PPP Configuration Commands.

PPP Configuration Commands
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. 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.This example configuration monitors the data dropped on the link and avoids frame looping:
 
R3(config)#interface serial 0/0
R3(config-if)#encapsulation ppp
R3(config-if)#ppp quality 80


Use the no ppp quality command to disable LQM.

Example 4: Load Balancing Across Links
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 inbound traffic, or on just outbound traffic, but not on the combined load of both inbound and outbound traffic.The following commands perform load balancing across multiple links:

Router(config)#interface serial 0/0
Router(config-if)#encapsulation ppp
Router(config-if)#ppp multilink


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