T1: A Survival Guide

Multi-Chassis MP (MMP)

Classic MP requires that all links in a bundle terminate into the same device. Fragments are held in memory and must be conveniently accessible to a single MP process for reassembly. For a circuit-switched path between two endpoints under the control of the same company, it is easy to have one machine on both ends of the link.

Service providers are a different story.^[32] No matter how large vendors can make dial-up termination equipment, there will always be a service provider who needs something bigger. Service providers may have several large routers terminating subscriber PPP sessions in a POP. All the T1 circuits providing the dial-up lines are aggregated together into one hunt group. At peak times, the dial-up channel a user receives is likely to be almost random; ISDN subscribers can easily have bearer channels terminated on separate routers.

Two major approaches have been developed to allow MP sessions to terminate correctly into a massive POP. One is to use additional protocols to direct subsequent calls to the access server already terminating the existing MP sessions. Alternatively, access servers can communicate with each other to determine when new sessions have landed on the “wrong” access server. The first chassis to accept an incoming session and instantiate the bundle becomes the bundle head and is responsible for performing bundle-related operations such as fragment reassembly. Layer-two tunneling techniques are used to forward fragments ...