User guide

A-18 Cisco 2500 Series Access Server User Guide

LANs and WANs

Point-to-Point Protocol

PPP encapsulation provides Cisco IOS software to devices that are not running Cisco IOS

software, connectivity over leased WAN lines. PPP uses a more complex model than HDLC

to ensure interoperability between networking vendors. This interoperability involves

several additional protocols, including the following:

• Link control protocol for negotiating basic line interoperability

• A family of network control protocols for negotiating individual Layer 3 protocols and

their options (such as IPCP for IP and options such as compression)

When the PPP link is negotiated, a link control protocol is negotiated to establish the link

and then additional network control protocols are negotiated. If IP, AppleTalk, or IPX, are

conﬁgured on the serial line, IP control protocol (IPCP), AppleTalk control protocol

(ATCP), or IPX control protocol (IPXCP), respectively, is negotiated to conform to the

protocols requirements.

Enter the command show interface to check the status of the link control protocol and the

network control protocol, and test the interoperability of the network layers. There are also

excellent debug ppp commands for troubleshooting. (See the publication Access and

Communication Servers Command Reference.)

To conﬁgure the serial line to use PPP, enter the interface subcommand encapsulation ppp

as follows:

Router(config)# interface s0

Router(config-if)# encapsulation ppp

Multipoint WAN Technologies

Advances in technology over the last decade have resulted in a number of additional WAN

solutions being available to network designers. These sections include multipoint WAN

technologies such as Frame Relay, SMDS, X.25, and ATM. (See Figure A-7.)

Selection of an appropriate WAN solutions should involve a discussion of the costs and

beneﬁts of each with your network designer and service providers.