Remember that HDLC is the default encapsulation protocol for Cisco routers. If PPP configuration is not done on Cisco routers, the default encapsulation will be used. To set PPP as the encapsulation method used by a serial interface we need PPP configuration on the serial interfaces. The PPP configuration for the below topology is following.
Router1
Router1#configure terminal
Router1(config)# interface serial 0/0/0
Router1(config-if)# encapsulation ppp
Router2
Router2#configure terminal
Router2(config)# interface serial 0/0/0
Router2(config-if)# encapsulation ppp
The encapsulation ppp interface command has no arguments. The figure billow illustrates that Router1 and Router2 have been configured with both an IPv4 and an IPv6 address on the serial interfaces. PPP is a Layer 2 encapsulation that supports different Layer 3 protocols.
Compression Configuration
The command syntax for compression configuration is “compress”. Compression can be configured after enabling PPP encapsulation on the serial interface of a Cisco router. This option invokes a software compression process; therefore it can affect system performance. If the traffic already contains compressed files, such as .zip, .tar, etc, never use this option. The command for configuration is as under:
Router1
Router1(config)# interface serial 0/0/0
Router1(config-if)# encapsulation ppp
Router1(config-if)# compress [ predictor | stac ]
Router2
Router2(config)# interface serial 0/0/0
Router2(config-if)# encapsulation ppp
Router2(config-if)# compress [ predictor | stac ]
Two keywords are available for “compress” command. With predictor keyword, the predictor algorithms are used. While stac is used for LSZ algorithms.
Link Quality PPP Configuration (LQM)
Quality is also the option of the LCP phase. LCP tests the link to verify whether the link quality is sufficient to use Layer 3 protocols or not. After configuring this option, the percentages are calculated for both incoming and outgoing directions. If the link quality percentage is not maintained, the link is supposed to be of poor quality and is taken down. The “ppp quality percentage” command is used to ensure that the link meets the quality requirement set; otherwise, the link closes down. The percentage range is from 1 to 100. The LQM configuration for the above network is as under:
Router1
Router1(config)# interface serial 0/0/0
Router1(config-if)# encapsulation ppp
Router1(config-if)# ppp quality 70
Router2
Router2(config)# interface serial 0/0/0
Router2(config-if)# encapsulation ppp
Router2(config-if)# ppp quality 70
Note:- We can cancel the LQM by using “no ppp quality” command.
