The All-New Switch Book: The Complete Guide to LAN Switching Technology, Second Edition

6.5. Source Routing Operation

There are two aspects to source routing operation:

• The Route Discovery process by which stations learn and select among the available paths for frames to traverse the catenet.

• The steady-state operation of the system for frames that follow the selected path.

In the sections that follow, we look at the details of the operation of both end stations and bridges with respect to both aspects of source routing operation.

6.5.1. Route Discovery

Before any pair of stations can communicate using Specifically Routed Frames across a catenet, they must first perform a Route Discovery procedure in which the stations learn which (if any) paths are available, and select one of those paths to use for frame exchanges during the instant session. Prior to the discovery of such a route, stations can still communicate across multiple rings by using Spanning Tree Explorer frames, although this obviates any benefits of source routing.

While it is theoretically possible to modify the source route dynamically over the duration of a session, in practice the Route Discovery process is invoked once and the selected route is used for the duration of the session, be it minutes or days. Source routes are generally not cached (remembered) between sessions; opening a new session with the same device invokes the Route Discovery process anew.

6.5.1.1. Route Discovery Algorithms

Before attempting to discover a route through the catenet, a station can try to communicate with its target ...