Information Technology Reference
In-Depth Information
Figure10-3 A Sample SRB Network
LAN 3
Bridge 3
LAN 2
Host Y
Bridge 1
Bridge 4
LAN 1
LAN 4
Bridge 2
Host X
Referring to Figure 10-3, assume that Host X wants to send a frame to Host Y. Initially, Host X does not
know whether Host Y resides on the same LAN or a different LAN. To determine this, Host X sends out
a test frame. If that frame returns to Host X without a positive indication that Host Y has seen it, Host
X must assume that Host Y is on a remote segment.
To determine the exact remote location of Host Y, Host X sends an explorer frame. Each bridge receiving
the explorer frame (Bridges 1 and 2, in this example) copies the frame onto all outbound ports. Route
information is added to the explorer frames as they travel through the internetwork. When Host X's
explorer frames reach Host Y, Host Y replies to each individually using the accumulated route
information. Upon receipt of all response frames, Host X chooses a path based on some predetermined
criteria.
In the example in Figure 10-3, this process will yield two routes:
•
LAN 1 to Bridge 1, to LAN 3, to Bridge 3, to LAN 2
•
LAN 1 to Bridge 2, to LAN 4, to Bridge 4, to LAN 2
Host X must select one of these two routes. The IEEE 802.5 specification does not mandate the criteria
that Host X should use in choosing a route, but it does make several suggestions, including the following:
•
First frame received
•
Response with the minimum number of hops
•
Response with the largest allowed frame size
•
Various combinations of these criteria
In most cases, the path contained in the first frame received will be used.
After a route is selected, it is inserted into frames destined for Host Y in the form of a routing
information field (RIF). A RIF is included only in those frames destined for other LANs. The presence
of routing information within the frame is indicated by the setting of the most significant bit within the
Source Address field, called the routing information indicator (RII) bit.
