is a brief introduction to routers the preferred devices
in large networks operating on different technologies
router is a device that operates at the network layer,
or layer 3 of the OSI model. When compared to hubs and
switches, routers are much smarter. Routers use a more
complete packet "address" to determine which
router or workstation should receive the next packet.
Routers can use either MAC addresses or administratively
assigned logical addresses (such as IP addresses) to
handle data routing.
Based on a network road map called a "routing table,"
routers can help ensure that packets travel the most
efficient path to their destinations. If a link between
two routers fails, the sending router can determine
an alternate route to keep traffic moving.
Routers are designed to link many network technologies,
and are frequently used in large campus and enterprise
networks. At the network layer of operation, you find
a wider range of mechanisms that are designed to deal
with the issues that can arise when building large network
systems. By operating at the network layer, routers
can easily deal with computers attached to everything
from slow-serial links to high-speed LAN systems.
sufficiently large and complex networks, and especially
networks with multiple network technologies, you may
wish to use a router for the advantages that it can
bring. Many vendors provide routers with multi-protocol
capabilities, making it possible to deal with a variety
of high-level protocols in a single device. Examples
of protocols include Internet Protocol (IP), Internet
Packet Exchange (IPX), and AppleTalk. While routers
are more complex to configure, the advantages tend to
offset the added complexity of their operation for many
In operation, a router unpacks all Ethernet frames sent
to it, and deals with the high-level protocol data carried
in the frame. When a router hears an Ethernet broadcast,
it does the same thing as all other stations on the
channel must do: it reads the frame in and tries to
figure out what to do with it. If the broadcast happens
to come from a client station attempting to discover
a service on a server station on that channel, for example,
the router will do what all other machines on the channel
other than the server will do: discard the broadcast
since it has nothing to do with them.
Therefore, unlike a switching hub, a router does not
automatically forward broadcast and multicast frames.
Because of this, routers limit the flow of broadcasts
and multicasts to the local LAN, thereby creating separate
broadcast domains and protecting a large network system
from the high multicast and broadcast traffic rates
that can occur when you link a large number of stations
together with a layer 2 switch. On large networks with
many stations, this is a decided advantage both for
the reduced traffic levels and for the reduction in
problems that can be caused by broadcasts.
vs. Switching Hubs: Which to Use?
Here we briefly describe some of the differences
in operation between routers and switching hubs, and
the different impact of routers and switching hubs on
the operation of your network. Although they are both
store and forward switches, and can both be used to
extend Ethernet by building larger network systems,
routers and layer 2 switching hubs operate in very different
ways. It's up to you to decide which device is best
suited to your needs, and which set of capabilities
is most important for your network design.
Bridging routers, or brouters, offer the best of both
worlds. Remember that some protocols are non-routeable,
such as NetBEUI. What if your network consisted of NetBEUI
traffic and TCP/IP traffic? In this scenario you could
use a brouter to route TCP/IP, and bridge the NetBEUI
Some advantages of routers are:
Routers block the flow of broadcasts. This, combined
with their ability to structure the flow of traffic
through a system based on layer 3 network protocol
address allows you to design more complex network
topologies while still retaining stability of network
operation as your network system grows and evolves.
Routers provide the ability to link multiple IP
networks without fragmentation problems.
Routers use routing protocols that can provide information
about paths such as the bandwidth of the path. Using
that information routers can provide best-path routing,
and can use multiple paths to provide load sharing.
Some disadvantages of routers are:
Routers are more complex to configure, and require
routing software for each high-level protocol suite
that you need to route.
Routers cannot provide support for non-routable
protocols that were designed to work on a single
LAN and that do not provide the information required
for routing (e.g., NetBIOS).
These advantages and disadvantages need to be considered
when deciding on the appropriate technology for extending
your Ethernet system. As always, the network world is
in rapid evolution, and things are changing all the
time. For example, as fast switching circuits continue
to drop in price, vendors are making routers available
at lower costs.
The world of switching hubs is rapidly evolving, and
switches are being given more router-like capabilities.
Switches that can base their forwarding decisions on
network layer protocol addresses are sometimes called
layer 3 switching hubs. In addition, "multilayer
switches" are being developed which combine layer
2 switching and layer 3 routing capabilities in the
same box. You need to carefully evaluate these approaches
to extending networks and decide how well they will
fit into your network system, given the requirements
at your site.
strip off the outer
layers of Ethernet or
Token- Ring data before they send a packet from one
LAN to the other, so they reduce the total number of
bits going across the inter-LAN communications link.
The remote router at the receiving end repackages the
data into a packet or frame appropriate for its LAN
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