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114 Chapter 5: Configuring PPP and Controlling Network Access

Foundation Summary

PPP was developed specifically for point-to-point connectivity, as its name implies. It has
become one of the more versatile protocols in use today. ISDN implementations, serial
connections, and other dial-up connections now implement PPP.

Configuring PPP is not a difficult process; however, it does have some intricate differences
when compared to other WAN technologies.

The options provided by PPP enable a level of control over network resources previously
unknown. LCP negotiation of authentication, callback, compression, and PPP Multilink make
this granularity possible.

PPP authentication enables the use of PAP or CHAP. PAP makes use of clear text passwords,
which could enable packet playback if captured by a protocol analyzer. CHAP implements an
MD5 hash challenge and response. Every challenge is unique, as is each response. At periodic
intervals (two minutes) during the course of the connection, additional challenges are issued. In
the event of a failed authentication, the call is immediately disconnected.

PPP Callback enables the centralization of call related costs. A central site provides callback
services to remote clients. Client devices dial it and are authenticated. Upon successful
authentication, the server disconnects the call and dials the client back.

PPP Compression enables a reduction in the delay associated with transmission of data over
lower-speed links. Care should be taken when using compression because memory utilization
on the router is greatly increased.

PPP Multilink enables the bundling of multiple bearer channels into one aggregate pipe. Traffic
is broken up and sent across the redundant pathways to the remote side where it is reassembled.

Any protocol can go across a PPP link. The only requirement is that the adjacent interfaces must
be configured with the protocols that need to cross the link.