Download - Ppp Protocols
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PPP and Data Links
PPP operates at the Data Link layer. Components ofPPP include:
A method for encapsulating packets datagrams! o"er serial links#he Link Control Protocol LCP! to esta$lish., maintain, test andterminate the data-link connection#he %et&ork Control Protocol %CP! to allo& simultaneousencapsulation of multiple net&ork layer protocols across thesame data-link &hich is refered to as protocol 'ultiple(ing
At the physical layer, PPP can $e used acrosssynchronous e.g., )SD%, leased lines! andasynchronous e.g., modem dialup! data links.
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PPP Layer *unctions
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*igure sho&s thatPPP is similar to an+DLC frame.#he Protocol fieldcontains the Layer protocol )D.
Figure 1Figure 1
Figure 2Figure 2
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PPP Layer *unctions
PhysicalPhysicalSynchronous or asynchronousSynchronous or asynchronous
physical media (Router tophysical media (Router toRouter, or Host to Network)Router, or Host to Network)
Data Link Data Link
High-Le el Data Link !ontrolHigh-Le el Data Link !ontrol
(HDL!)(HDL!)
Network !ontrol ProtocolNetwork !ontrol Protocol(N!P)(N!P)
(speci"c to ea# Network layer(speci"c to ea# Network layerprotocol e$# %P!P, %P&!P)protocol e$# %P!P, %P&!P)
Link !ontrol Protocol (L!P)Link !ontrol Protocol (L!P)('uthentication, !ompression, LP,('uthentication, !ompression, LP,
and !all ack)and !all ack)
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PPP Sessionsta$lishment
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Phases of PPP sta$lishment
PPP goes through four distinct phases topro"ide communications o"er point-to-point
links. Link esta$lishment / configuration negotiation
0. Link 1uality determination. %et&ork-layer protocol configuration negotiation
4. Link termination
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. Link sta$lishment /
Configuration %egotiationLink esta$lishment occurs at the data link layer &ith each PPPde"ice sending LCP packets.#he Link Control Protocol packets contain a configuration field for
options such as...'#2compressionlink authentication'LPCall $ack
LCP must esta$lish the link $efore any net&ork layer protocols can$e e(changed#his phase is completed &hen a configuration ackno&ledgementhas $een sent and recei"ed.
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0. Link-3uality DeterminationPPP pro"ides optional testing to determine &hetherthe link is good enough to $ring up net&ork layerprotocols.
)n addition, if authentication is re1uired it occursduring this phase:PAP Pass&ord Authentication Protocol!C+AP Challenge +andshake Authentication Protocol!
Authentication occurs $efore the net&ork layerconfiguration phase $egins.
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. %et&ork-Layer %egotiation
nce LCP finishes the link-1uality phase, net&orklayer protocols can $e configured $y the appropriate%CP%CPs are sent for each protocol e.g., )P, )P5,
Apple#alk!)f LCP terminates the link, it informs %CP so it can
take the appropriate action#o "ie& the status of LCP and %CP, use the showinterfaces command
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4. Link #ermination
LCP can terminate the link at any time.6easons include:
6e1uested $y user closing internetconnection!Loss of carrier at the physical layer
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na$ling PPP
7ou ena$le PPP encapsulation on an interface $y usingthe follo&ing command:
Router(config-if)#encapsulation ppp
#hus, if you &ant dial-in hosts on terminal line 0 to usePPP, you &ould enter the follo&ing commands:
RTA(config)#interface async 2RTA(config-if)#encapsulation ppp
%ote that the encapsulation command is issued ininterface configuration mode, not line configurationmode.
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Dialup PPP "s. Dialup 5 C
Sessions5 C Sessions: %o )P
addressing or PPP encapsulationis needed for this type ofconnection. Data is sent asasynchronous characters.Dialup PPP: a remote host candial in to an access ser"er andsend a Layer protocol packetencapsulated $y PPP. #his type ofconnection allo&s the remote userto access net&ork resources suchas file ser"ers and mail ser"ers7ou can also configure the router8sasynchronous interface toautomatically select $et&een PPPdata sessions and 5 Csessions.
