What is PPP? How to Configure PPP on Cisco Router Example

By | 9th November 2015

You can configure point-to-point encapsulation, software compression, link quality monitoring, load balancing across links (multilinks) on R1, R2 and R3 serial interface below.

Before you actually configure PPP on a serial interface, we will look at the commands and the syntax of these commands as shown below. This series of examples shows you how to configure PPP and some of the options.

ppp config

1: How to enable 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 0/0/0:

R1#config t

R1(config)#interface se 0/0/0

R1(config-if)#encapsulation ppp


 The encapsulation ppp command has no arguments, however, you must first configure the router with an IP routing protocol (RIP, EIGRP or OSPF) 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.

2: How to configure point-to-point software 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.

To configure compression over PPP, enter the following commands:

R1(config)#interface serial 0/0/0

R1(config-if)#encapsulation ppp

R1(config-if)#compress [predictor | stac]


3: How to configure Link Quality Monitoring

One of the primary functions of LCP when establishing a PPP session includes testing of a link to determine whether the link quality is sufficient to use Layer 3 protocols. The command ppp quality {percentage} ensures that the link meets the quality requirement you set; otherwise, the link closes down.

How Link Percentage is calculated.
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.

Use the configuration commands below to monitor the data traffic on the link and avoids frame looping:

R1(config)#interface serial 0/0/0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp quality 80


Use the no ppp quality command to disable LQM.

4:How to Configure Load Balancing Across Links (MultiLinks)

Multilink PPP (also referred to as MP, MPPP, MLP, or Multilink) provides a method for spreading traffic across multiple physical WAN links while providing packet fragmentation and reassembly, proper sequencing, multivendor interoperability, and load balancing on inbound and outbound traffic.

MPPP allows packets to be fragmented and sends these fragments simultaneously over multiple point-to-point links to the same remote address. The multiple physical links come up in response to a user-defined load threshold. MPPP can measure the load on just traffic into the network, or on just traffic going out, but not on the combined load of both inbound and outbound traffic.

Use the following commands to perform load balancing across multiple links:

R1#config t

R1(config)#interface se0/0/0

R1(config-if)#encapsulation ppp

R1(config-if)#ppp multilink


The multilink command has no arguments. To disable PPP multilink, use the no ppp multilink command.

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