C H A P T E R25
• Describe when to use source-route bridging.
• Understand the difference between SRB and transparent bridging.
• Know the mechanism that end stations use to specify a source-route.
• Understand the basics of source-route frame formats.
The source-route bridging (SRB) algorithm was developed by IBM and was proposed to the IEEE 802.5
committee as the means to bridge between all LANs. 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, proposing that the two types of LAN bridges be transparent bridges and
SRT bridges. Although SRT bridging has achieved support, SRBs are still widely deployed. SRT is
covered in Chapter 24, “Mixed-Media Bridging.” This chapter summarizes the basic SRB
frame-forwarding algorithm and describes SRB frame fields.
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 25-1 illustrates a sample SRB network.
In Figure 25-1, 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
assumes that Host Y is on a remote segment.
rnetworking Technologies Handbook
Figure 25-1 An SRB Network Contains LANs and Bridges
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 internet