Layer 2 Switching Configuration
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OmniAccess 5740 Unified Services Gateway CLI Configuration Guide
Alcatel-Lucent
SWITCHING OVERVIEW
Bridges and switches are data communication devices that operate principally at
Layer 2 of the OSI reference model. As such they are widely referred to as Data
Link Layer devices.
Bridges became commercially available in the early 1980s. At the time of their
introduction, bridges connected and enabled packet forwarding between
homogeneous networks. More recently, bridging between different networks has
also been defined and standardized. Several kinds of bridges have proven
important as internetworking devices. ‘Transparent bridging’ is found primarily in
Ethernet environments. ‘Translational bridging’ provides translation between
the formats and transit principles of different media types.
Note: The OmniAccess 5740 USG supports only transparent bridging.
Bridging and switching occur at the link layer, which controls data flow, handles
transmission errors, provides physical addressing, and manages access to the
physical medium. By dividing large networks into self-contained units, bridges and
switches provide several advantages.
The switch acts as a firewall for some potentially damaging network errors and will
accommodate communication between a larger number of devices than would be
supported on any single LAN connected to the bridge. Bridges and switches
extend the effective length of a LAN, permitting the attachment of distant stations
that was not previously permitted.
Since the only devices on the segments are the switch and the end station, the
switch picks up every transmission before it reaches another node. The switch
then forwards the frame over the appropriate segment, just like a bridge, but since
any segment contains only a single node, the frame only reaches the intended
recipient. This allows many conversations to occur simultaneously on a switched
network.
Some switches support cut-through switching, which reduces latency and delays
in the network, while bridges support only store-and-forward traffic switching.
Switches also reduce collisions on network segments because they provide
dedicated bandwidth to each network segment. Some bridges are MAC-layer
bridges, which bridge between homogeneous networks, while other bridges can
translate between different link layer protocols. The basic mechanics of such a
translation is depicted in the graphic below.
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