Bridging
Source Routing Information
Source Routing is a bridging technique developed by IBM and the 802.5 standards committee in which a bridge routes frames based on the contents of their media access control frame header, rather than by maintaining a Þltering database to determine whether a packet should be forwarded or Þltered. Source Routing functions as follows:
¥An end point station transmits discovery (explorer) frames to a particular destination address in order to seek the best route through a bridged topology to that node. These frames are broadcast over the entire network.
In a network topology with parallel bridges, multiple paths may be available to the same destination. In this case, the explorer frame may be further deÞned as:
-All Routes Explorer, so that all possible routes to the destination are recorded, and multiple explorer frames can reach the same segment.
-Spanning Tree Explorer (also known as Single Route Broadcast), so that only one path is possible to a segment (i.e., through a designated bridge in a Spanning Tree topology), and only one explorer frame will be forwarded onto each segment. The Spanning Tree can be conÞgured either automatically (i.e., by algorithm) or manually.
¥When a source routing bridge processes the explorer frame, it adds a unique identiÞer to the frame in a reserved portion of the frame. This identiÞes the segment the frame was received from, followed by the speciÞc bridge, and Þnally the segment it was forwarded onto.
¥When the discovery frame (or frames if more than one route is possible) reaches its destination, it contains a complete record of bridge hops on its route.
¥The destination address then returns All Routes Explorer (using speciÞcally routed frames) and Single Route (Spanning Tree) Explorer frames (using All Routes Broadcast frames), to the source address.
¥The source station selects one path from the returned explorer frames, and includes that path speciÞcation (with bridge and segment identiÞers) in subsequent transmissions to that particular destination.
All bridges in the topology then examine the routing information Þeld of a speciÞcally source routed frame and either forward it if there is a match in the routing informationÑor if it is an All Routes explorer frameÑor discard it.
Source Route Configuration
The Source Route ConÞguration window allows you to view IP address and routing information, and to view and set source route bridging parameters for any bridging device which supports this menu option.
Source Route Configuration |
