An end station can be configured to begin the route discovery process using either of two general methods:
NOTE: This guide uses the terms all-routes explorer frame and single-route explorer frame . In IBM literature, these frames are called all-routes broadcast frame and single-route broadcast frame , respectively. In RFCs, the term spanning tree explorer frame is equivalent to the term single-route explorer frame.
For more information about explorer frames, refer to:
By default, the Novell end station implementation originates route discovery using single-route explorer frames. When the end station sends out a single-route explorer frame, it travels over a single route that is configured manually or determined automatically by the Spanning Tree Protocol. This protocol uses the configuration of each bridge in the network to determine a single route, as described in Spanning Tree Protocol.
With this method, the destination receives only one explorer frame, resulting in considerably less traffic than the use of all-routes explorer frames (as described in the next section).
When the destination receives an explorer frame, it can respond in one of three ways:
A specifically routed frame uses the information contained in the original explorer frame to take the same route back to the source. An all-routes explorer frame is propagated and takes all possible routes back to the source. Single-route explorer frames behave as described at the beginning of this section.
NOTE: The Novell implementation does not support a response with a single-route explorer frame. It responds with only a specifically routed frame or an all-routes explorer frame, the default being an all-routes explorer frame.
When the all-routes explorer frame is received by the destination, it chooses the best route based on one of the following criteria:
The criteria used to choose the route is determined by the end station implementation. Novell's implementation chooses the route traveled by the frame that is received first (the fastest route).
Route discovery begins when the source route end station generates an all-routes explorer frame and sends it to its local ring. As the process continues, each bridge examines the all-routes explorer frame to determine whether it has already been on any of the other rings attached to the bridge. If the frame has not been on one of the attached rings, it is forwarded to that ring. Because frames are not transmitted on rings on which they have previously traveled, no frame can follow the same route twice. With this mechanism, a frame is propagated in such a way that all possible routes to a destination are discovered, but no routes containing loops are received by the destination.
With this method, the destination receives one explorer frame for each possible route to the destination, resulting in considerably more traffic than the use of single-route explorer frames.
Depending on the implementation, the destination responds with one of three types of frames described earlier. The source then chooses the best route from all the frames it receives, based on the same criteria described in the previous section. As before, the criteria used are determined by the end station implementation.
In comparing the two methods of route determination, using single-route explorer frames creates the least amount of traffic in the network. This method produces only one frame on the destination ring.
In contrast, the number of duplicate frames created by using all-routes explorer frames can increase route determination traffic to the point of significantly degrading network performance. However, if your network can handle the extra route discovery traffic, the use of all-routes explorer frames has two advantages:
IMPORTANT: If the destination responds with a single-route explorer frame or specifically routed frame to a single-route explorer frame, the source cannot choose the best route. For the source to choose the best route, the destination must respond with an all-routes explorer frame.