Network devices have one primary purpose: to pass network traffic from one
segment to another. (There are exceptions, of course, such as network analyzers,
which inspect traffic as it goes by.) With devices that independently make
forwarding decisions, traffic can travel from its source to the destination. The
higher up the OSI model a device operates, the deeper it looks into a packet to
make a forwarding decision. Railroad-switching stations provide a similar
example. The switches enable a train to enter the appropriate tracks (path)
that take it to its final destination. If the switches are set wrong, a train can
end up traveling to the wrong destination or traveling in a circle.
Switching technology emerged as the replacement for bridging. Switches provide
all the features of traditional bridging and more. Compared to bridges,
switches provide superior throughput performance, higher port density, and
lower per-port cost.
The different types of bridging include the following:
• Transparent bridging primarily occurs in Ethernet networks.
• Source-route bridging occurs in Token Ring networks.
• Translational bridging occurs between different media. For example, a translational
bridge might connect a Token Ring network to an Ethernet network.
Bridging and switching occur at the data link layer (Layer 2 in the OSI model),
which means that bridges control data flow, provide transmission error handling,
and enable access to physical media. Basic bridging is not complicated:
A bridge or switch analyzes an incoming frame, determines where to forward
the frame based on the packet’s header information (which contains information
on the source and destination addresses), and forwards the frame toward
its destination. With transparent bridging, forwarding decisions happen one
hop (or network segment) at a time. With source-route bridging, the frame
contains a predetermined path to the destination.
Bridges and switches divide networks into smaller, self-contained units.
Because only a portion of the traffic is forwarded, bridging reduces the overall
traffic that devices see on each connected network. The bridge acts as a kind
of firewall in that it prevents frame-level errors from propagating from one
segment to another. Bridges also accommodate communication among more
devices than are supported on a single segment or ring.
Bridges and switches essentially extend the effective length of a LAN, permitting
more workstations to communicate with each other within a single broadcast
domain. The primary difference between switches and bridges is that
bridges segment a LAN into a few smaller segments. Switches, through their
increased port density and speed, permit segmentation on a much larger scale.
Modern-day switches used in corporate networks have hundreds of ports per
chassis (unlike the four-port box connected to your cable modem).
Additionally, modern-day switches interconnect LAN segments operating at
different speeds.
Switching describes technologies that are an extension of traditional bridges.
Switches connect two or more LAN segments and make forwarding decisions
about whether to transmit packets from one segment to another. When a
frame arrives, the switch inspects the destination and source Media Access
Control (MAC) addresses in the packet. (This is an example of store-andforward
switching.) The switch places an entry in a table indicating that the
source MAC address is located off the switch interface on which the packet
arrived. The bridge then consults the same table for an entry for the destination
MAC address. If it has an entry for the destination MAC address, and the
entry indicates that the MAC address is located on a different port from which
the packet was received, the switch forwards the frame to the specified port.
If the switch table indicates that the destination MAC address is located on the
same interface on which the frame was just received, the bridge does not forward
the frame. Why send it back onto the network segment from which it
came? This decision is where a switch reduces network congestion. Finally, if
the destination MAC address is not in the switch’s table, this indicates that the
switch has not yet seen a frame destined for this MAC address. In this case,
the switch then forwards the frames out all other ports (called flooding) except
the one on which the packet was received.
At their core, switches are multiport bridges. However, switches have radically
matured into intelligent devices, replacing both bridges and hubs. Switches not
only reduce traffic through the use of bridge tables, but also offer new functionality
that supports high-speed connections, virtual LANs, and even traditional
routing.