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Bridging
2-58 Source Route Configuration
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.