Point-to-Point Protocol (PPP)
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Link Control Protocol (LCP) Configuration Options
Link Control Protocol offers different PPP encapsulation options, including
the following:
Authentication
This option tells the calling side of the link to send infor-
mation that can identify the user. The two methods relevant to the CCNA
exam are PAP and CHAP.
Compression
This is used to increase the throughput of PPP connec-
tions. PPP decompresses the data frame on the receiving end. Cisco uses
the Stacker and Predictor compression methods, discussed in
CCNP:
Routing Study Guide
(Sybex, 2000).
Error detection
PPP uses Quality and Magic Number options to ensure
a reliable, loop-free data link.
Multilink
Starting in IOS version 11.1, multilink is supported on PPP
links with Cisco routers. This splits the load for PPP over two or more
parallel circuits and is called a bundle.
PPP Session Establishment
PPP can be used with authentication. This means that communicating routers
must provide information to identify the link as a valid communication
link. When PPP connections are started, the links go through three phases of
session establishment:
Link-establishment phase
LCP packets are sent by each PPP device to
configure and test the link. The LCP packets contain a field called the
Configuration Option that allows each device to see the size of the data,
compression, and authentication. If no Configuration Option field is
present, then the default configurations are used.
Authentication phase
If configured, either CHAP or PAP can be used
to authenticate a link. Authentication takes place before Network layer
protocol information is read.
Network layer protocol phase
PPP uses the Network Control Protocol
to allow multiple Network layer protocols to be encapsulated and sent
over a PPP data link.
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