Ppp Hdlc Pap Chap

8/18/2019 Ppp Hdlc Pap Chap

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The High-Level Data-Link Control (HDLC) protocol is a popular ISO-

standard, bit-oriented Data Link laer protocol!

It speci"ies an encapsulation #ethod "or data on snchronous serial data

links using "ra#e characters and checksu#s!

HDLC is a point-to-point protocol used on leased lines! $o authentication can

be used %ith HDLC!

HDLC is the de"ault encapsulation used b Cisco routers over snchronous

serial links!

Cisco&s HDLC is proprietar'it %on&t co##unicate %ith an other vendor&s

HDLC i#ple#entation!

ach vendor has a di""erent %a "or the HDLC protocol to encapsulate

#ultiple $et%ork laer protocols!


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Cisco HDLC frame format


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oint-to-oint rotocol () is a Data Link laer protocol

that can be used over either asnchronous serial (dial-up) or

snchronous serial (ISD$) #edia!

It uses the LC (Link Control rotocol) to build and

#aintain data-link connections!

$et%ork Control rotocol ($C) is used to allo% #ultiple

$et%ork laer protocols (routed protocols) to be used on a

point-to-point connection!

The basic purpose o" is to transport laer * packets

across a Data Link laer point-to-point link!


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contains "our #ain co#ponents+

EIA/TIA-232-C, V.24, V.35, and ISD - hsical laer international

standard "or serial co##unication!

HDLC - #ethod "or encapsulating datagra#s over serial links!

LC! - #ethod o" establishing, con"iguring, #aintaining, and

ter#inating the point-to-point connection!

C! - #ethod o" establishing and con"iguring di""erent $et%ork

laer protocols! $C is designed to allo% the si#ultaneous use o"#ultiple $et%ork laer protocols! So#e ea#ples o" protocols here are

IC (Internet rotocol Control rotocol) and I.C (Internet%ork

acket change Control rotocol)!


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Protocol stack compared to the OSI reference model

oint-to-oint rotocol stack

It is i#portant to understand that the protocol stack is speci"ied at the hsical

and Data Link laers onl!

$C is used to allo% co##unication o" #ultiple $et%ork laer protocols bencapsulating the protocols across a data link!


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A"t#entication This option tells the calling side o" the link to send in"or#ation

that can identi" the user! The t%o #ethods are and CH!

Com$ression This is used to increase the throughput o" connections b

co#pressing the data or paload prior to trans#ission! deco#presses the

data "ra#e on the receiving end!Error detection uses /ualit and 0agic $u#ber options to ensure a

reliable, loop-"ree data link!

%"&ti&in' Starting in IOS version 11!1, #ultilink is supported on links %ith

Cisco routers! This option allo%s several separate phsical paths to appear to

be one logical path at laer *! 2or ea#ple, t%o T1s running #ultilink

%ould appear as a single *0bps path to a laer * routing protocol!!!! ca&&(ac' can be con"igured to call back a"ter success"ul

authentication! 3ith callback enabled, a calling router (client)

%ill contact a re#ote router (server) and authenticate as described in the

previous section! 4oth routers #ust be con"igured "or the callback "eature!

Once authentication is co#pleted, the re#ote router %ill ter#inate the

connection and then re-initiate a connection to the calling router "ro# there#ote router!


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3hen connections are started, the links go through three phases o"session establish#ent!


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Lin'-esta(&is#ment $#ase LC packets are sent b each device to

con"igure and test the link! These packets contain a "ield called the

Con"iguration Option that allo%s each device to see the si5e o" the data,co#pression, and authentication! I" no Con"iguration Option "ield is

present, then the de"ault con"igurations are used!

A"t#entication $#ase I" re6uired, either CH or can be used to

authenticate a link! uthentication takes place be"ore $et%ork laer protocol

in"or#ation is read! It is possible that link-6ualit deter#ination #a occur atthis sa#e ti#e!

et)or' &a*er $rotoco& $#ase uses the Network Control Protocol

(NCP) to allo% #ultiple $et%ork laer protocols to be encapsulated and sent

over a data link! ach $et%ork laer protocol (e!g!, I, I., ppleTalk,

%hich are routed protocols) establishes a service %ith $C!


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There are t%o #ethods o" authentication that can be used %ith links+

!ass)ord A"t#entication !rotoco& +!A! The Password Authentication

Protocol (PAP) is the less secure o" the t%o #ethods! ass%ords are sent in

clear tet, and is onl per"or#ed upon the initial link establish#ent! 3hen

the link is "irst established, the re#ote node sends back to the originating

router the userna#e and pass%ord until authentication is ackno%ledged!


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C#a&&ene Hands#a'e A"t#entication !rotoco& +CHA! The Challenge Handshake

Authentication Protocol (CHAP) is used at the initial startup o" a link and at periodic

checkups on the link to #ake sure the router is still co##unicating %ith the sa#e host! "ter "inishes its initial link-establish#ent phase, the local router sends a challenge

re6uest to the re#ote device! The re#ote device sends a value calculated using a one-

%a hash "unction called 0D7! The local router checks this hash value to #ake sure it

#atches! I" the values don&t #atch, the link is i##ediatel ter#inated!


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Ppp Hdlc Pap Chap