Before you actually configure PPP on a serial interface, we will look at the commands and the syntax of these commands as shown in the figure. This series of examples shows you how to configure PPP and some of the options.
Example 1: Enabling PPP on an Interface
To set PPP as the encapsulation method used by a serial or ISDN interface, use the encapsulation ppp interface configuration command. The following example enables PPP encapsulation on serial interface 0/0:
R3#configure terminal
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
The encapsulation ppp command has no arguments, however, you must first configure the router with an IP routing protocol to use PPP encapsulation. You should recall that if you do not configure PPP on a Cisco router, the default encapsulation for serial interfaces is HLDC.
Example 2: Compression
You can configure point-to-point software compression on serial interfaces after you have enabled PPP encapsulation. Because this option invokes a software compression process, it can affect system performance. If the traffic already consists of compressed files (.zip, .tar, or .mpeg, for example), do not use this option. The figure shows the command syntax for the compress command. To configure compression over PPP, enter the following commands:
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
R3(config-if)#compress [predictor | stac]
Example 3: Link Quality Monitoring
Recall from our discussion on LCP phases that LCP provides an optional link quality determination phase. In this phase, LCP tests the link to determine whether the link quality is sufficient to use Layer 3 protocols. The command ppp qualitypercentage ensures that the link meets the quality requirement you set; otherwise, the link closes down.The percentages are calculated for both incoming and outgoing directions.
