T1: A Survival Guide

Appendix B. Multilink PPP

Art necessarily presupposes knowledge.

—John Stuart Mill, System of Logic

Bandwidth is like disk space or closet space—you can never have too much. Eventually, most observers say, fiber to the home will solve the last-mile bandwidth crunch. In the interim, however, demand for last-mile bandwidth has driven a great deal of research and debate (not to mention marketing) for possible solutions.

Certain types of access channels, most notably ISDN, offer the ability to have different levels of service between two points. Higher capacity comes only with increased cost, but the telco’s tariffs may be structured in such a way that it is cheaper to use switched access on demand rather than pay for sufficient dedicated capacity to cover peak usage.

Several approaches exist for taking advantage of multiple links. This appendix focuses on Multilink PPP (MP), the set of PPP extensions that are used to aggregate several distinct lower-layer links into a single logical link. ISPs commonly use MP to offer 128-kbps ISDN access. When MP was first deployed, ISPs with large customer bases often had trouble offering 128-kbps ISDN service because both subscriber calls needed to land on the same access server at the POP. To lift this restriction, a variety of methods were developed to allow individual links in an MP session to land on different devices. This broad collection of techniques is referred to as Multi-Chassis MP (MMP) because it allows links to be spread across ...