A transparent bridge floods all multicast frames, broadcast frames, and frames with an unknown destination address out every port except the one on which the frame was received. Broadcasts and multicast create scalability issues that can be controlled only with routing.
Transparent bridges implement the Spanning-Tree Algorithm, which is specified in IEEE 802.1d. In the sample network in Figure 6-8, the Spanning-Tree Protocol states that only one active path exists between two stations. If a physical loop exists in the network, the Spanning-Tree Protocol handles this loop by disabling bridge ports.
Figure 6-8 Spanning-Tree Protocol
Transparent bridges send Bridge Protocol Data Unit (BPDU) frames to each other to build and maintain a Spanning Tree. The BPDU frames are sent every two seconds. The amount of traffic caused by BPDU frames can be a scalability issue on large flat networks with numerous switches and bridges. For more information on the Spanning-Tree Protocol, see Chapter 4, "Network Topologies and LAN Design."
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