Information Technology Reference
In-Depth Information
Explorer Frames
When a source node wants to send information to a destination that is not on the local LAN, it
sends out an explorer frame. Bridges pick up the explorer frames and forward them out the other
interfaces. The bridges add route information to the frames as they travel through the network.
The route information includes a Token Ring number and bridge number pair that the explorer
frame travels on. As seen in Figure 4-21, when the explorer frame reaches Bridge 6, a route
descriptor is added to the frame that includes Ring 7/Bridge 6. At Bridge 4, another route
descriptor with Ring 3/Bridge 4 is added. Finally, Ring 2 is added at the destination node with
the bridge value set to 0 to indicate that the destination has been reached.
SRB Ring/Bridge Routing Information
Figure 4-21
Host A
Ring 7
Bridge 6
Ring 3
Bridge 4
Ring 2
Host B
ring7 bridge6 ring3 bridge4 ring2 bridge0
RIF routing information
When the explorer frames arrive at the destination, the destination frame sets the direction bit
(also known as the D-bit) to 1 and sends the frame back to the source node through the same
route that was used to arrive to the destination. When multiple frames reach back to the source
node, it usually uses the route of the first frame received. Other decision metrics include the
minimum number of hops and the path with the largest MTU allowed.
There are two types of explorer frames. The first is the all-routes explorer (ARE) frame. This
explorer takes all possible paths to the destination. The second type is the spanning-tree
explorer (STE) frame; these explorer frames use a spanning tree to reach a destination to
prevent loops.
RIF
The RIF is contained in the 802.5 frame and is composed of a 2-byte RIF header. The RIF can
contain one or more route descriptors. Figure 4-22 shows the RIF header and route descriptor
format.
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