T1: A Survival Guide

Configuring PPP

PPP presents a more complex set of configuration parameters to the network administrator than HDLC. Many of these parameters are better suited to use with low-speed asynchronous dial-up lines. Of all the PPP configuration options, you are most likely to need these:

Enable PPP encapsulation

Not all vendors default to PPP encapsulation on serial links, but several do not. PPP encapsulation must often be explicitly configured.

Configure IP networking (addresses, encapsulation, and routing)

Depending on the vendor’s implementation, IP encapsulation may be on by default or implied when an IP address is assigned to an interface. ISPs do not usually assign addresses on leased lines automatically. You must also enable routing on the interface. Routing often consists of a simple static default route to the other end of the link, though many WANs make use of dynamic routing protocols.

Magic number negotiation

Magic numbers assist in detecting looped-back links. Leased lines may be looped back at several locations along the path, and administrators do not have access to all of these locations. Loopbacks at the CSU/DSU can be corrected, but any others require phone calls. To make phone calls for looped-back links successful, arm yourself with a PPP log that shows identical magic numbers being transmitted and received.

Link Quality Monitoring (LQM)

This is typically enabled for leased lines. Users do not always have access to leased-line performance data such as the T1 PRM ...