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Intranets
Chapter 3
How Bridges Work
CONTENTS
An intranet-especially one in a large corporation-can be a remarkably
complex endeavor, consisting of many networks, including departmental
local area networks, as well as larger subnetworks, which
each are in essence collections of networks.
Bridges connect networks. When bridges were first developed they
could not connect incompatible networking architectures. Early
bridges functioned at the media-access control (MAC) section of
the data-link layer of the OSI model (discussed in Chapter 2.
Ethernet and Token Ring architecture differ at the MAC section
of the data-link layer, hence their incompatibility. Newer bridges
function at the logical-link control (LLC) portion of the data-link
layer. As long as the communication protocol on both networks
is the same (IPX to IPX, for example), Ethernet and Token Ring
networks can be linked by bridges. Connecting divergent networks
is an essential requirement for intranets.
Bridges are also used to connect networks indirectly by long-distance,
usually leased, lines. Remote bridges on Ethernet networks use
a transparent routing technique to handle traffic. It is
the destination address of each packet that is read to determine
the action to be taken. If the destination is on the same network,
bridges ignore the packet; bridges pass only packets that need
to go to other networks. Token Ring operates somewhat differently
in that it uses source routing. In this case, the bridge
uses a test message to calculate the best path between the source
and the destination.
In order to determine whether to pass or drop a packet, bridges
refer to a table, which is basically a list of addresses. These
tables originally had to be built manually, causing considerable
maintenance every time a replacement or additional NIC address
was put on the network. This led to the development of learning
bridges, that is, bridges with software algorithms that could
build these tables automatically. Today's bridges build and maintain
their tables by listening to cable traffic and checking packets
for source addresses.
Bridges are relatively simple to set up and maintain, and they
can operate fairly fast since they simply decide whether to pass
or drop the packet. In the next chapter a similar but more complex
piece of equipment, the router, is discussed. Bridges are often
used to segment and reduce local network traffic. Routers are
usually used for connecting entire networks and subnetworks together.
Bridges and routers are sometimes combined into a single product
called a brouter. A brouter combines the functions of both
bridge and router. It examines the outermost address to see if
the data needs to be sent to another LAN over a bridge, and delivers
information that way if possible. If, instead, it needs to be
sent using IP technology, it will act as a router does.
Bridges are hardware and software combinations that connect different
parts of a single network, such as different sections of an intranet.
They connect local area networks (LANs) to each other. They are
generally not used, however, for connecting entire networks to
each other, for example, for connecting an intranet to the Internet,
or an intranet to an intranet, or to connect an entire subnetwork
to an entire subnetwork. To do that, more sophisticated pieces
of technology called routers are used. (See Chapter 4for more
information on how routers work.)
- When there is a great amount of traffic on an Ethernet local
area network, packets can collide with one another, reducing
the efficiency of the network, and slowing down network traffic.
Packets can collide because so much of the traffic is routed among
all the workstations on the network.
- In order to cut down on the collision rate, a single LAN can
be subdivided into two or more LANs. For example, a single LAN
can be subdivided into several departmental LANs. Most of the
traffic in each departmental LAN stays within the department LAN,
and so it needn't travel through all the workstations on all the
LANs on the network. In this way, collisions are reduced. Bridges
are used to link the LANs. The only traffic that needs to travel
across bridges is traffic bound for another LAN. Any traffic within
the LAN need not travel across a bridge.
- Each packet of data on an intranet has more information in
it than just the IP information. It also includes addressing information
required for other underlying network architecture, such as for
an Ethernet network. Bridges look at this outer network addressing
information and deliver the packet to the proper address on a
LAN
- Bridges consult a learning table that has the addresses
of all the network nodes in it. If a bridge finds that a packet
belongs on its own LAN, it keeps the packet inside the LAN. If
it finds that the workstation is on another LAN, it forwards the
packet. The bridge constantly updates the l
earning table as it monitors and routes traffic.
- Bridges can connect LANs in a variety of different ways. They
can connect LANs using serial connections over traditional phone
lines and modems, over ISDN lines, and over direct cable connections.
CSU/DSU units are used to connect bridges to telephone lines for
remote connectivity.
- Bridges and routers are sometimes combined into a single product
called a brouter. A brouter handles both bridging and routing
tasks. If the data needs to be sent only to another LAN on the
network or subnetwork, it will act only as a bridge delivering
the data based on the Ethernet address. If the destination is
another network entirely, it will act as a router, examining the
IP packets and routing the data based on the IP address.
Contact
reference@developer.com with questions or comments.
Copyright 1998
EarthWeb Inc., All rights reserved.
PLEASE READ THE ACCEPTABLE USAGE STATEMENT.
Copyright 1998 Macmillan Computer Publishing. All rights reserved.
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