3Com manual 11-6CHAPTER 11 Ethernet Lans and Bridges in the 7600 Card

11-6CHAPTER 11: ETHERNET LANS AND BRIDGES IN THE 7600 CARD

Transparent Bridges The transparent bridge is a device that connects to two or more LANs and provides a selective data packet transfer capability between them which allows them to work in a bridged-LAN environment. The bridged-LAN can support more stations and a greater amount of traffic than can a simple LAN and, at the same time, it eliminates the cable length restriction between stations.

Bridge ports A LAN is attached to a bridge through a bridge port which handles traffic between the LAN and the bridge. Each bridge port has a unique address. The bridge ports are numbered internally by the bridge. In the 7600 Card, bridge ports are numbered differently from the Ethernet ports to which the Ethernet LANs are physically attached. LANs are assigned to bridges by associating physical port numbers to bridge port numbers when setting up the device. Figure 11-4 below shows LANs connected to a bridge to form a bridged-LAN environment.

Figure 11-3Forming a bridged-LAN environment

Store-and-ForwardThe basic idea of the transparent bridge, as specified in the IEEE 802.1d bridging standard, is that it listens to every packet transmitted by the LANs to which it is attached, storing each received packet until it can be transmitted on the LANs other than the one on which it was received. Since the entire packet is stored in the bridge and retransmitted only when the receiving LANs are idle, the cable length restriction needed to avoid collisions in a single LAN can be exceeded between stations in the bridged LAN. In addition, the total number of stations in the bridged LAN can exceed the permissible number in a single LAN because each LAN is treated separately by the bridge.