Cabletron Systems EMM-E6 Ethernet manual Bridges, OSI Model

BRIDGES

1.6BRIDGES

An Ethernet bridge is a device that allows the expansion of a network beyond the limitations of the IEEE 802.3 specified limits for repeated Ethernet networks. If an Ethernet network has a repeater hop of four repeaters or a round trip propagation delay near the 51.2 ∝s maximum, a bridge can be used to build an extended network. Ethernet bridges read in packets and decide to filter or forward based on the destination address of the packet. The simple forward/filter decision process allows a bridge to segment traffic between two networks, keeping local traffic local. This process increases the availability of each network while still allowing traffic destined for the opposite side of the bridge to pass.

Bridges can also connect similar networks together such as Ethernet, Token Ring, and Fiber Distributed Data Interface (FDDI) together. Note that similar networks means that the upper five layers of the OSI model, see Figure 1-2, are the same but that different Data Link and Physical layers may be used by the architecture. The Bridge operates at the Data Link level of the OSI model. It stores packets and based on the packet destination address, forwards or filters the packets. Because bridges work at layer 2 of the OSI model, bridges are protocol independent. A bridge must read the complete data frame, check for errors, and make forward or filter decisions based on recognized addresses stored in its source address table.

7.Application

6.Presentation

5.Session

4.Transport

3.Network

2.Data Link

1.Physical

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Bridge

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Figure 1-2. OSI Model

EMM-E6 User’s Guide 1-17