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Async )nterface Commandsna$ling this feature re1uires t&o steps. *irst, you must configure the
asynchronous interface s! &ith the async mode interactive commandin interface configuration mode. #his command configures the router so thatit allo&s the remote host to choose either a PPP session or an 5 Csession. #he follo&ing e(ample sho&s ho& to configure interface async :
RTA(config)#interface async 1RTA(config-if)#encapsulation pppRTA(config-if)#async mode interactive
Second, you must configure the corresponding terminal line s! &ith theautoselect ppp command in line configuration mode. #o complete thee(ample configuration, you &ould enter the follo&ing commands:
RTA(config)# line 1 RTA(config-line)# autoselect ppp during-login
#he autoselect command permits the access ser"er to allo& an appropriateprocess to start automatically &hen a starting character is recei"ed. )f thestart character is a return character, then the access ser"er starts an 5 Csession. n the other hand, if the access ser"er recogni9es the startcharacter as PPP, SL)P, or A6AP, it &ill $egin a session for &hiche"erprotocol it detects . So, if an end user is using a program that sends a PPPframe &hich has a flag character in he(adecimal or ; ; in $inary!format, the access ser"er &ill automatically start a PPP session.
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Dedicated 'ode
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Assigning An )P address to #he Async
)nterface and #o #he 6emote 2ser RTA(config)# interface async 1RTA(config-if)# ip address 10.1.1.1 255.255.255.0
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LCP options. Authentication
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AuthenticationAuthentication , &ith PAP orC+AP, is used as a securitymeasure &ith PPP. Authenticationallo&s the dial-up target to identifythat any gi"en dialup client is a"alid client &ith a preassignedusername and pass&ord.)f chosen, occurs during the link-1uality determination phase.6e1uires that the calling side ofthe link Pro"ide authenticationinformation.#he t&o authentication options
supported $y PPP are:PAP Pass&ord AuthenticationProtocol!C+AP Challenge +andshake
Authentication Protocol!
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PAP )s %ot As =ood As C+APPass&ord Authecntication Protocol
PAP!:Pass&ords are sent across the link inclear te(t#he remote node is in control of thefre1uency and timing of the loginattempts.
Challenge +andshake Authentication Protocol C+AP!:
ncryption and +ashing#he access ser"er is in charge of thefre1uency and timing of the loginattempts
Authentication is done upon initial linkesta$lishment and may $e repeated
any time after the link has $eenesta$lished.
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Configuring PAP'utual Authentication
n each router, define the user name and pass&ord to e(cept from the remote router.na$le PPP and PAP on the interface.
La -A(config)#username La -! password class
La -A(config-if)#encap pppLa -A(config-if)#ppp authentication pap
La -A(config-if)#ppp pap sent-username La -A password cisco---------La -!(config)#username La -A password cisco
La -!(config-if)#encap pppLa -!(config-if)#ppp authentication pap
La -!(config-if)#ppp pap sent-username La -! password class
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Configuring C+AP
La -A(config)#username La -! password cisco
La -A(config-if)#encap pppLa -A(config-if)#ppp authentication chap
---------La -!(config)#username La -A password cisco
La -!(config-if)#encap pppLa -!(config-if)#ppp authentication chap
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s ) S.
Close all telnet sessions first to return to the originalconsoled router.
)n Pri"ileged (ec. 'ode, enter the command?La -A#de ug ppp authentication=o to the ppp interface. Shut it do&n and then $ring it $ackup. 7ou should see PAP or C+AP info come across the linkas it comes $ack up and the routers authenticate each other.
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LCP options0. Compression
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Data Compression
PPP can also ma(imi9e performance $y usingdata compression, &hich may pro"ide higherdata throughput across lo&-speed links
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Data CompressionCompression is an option that is negotiated $y LCP. So, if theparty you are calling is not configured for compression, nocompression &ill take place.#ypically, you should only configure compression on low-speedlinks $ecause the router compresses data using soft&are, &hichre1uires router CP2 time and memory. Some algorithms are morememory-intensi"e@ others are more CP2-intensi"e. )n either case,the router8s a$ility to route packets is impaired $y the drain on itsresources.)f you fre1uently transfer already compressed data, such asgraphics and "ideo, you need to consider &hether you &ant to setup compression. #rying to compress already compressed data can
take longer than transferring the data &ithout compression. )deally,you can attain a 2:1 or 3:1 compression ratio for information that&as not pre"iously compressed. (pect an a"erage of 1.6:1 compression for mixed compressed and uncompressed sourcedata. #he ratio for compressed data is : .
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Compression #ypes
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Configuring Compression
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6e"ie&
PPP 'ainComponents
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PPP
ncapsulationSynchronous and asynchronous
PPP pro"ides error detection
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Link Control Protocol
sta$lishes, configures, test, andterminate the data link connection.
Authenticates the identity of a the peer onthe link.Performs data compression%egotiates ptions
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%et&ork Control Protocol
pens, configures, and terminates net&orklayer protocol.
)nternet Protocol Control Protocol )PCP! is asu$ protocol of %CP.)PCP configures )P at the net&ork layer andcan negotiate )P address such as using D+CP.
