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This chapter introduces the technologies
employed in devices loosely referred to as bridges and switches.
Topics summarized here include general link-layer device operations,
local and remote bridging, ATM switching, and LAN switching. Chapters in
Part 4, "Bridging and Switching," of this book address
specific technologies in more detail.
Bridges and switches are
data communications 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 bridging have proven
important as internetworking devices.
Transparent bridging is found primarily in Ethernet
environments, while source-route bridging occurs primarily in
Token Ring environments. Translational bridging provides
translation between the formats and transit principles of different
media types (usually Ethernet and Token Ring). Finally, source-route
transparent bridging combines the algorithms of
transparent bridging and source-route bridging to enable communication
in mixed Ethernet/Token Ring environments.
Today, switching technology has emerged
as the evolutionary heir to bridging based internetworking solutions.
Switching implementations now dominate applications in which bridging
technologies were implemented in prior network designs. Superior
throughput performance, higher port density, lower per-port cost, and
greater flexibility have contributed to the emergence of switches as
replacement technology for bridges and as complements to routing
technology.
Bridging and switching
occur at the link layer, which controls data flow,
handles transmission errors, provides physical (as opposed to logical)
addressing, and manages access to the physical medium. Bridges provide
these functions by using various link-layer
protocols that dictate specific flow control, error handling,
addressing, and media-access algorithms. Examples of popular link-layer
protocols include Ethernet, Token Ring, and FDDI.
Bridges and switches are not complicated
devices. They analyze incoming frames, make forwarding decisions based
on information contained in the frames, and forward the frames toward
the destination. In some cases, such as source-route bridging, the
entire path to the destination is contained in each frame. In other
cases, such as transparent bridging, frames are forwarded one hop at a
time 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. This means that they can rapidly forward
traffic representing any network-layer protocol. It is not uncommon for
a bridge to move AppleTalk, DECnet, TCP/IP, XNS, and other traffic
between two or more networks.
Bridges are capable of filtering frames
based on any Layer 2 fields. A bridge, for example, can be programmed to
reject (not forward) all frames sourced from a particular network.
Because link-layer information often includes a reference to an
upper-layer protocol, bridges usually
can filter on this parameter. Furthermore, filters can be helpful in
dealing with unnecessary broadcast and multicast packets.
By dividing large networks into
self-contained units, bridges and switches provide several advantages.
Because only a certain percentage of traffic is forwarded, a bridge or
switch diminishes the traffic experienced by devices on all connected
segments. The bridge or switch will act as a firewall for some
potentially damaging network errors, and both 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 were
not previously permitted.
Although bridges and switches share most
relevant attributes, several distinctions differentiate these
technologies. Switches are significantly faster because they switch in
hardware, while bridges switch in software and can interconnect LANs of
unlike bandwidth. A 10-Mbps
Ethernet LAN and a 100-Mbps Ethernet LAN, for example, can be connected
using a switch. Switches also can support higher port densities than
bridges. Some switches support cut-through switching, which reduces
latency and delays in the network, while bridges support only
store-and-forward traffic switching. Finally, switches reduce collisions
on network segments because they provide dedicated bandwidth to each
network segment.
Bridges can be grouped into categories
based on various product characteristics. Using one popular
classification scheme, bridges are either local
or remote. Local bridges provide a direct connection between
multiple LAN segments in the same area. Remote bridges connect multiple
LAN segments in different areas, usually over telecommunications lines. Figure
4-1 illustrates these two configurations.
Figure 4-1: Local and remote
bridges connect LAN segments in specific areas.
Remote bridging presents several unique
internetworking challenges, one of which is the difference between LAN
and WAN speeds. Although several fast WAN technologies now are
establishing a presence in geographically dispersed internetworks, LAN
speeds are often an order of magnitude faster than WAN speeds. Vast
differences in LAN and WAN speeds can prevent users from running
delay-sensitive LAN applications over the WAN.
Remote bridges cannot improve WAN speeds,
but they can compensate for speed discrepancies through a sufficient
buffering capability. If a LAN device capable of a 3-Mbps transmission
rate wants to communicate with a device on a remote LAN, the local
bridge must regulate the 3-Mbps data stream so that it does not
overwhelm the 64-kbps serial link. This is done by storing the incoming
data in on-board buffers and sending it over the serial link at a rate
that the serial link can accommodate. This buffering can be achieved
only for short bursts of data that do not overwhelm the bridge's
buffering capability.
The Institute of Electrical and
Electronic Engineers (IEEE) differentiates the OSI link layer into two
separate sublayers: the Media Access Control ( MAC)
sublayer and the Logical Link Control (LLC)
sublayer. The MAC
sublayer permits and orchestrates media access, such as contention and
token passing, while the LLC sublayer deals with framing, flow control,
error control, and MAC-sublayer addressing.
Some bridges are MAC-layer
bridges, which bridge between homogeneous networks (for example,
IEEE 802.3 and IEEE 802.3), while other bridges can translate between
different link-layer protocols (for example, IEEE 802.3 and IEEE 802.5).
The basic mechanics of such a translation are shown in Figure 4-2 .
Figure 4-2 illustrates an IEEE 802.3 host
(Host A) formulating a packet that contains application information and
encapsulating the packet in an IEEE 802.3-compatible frame for transit
over the IEEE 802.3 medium to the bridge. At the bridge, the frame is
stripped of its IEEE 802.3 header at the MAC sublayer of the link layer
and is subsequently passed up to the LLC sublayer for further
processing. After this processing, the packet is passed back down to an
IEEE 802.5 implementation, which encapsulates
the packet in an IEEE 802.5 header for transmission on the IEEE 802.5
network to the IEEE 802.5 host (Host B).
A bridge's translation between networks
of different types is never perfect because one network likely will
support certain frame fields and protocol functions not supported by the
other network.
Figure 4-2: A MAC-layer
bridge connects the IEEE 802.3 and IEEE 802.5 networks.
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