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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