Bridges and switches analyze incoming frames, make forwarding decisions based on information contained in the frames, and forward the frames toward the destination. Upper-layer protocol transparency is a primary advantage of both bridging and switching. Because both device types operate at the link layer, they are not required to examine upper-layer information. Bridges are also capable of filtering frames based on any Layer 2 fields.
Although bridges and switches share most relevant attributes, several distinctions differentiate these technologies. Switches are significantly faster because they switch in hardware. Switches also can support higher port densities than bridges. And, some switches support cut-through switching, which reduces latency and delays in the network, while bridges support only store-and-forward traffic switching. The primary differences, though, are that bridges perform switching via software (as opposed to hardware) and switches have a higher port density.
Layer 2 switching is basically
hardware-based bridging. -
In a switch, frame forwarding is handled by specialized hardware
called Application-Specific Integrated Circuits (ASICs). The ASIC
technology engineered for switches allows for scalability up to
gigabit speeds, with low latency at costs significantly lower than
Ethernet bridges.
Layer 2 switches provide network
managers with the ability to increase bandwidth without adding
complexity to the network. Layer 2 data frames consist of both
control information, such as MAC addresses, and end-user content. At
Layer 2, no modification of the frame control information is
required when moving between similar Layer 1 interfaces, such as
Ethernet and Fast Ethernet. However, changes to control information
may occur when bridging between unlike media types such Fiber
Distributed Data Interface (FDDI) or ATM and Ethernet.
Workgroup connectivity and network
segmentation are the two primary uses for Layer 2 switches. The high
performance of a Layer 2 switch allows for network designs that
significantly decrease the number of hosts per physical segment.
Decreasing the number of hosts per segment leads to a flatter design
with more segments in the campus network.
However, despite the advantages of
Layer 2 switching, it still has all the same characteristics and
limitations of legacy bridging. Broadcast domains are unaffected by
the incorporation of Layer 2 switches - the same scaling and
performance issues exist in this respect as in the large bridged
networks of the past. The broadcast and multicast radius increases
with the number of hosts, and broadcasts are still propagated across
the network.
Thus, it is evident that Layer 3
functionality is still needed within the network. Hence, we
introduce Layer 3 switching!
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