Recall 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 ppp quality percentage command ensures that the link meets the quality requirement set; otherwise, the link closes down.
The percentages are calculated for both incoming and outgoing directions. The outgoing quality is calculated by comparing the total number of packets and bytes sent to the total number of packets and bytes received by the destination node. The incoming quality is calculated by comparing the total number of packets and bytes received to the total number of packets and bytes sent by the destination node.
If the link quality percentage is not maintained, the link is deemed to be of poor quality and is taken down. Link Quality Monitoring (LQM) implements a time lag so that the link does not bounce up and down.
The following configuration example monitors the data dropped on the link and avoids frame looping, as shown in Figure 1:
R3(config)# interface serial 0/0/0
R3(config-if)# encapsulation ppp
R3(config-if)# ppp quality 80
Use the no ppp quality command to disable LQM.
Use the Syntax Checker in Figure 2 to configure PPP encapsulation, compression, and LQM on router R1’s Serial 0/0/1 interface.