3Com TR manual Transparent Bridging TP, Bridge Table

the destination ring station. A route is simply the path a packet takes through a source route bridged net- work from the source ring station to the destination ring station.

In a multiple ring environments, nodes on different rings need additional bridging information before they can communicate with each other. A source ring station must first determine if one or more routes exist to another station on a remote ring.

In general, the source ring station determines the route by sending a “discovery” packet out across the multi-ring network. Source routing bridges forward this “discovery” packet while adding path informa- tion. Likewise, switches forward any response from the destination ring station to the “discovery” packet originator. When a source ring station receives a response it updates its own bridging table with the information. After determining the optimal path, the ring stations include the path information in every packet transmitted between them.

Switching provides connectivity between LANs, form- ing enterprise-wide networks. In a Token Ring envi- ronment, source routing switches connect Token Ring LANs and enable peer-to-peer and terminal-to-host communications across both local and wide area Token Ring networks. Both PC workgroup protocols and IBM’s Systems Network Architecture (SNA) net- works are supported by source routing switches.

Another advantage of switching is that it allows the segmentation of the Token Ring network into multi- ple rings to reduce traffic on any one particular ring segment. The adjacent rings may be connected by

Token Ring Switching Concepts 1-15

parallel source routing switches to provide fault toler- ance. In the event one switch fails, the other parallel switch can be configured to automatically maintain connectivity, providing an alternate route for data between the two rings.

Transparent Bridging (TP)

Transparent bridging provides the simplest data com- munication method. In transparent bridging end sta- tions are not aware of existing intermediate bridges. As such, a transparent bridge learns about its sur- rounding network from the source addresses of the packets that it receives. Forwarding decisions are based on the destination addresses contained in the MAC header. Transparent bridges consult their bridg- ing table (see “Bridge Table) to determine which port should forward the traffic on to the destination port.

Bridge Table

A switch dynamically manages and updates its bridg- ing table. All switches receive every packet transmit- ted on the segments attached to its ports. As a result, a switch is able to “learn” the source MAC addresses of each station that transmits packets on its attached segments. Since a switch never places its own MAC address in a packet that it forwards, the received source MAC address always identifies the original transmitter of the packet.

A switch uses the received source address information to construct its bridging table. The bridging table con- tains a list of all received MAC addresses and the ports on which they were learned. From its bridging