What Are Bridges and Switches?
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.
Link Layer Device Overview
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-layerprotocoltransparencyisaprimaryadvantageofbothbridgingandswitching.Becauseboth
device types operate at the link layer, they are not required to examine upper-layer information. This
meansthattheycanrapidlyforwardtrafficrepresentinganynetworklayerprotocol.Itisnotuncommon
forabridgetomoveAppleTalk,DECnet,TCP/IP,XNS,andothertrafficbetweentwoormorenetworks.
Bridges are capable of filtering frames based on any Layer 2 fields. For example, a bridge 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
potentiallydamagingnetworkerrorsandwillaccommodatecommunicationbetweenalargernumberof
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.
Although bridges and switches share most relevant attributes, several distinctions differentiate these
technologies.Bridgesare generallyusedtosegmentaLAN intoacoupleofsmaller segments.Switches
are generally used to segment a large LAN into many smaller segments. Bridges generally have only a
few ports for LAN connectivity, whereas switches generally have many. Small switches such as the
Cisco Catalyst 2924XL have 24 ports capable of creating 24 different network segments for a LAN.
Larger switches such as the Cisco Catalyst 6500 can have hundreds of ports. Switches can also be used
toconnectLANswithdifferentmedia—forexample,a10-MbpsEthernetLANanda100-MbpsEthernet
LAN can be connected using a switch. 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.
Types of Bridges
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.
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 much 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 onboard
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.
SomebridgesareMAC-layerbridges,whichbridgebetweenhomogeneousnetworks(forexample,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).
Types of Switches
Switches are data link layer devices that, like bridges, enable multiple physical LAN segments to be
interconnected into a single larger network. Similar to bridges, switches forward and flood traffic based
onMACaddresses.Anynetworkdevicewillcreatesomelatency.Switchescanusedifferentforwarding
techniques—two of these are store-and-forward switching and cut-through switching.
Instore-and-forwardswitching,anentireframemustbereceivedbeforeitisforwarded.Thismeansthat
thelatencythroughtheswitchisrelativetotheframesize—thelargertheframesize,thelongerthedelay
throughtheswitch.Cut-throughswitchingallowstheswitchtobeginforwardingtheframewhenenough
of the frame is received to make a forwarding decision. This reduces the latency through the switch.
Store-and-forward switching gives the switch the opportunity to evaluate the frame for errors before
forwarding it. This capability to not forward frames containing errors is one of the advantages of
switches over hubs. Cut-through switching does not offer this advantage, so the switch might forward
frames containing errors. Many types of switches exist, including ATM switches, LAN switches, and
various types of WAN switches.
ATM Switch
Asynchronous Transfer Mode (ATM) switches provide high-speed switching and scalable bandwidths in
theworkgroup,theenterprisenetworkbackbone,andthewidearea.ATMswitchessupportvoice,video,
anddataapplications,andaredesignedtoswitchfixed-sizeinformationunitscalledcells,whichareused
in ATM communications. Figure 4-3 illustrates an enterprise network comprised of multiple LANs
interconnected across an ATM backbone.
LAN Switch
LAN switches are used to interconnect multiple LAN segments. LAN switching provides dedicated,
collision-free communication between network devices, with support for multiple simultaneous
conversations. LAN switches are designed to switch data frames at high speeds. Figure 4-4 illustrates a
simple network in which a LAN switch interconnects a 10-Mbps and a 100-Mbps Ethernet LAN.
Bridging and Switching Basics
