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

6.1. Overview of Source Routing Operation

Consider the source-routed catenet depicted in Figure 6-1, and the communications among stations X, Y, and Z.

Figure 6.1. Source routing

Stations X and Y are on the same ring (ring 1). As a result, they can communicate directly without the use of source routing or any other type of bridge. Frames sent from station X to station Y carry a Destination Address of Y, a Source Address of X, and the data payload associated with the frame. Because the frame does not need to pass through any bridges, no source routing information is required.

Stations X and Z can only communicate through a series of intervening rings and bridges. Four different paths are available between these two stations:

• Ring 1–bridge 1–ring 2–bridge 3–ring 3–bridge 4–ring 5.

• Ring 1–bridge 1–ring 2–bridge 3–ring 3–bridge 2–ring 5.

• Ring 1–bridge 2–ring 2–bridge 3–ring 3–bridge 4–ring 5.

• Ring 1–bridge 2–ring 2–bridge 3–ring 3–bridge 2–ring 5.

Prior to communicating with station Z, station X (not the bridges) must learn the routes that are available using a Route Discovery process. Station X then selects one of the routes (in the example shown, station X has chosen the fourth route from the preceding list) and inserts the explicit list of intervening rings and bridges in all frames sent to station Z. That is, the frame itself is modified to include routing information that describes ...