Source-Route Bridging (SRB)

Source-route bridging is implemented by IBM and compatible bridge products for use over token-ring LAN segments.

Source-routing requires a sending device to specify the path that should be taken by a frame across an internetwork, rather than allowing the decision to be made by individual bridges. To do this a sending device must determine the best path to a destination and include it in all frames to that destination. The best path to a destination is found using a discovery process, one implementation of which is described in this section.

A sending device sends a discovery frame to the intended destination device marked single-route broadcast. Bridges in a token-ring internetwork should be configured using the token-ring spanning tree algorithm to permit only one path for single-route broadcast frames between devices. The destination device should therefore receive only a single copy of the discovery frame.

The destination device responds to the discovery frame with a discovery response frame, marked all-routes broadcast. This will contain the most significant bit (the route information indicator, also called RII) set in the source MAC address field, and an entry in the routing information field (RIF). This will initially contain zero in the bridge number field, and the number of the networks to which the destination device is attached in the segment number field.

The discovery response frame, because it is marked all-routes broadcast, will pass through all bridges on its way back to the original sending device. Each bridge that the frame passes through must insert its bridge number and LAN segment, and hence the frames that return to the original sending device contain the routes they have taken through the bridged internetwork.

The routing information field can currently only hold data for about seven bridges and eight LAN segments. If a frame is received by a bridge with this field full, it is discarded. This limits the number of bridge hops in the network to seven, and consequently the maximum size of source-route bridged internetworks.

The original sending device therefore receives one or more discovery response frames. These frames contain routing control and bridge and LAN segment numbers in their routing information fields. The routing control field indicates the number of bridge/LAN segments in the routing information field and also the maximum frame size that is supported by the route.

The sending device can now select the best route to use through the internetwork to reach the destination device. Current implementations select the route in the first received discovery response frame (the fastest path at the time of the discovery process), although the architecture allows route selection based on other criteria, for example, maximum frame size supported by the route.