'ultilink Protocol 'LP! links the LCP and %CPlayers.
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PPP ) SCommands
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2ser Control
Configuring the router to pro"ide a promptfor entering encapsulation information and
an )P address,r, you can take the prompt a&ay and
assign e"erything $y the router
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Async 'ode Dedicated
#he e(ec Prompt does not appear #he interface &ill use either SL)P or PPP
encapsulation as configured $y the net&orkengineer.
(ample:)nterface Async 4
Async mode dedicatedncapsulation SL)P
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Async 'ode )nteracti"e
Allo&s SL)P and PPP 5 C commandsfor the user.
(ample:)nterface Async
Async mode interacti"e
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Pro"iding )P address
Peer default )P address#he ser"er router gi"es an )P address to the
client.(ample:
)nterface async Peer default )P-address 0. .40.0
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Pooling Local A set of )P addresses is defined in a local data$aseSimplest mechanism for assigning )P addresses.Suita$le &hen there is only one access ser"er
pro"iding access to the net&ork.(ample:
config!B )P address-Pool Localconfig!B )P local pool Sales 0. . ;. 0. . ;.config!B)nt async config-if! Peer default )P-address pool Sales
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)SD% e(ample-Address pool
config!B username $ill pass&ord $aileyconfig!B )P local-pool isdnpool 0. . ;.0
0. . ;.config!B )P address-pool localconfig!B )nt $ri;
config-if! encapsulation PPP
config-if! )P address 0. . ;. 0EE.0EE.0EE.;config-if! per default )P address pool isdnpoolconfig-if! PPP authentication chap
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Pool-D+CP
A pool of )P addresses is defined inside a centrali9ed )Paddress ser"er, called D+CP ser"er.#his central data$ase can ser"e addresses to se"eral
different access ser"ers at the same time7ou can ena$le D+CP address pooling on an accessser"er $y performing the follo&ing commands:
Specify that the access ser"er uses the D+CP client-pro(y on allasynchronous interfaces $y using the command:
config!B)P address-pool dhcp-pro(y-client.
Specify at least one and up to ten )P addresses of D+CP ser"ers.config!B)P D+CP-ser"er 0, .E.E
Configure the appropriate interfaces using the command:config-if!B peer default ip-address dhcp
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(ample D+CP Pooling
config!B )P address-pool dhcp-pro(y-client
config!B )nterface group-async config-if! encapsulation PPP
config-if! Peer default )P-address dhcp
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=roup Asynchronous )nterfaces
=ather asynchronous interfaces into a groupinterface and configure only the group interface toeliminate manual configuration duplication.
(ample:config!B )nterface group-async ;config!B =roup-range 0
config!B )nterface group-async ;config-if!B encapsulation PPPconfig-if!B mem$er async default )P address 0. ;. .
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Link Control Protocol ptions
AuthenticationCall$ackCompression'ultilink
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PPP Call$ack
FhyG:'inimi9ing costCentrali9ed $illing
Process that occur during a PPP call$ackconnection:. )nitiation of a call $y a client. #he client re1uests call$ack
as one of the options during the LCP negotiation phase.
0. Call$ack re1uest is ackno&ledged $y the ser"er, and theser"er checks its configurations to see if the call isallo&ed.
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PPP Call$ack
Process that occur during a PPP call$ackconnection:
. 2ser authentication occurs, and the clientusername is used in the dialer map command toidentify the dial string to $e used in the return call.
4. )f the authentication is successful $ut there is nocall$ack option, the call continues $ut the clientpays for the call@ other&ise, the call isdisconnected $y the ser"er.
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PPP Call$ack
Process that occur during a PPPcall$ack connection:
E. Client is called $y the ser"er using thedial string.
. Authentication occurs again.
. #he connection continues.
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(ample-async PPP Call$ack
6outer Call$ack ser"er!config!B username callman call$ack-
dialstring EEE 0 4 pass&ord ciscoconfig!B )nt async config-if! PPP call $ack accept
PC'ust support 6*C E ; for PPP call$ack.
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'ultilink PPP 'LP!
)t allo&s you to com$ine channels into a multilink$undle so that data could $e sent at a higher rates.
(ample:)nterface $ri;)P address .0. .4 0EE.0EE.0EE.;
ncapsulation PPPPPP authentication chap
PPP multilinkDialer map )P .;.;.E name mlpPeer EEE4444Dialer load threshold 0 either
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PPP compressionData compression:
Stacker: 'ore CP2 intensi"e, and lessmemory intensi"e
Predictor: Less CP2 intensi"e and morememory intensi"e
config-if! PPP compress predictor
#CP header compressionconfig-if! )P #CP header-compression passi"e
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