System information

SRB
Book Title
10-214
Since its initial proposal, IBM has offered a new bridging standard to the IEEE 802 committee: the
source-route transparent (SRT) bridging solution. SRT bridging eliminates pure SRBs entirely,
proposing that the two types of LAN bridges be transparent bridges and SRT bridges. Although SRT
bridging has support, SRBs are still widely deployed.
SRB Algorithm
SRBs are so named because they assume that the complete source-to-destination route is placed in
all inter-LAN frames sent by the source. SRBs store and forward the frames as indicated by the route
appearing in the appropriate frame field. Figure 10-3 illustrates a sample SRB network.
Figure 10-3 A Sample SRB Network
Referring to Figure 10-3, assume that Host X wishes to send a frame to Host Y. Initially, Host X does
not know whether Host Y resides on the same 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 Host X should use in choosing a route, but it does make several suggestions, including the
following:
Bridge 1
LAN 1
Bridge 2
LAN 3 LAN 2
LAN 4
Bridge 3
Bridge 4
Host Y
Host X