Selecting Bridging and Switching Protocols 223
You can fill in Table 7-1 first by simply putting an X in each option that meets a critical goal.
Any options that do not meet critical goals can immediately be eliminated. Other options
can be evaluated on how well they meet other goals, on a scale from 1 to 10.
After a decision has been made, you should troubleshoot the decision. Ask yourself the
If this option is chosen, what could go wrong?
Has this option been tried before (possibly with other customers)? If so, what
problems occurred?
How will the customer react to this decision?
What are the contingency plans if the customer does not approve of the decision?
This decision-making process can by used during both the logical and physical network
design phases. You can use this process to help you select protocols, technologies, and
devices that will meet a customer’s requirements.
Selecting Bridging and Switching Protocols
If your network design includes Ethernet switches, you will most likely use transparent
bridging with the spanning-tree algorithm. You might also need a protocol for connecting
switches that supports virtual LANs (VLANs). This protocol could be the Cisco Inter-
Switch Link (ISL) protocol or the Institute of Electrical and Electronic Engineers (IEEE)
802.1Q protocol.
With Token Ring networks, your options include source-route bridging (SRB), source-
route transparent (SRT) bridging, and source-route switching (SRS). To connect Token
Ring and Ethernet LANs (or other dissimilar networks), you can use translational or
encapsulating bridging.
IBM developed SRB for Token Ring networks in the 1980s. In the early 1990s, IBM pre-
sented the SRB protocols to the IEEE. The protocols became the SRT standard, which is
documented in Annex C of the IEEE 802.1D document. An SRT bridge can act like a trans-
parent bridge or a source-routing bridge, depending on whether source-routing information
is included in a frame. SRS improves the efficiency of an SRT bridge and allows parallel
source-routing paths, which SRT does not support.
Although bridged and switched Token Ring networks still exist, most of them are being
migrated to Ethernet switched and routed networks. The rest of this section focuses on the
switching protocols used on typical Ethernet networks.

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