Three Labs in One PPP Encapsulation Configuration Troubleshooting


In these three labs, you will learn how to configure and troubleshoot PPP encapsulation on serial links using the network shown in the topology diagram. You will also learn how to restore serial links to their default HDLC encapsulation. Pay special attention to what the output of the router looks like when you intentionally break PPP encapsulation. This will assist you in the Troubleshooting lab associated with this chapter. Finally, you will configure PPP PAP authentication and PPP CHAP authentication.

Learning Objectives:

  • Upon completion of this lab, you will be able to:
  • Cable a network according to the topology diagram.
  • Erase the startup configuration and reload a router to the default state.
  • Perform basic configuration tasks on a router.
  • Configure and activate interfaces.
  • Configure OSPF routing on all routers.
  • Configure PPP encapsulation on all serial interfaces.
  • Learn about the debug ppp negotiation and debug ppp packet commands.
  • Learn how to change the encapsulation on the serial interfaces from PPP to HDLC.
  • Intentionally break and restore PPP encapsulation.
  • Configure PPP PAP and CHAP authentication.
  • Intentionally break and restore PPP PAP and CHAP authentication. 

PPP Overview:

In networking, the Point-to-Point Protocol (PPP) is a data link protocol commonly used in establishing a direct connection between two networking nodes. It can provide connection authentication, transmission encryption (using ECP, RFC 1968), and compression. 

PPP permits multiple network layer protocols to operate on the same communication link. For every network layer protocol used, a separate Network Control Protocol (NCP) is provided in order to encapsulate and negotiate options for the multiple network layer protocols. It negotiates network-layer information, e.g. network address or compression options, after the connection has been established.

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
  • R1(config-if)#end

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.

How to configure 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]
  • R1(config-if)#end 

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
  • R1(config-if)#end

Use the no ppp quality command to disable LQM. 

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
  • R1(config-if)#end

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

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