MORE ABOUT BRIDGES
EMME USER’S GUIDE
Page 1-11
1.5
MORE ABOUT BRIDGES
A bridge is a device that can be added to a network to allow expansion
beyond the limitations of IEEE 802.3. If an Ethernet network has a
repeater hop (count) of four repeaters or a propagation delay near the
51.2-microsecond maximum, a bridge can be used to build an extended
network. Ethernet bridges read in packets and decide to filter or forward
them 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 are also used to connect similar networks 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 may have different Data Link and Physical
layers. 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. Bridges are slower than repeaters
because 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.
Figure 1-2. OSI Model
BRIDGE
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7. APPLICATION
6. PRESENTATION
5. SESSION
4. TRANSPORT
3. NETWORK
2. DATA LINK
1. PHYSICAL
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