.2 255.255.255.0 ip address 172.25.3.1 255.255.255.0
encapsulation ppp encapsulation ppp
ppp authentication pap ppp authentication pap
ppp pap sent-username SantaCruz ppp pap sent-username HQ
password SantaCruzpass password HQpass


 Configuring PAP

DTE 1 7 2 .2 5 .3 .0 /2 4 DCE
.2 /S 0 S e r ia l .1 /S 0
username HQ password HQpass username SantaCruz password SantaCruzpass

ppp authentication pap ppp authentication pap
ppp pap sent-username SantaCruz ppp pap sent-username HQ
password SantaCruzpass password HQpass

Notes: sent-username and password must match remote username and
password. Passwords are case-sensitive, but usernames are not.
Hostnames are not involved.

 Configuring CHAP

DTE 1 7 2 .2 5 .3 .0 /2 4 DCE
.2 /S 0 S e r ia l .1 /S 0
username HQ password boardwalk username SantaCruz password boardwalk

ppp authentication chap ppp authentication chap


 Explain how to configure a PPP connection with
authentication


 Explain the output of the debug ppp authentication
command


Summary

 PPP is a widely used WAN protocol
 PPP provides multi-protocol LAN to WAN connections
 PPP session establishment – 4 phases
Link establishment
Link quality determination
Network layer protocol configuration negotiation
Link termination
 WAN Encapsulation
–HDLC default encapsulation
–PPP


Summary
 PPP authentication
–PAP
•2 way handshake
–CHAP
•3 way handshake
–Use debug ppp authentication to confirm authentication
configuration
 PPP configuration
–Done on a serial interface

 After PPP configuration, use show interfaces command
to display:
–LCP state
–NCP state


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