Specifications

TOKEN RING OVERVIEW
2-20
SOURCE-ROUTE BRIDGING METHOD
Source-route bridging is the predominant method used for linking Token
Ring LANs. Source-route bridging is a process whereby the source device,
or the sending station, rather than the bridge determines the route to other
stations used for sending messages. Bridging is implemented in the Token
Ring LAN environment typically for the following reasons:
• To link Token Rings running at different ring speeds
• To create more than one ring as the number of stations exceeds the IEEE
802.5 maximum for a single ring
Route Discovery
Source stations discover the routes to destination stations by sending a
source-route broadcast frame, single-route or all-routes, to the destination
station. If the source-route frame determines that the destination is not on
the local ring, the source-route frame is sent through bridges to other rings
until it arrives at the destination station.
Along the route to the destination station, bridges write ring numbers,
bridge numbers, and, in some cases, the maximum frame size the bridge
can process to the Routing Information Field (RIF) of the source-route
broadcast frame. Moreover, a bit called the Routing Information Indicator
(RII) bit in the source-route broadcast frame’s source address field is set
from zero (0) to one (1) to indicate that the source-route broadcast frame
contains routing information in its RIF field.
Upon receiving the source-route broadcast frame, the destination station
reverses the direction bit in the routing control field, an entity included in
the RIF, and transmits the source-route broadcast frame to the source
station along the route specified in the RIF. The source station then
appends data intended for the destination station to the source-route
broadcast frame, transforming it into a Specifically Routed Frame (SRF).
The RIF in the SRF indicates the exact route all further communications
between the source station and the destination must traverse. If the
internetwork configuration changes, that is for example, if a bridge along
the route is removed, then the discovery process must be repeated to
re-establish a new route between the source and destination stations.