BRIDGESEMM-E6 User’s Guide1-171.6 BRIDGESAn Ethernet bridge is a device that allows the expansion of a networkbeyond the limitations of the IEEE 802.3 specified limits for repeatedEthernet networks. If an Ethernet network has a repeater hop of fourrepeaters or a round trip propagation delay near the 51.2 μs maximum, abridge can be used to build an extended network. Ethernet bridges read inpackets and decide to filter or forward based on the destination address ofthe packet. The simple forward/filter decision process allows a bridge tosegment traffic between two networks, keeping local traffic local. Thisprocess increases the availability of each network while still allowingtraffic 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. Notethat 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 Physicallayers may be used by the architecture. The Bridge operates at the DataLink level of the OSI model. It stores packets and based on the packetdestination address, forwards or filters the packets. Because bridges workat layer 2 of the OSI model, bridges are protocol independent. A bridgemust read the complete data frame, check for errors, and make forward orfilter decisions based on recognized addresses stored in its source addresstable.Figure 1-2. OSI Model765432121Bridge7. Application6. Presentation5. Session4. Transport3. Network2. Data Link1. Physical7654321
