networking devices 21
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Network Devices
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Ethernet - Architecture
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What are internetworking
devices?
Internetworking devices are
products used to connect networks As computer networks grow in size
and complexity, so do the
internetworking devices used toconnect them.
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The purposes of having
devices First, they allow a greater number of nodes
to be connected to the network.
Second, they extend the distance over which
a network can extend.
Third, they localize traffic on the network.
Fourth, they can merge existing networks.
Fifth, they isolate network problems so that
they can be diagnosed more easily.
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What internetworking devices operate at
the physical layer (layer 1) of the OSImodel?
A repeater can provide a simple solution if either
of these two problems exists.
When signals first leave a transmitting station,they are clean and easily recognizable
However, the longer the cable length, the weaker
and more deteriorated the signals become as they
pass along the networking media.
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What is the disadvantage associated with
using a repeater?
it can't filter network traffic. Data,
sometimes referred to as bits, arriving at
one port of a repeater gets sent out on all
other ports
data gets passed along by a repeater to all
other LAN segments of a networkregardless of whether it needs to go there or
no
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What problem could occur as a result of
too much traffic on a network?
if segments of a network are only connected
by non-filtering devices such as repeaters,
this can result in more than one user trying
to send data on the network at the same time
If more than one node attempts to transmit
at the same time, a collision will occur.
When a collision occurs, the data from each
device impact and are damaged
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Hub Multi-port repeaters are often called hubs
Hubs are very common internetworking
devices
Generally speaking, the term hub is used
instead of repeater when referring to the
device that serves as the center of a star
topology network.
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Hubs Intelligent hubs have console ports, to allow
monitoring of the hubs status and port activity.
Passive hubs just repeat any incoming signals to
every port available, therefore does not act as a line
repeater. Passive hubs just split signals to multiple ports but
do not regenerate the signals, which means that
they do not extend a cables length. They only allow
two or more hosts to connect to the same cable
segment.
Active hubs regenerate signals.
Hubs utilise star topology.
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Advantages As an active hubs regenerate signals, it
increases the distance that can be spanned by
the LAN (up to 100 meters per segment). Hubs can also be connected locally to a
maximum of two other hubs, therebyincreasing the number of devices that can beattached to the LAN.
Active hubs are usually used againstattenuation, which is a decrease in thestrength of the signal over distance.
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Disadvantages
Bandwidth is shared by all hosts i.e. 10Mbs
shared by 25 ports/users.
Can create bottlenecks when used with
switches.
Have no layer 3 switching capability.
Most Hubs are unable to utilise VLANS.
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What internetworking device can be
used to filter traffic on the network?
One way to solve the problems of too much
traffic on a network and too many collisions
is to use an internetworking device called a
bridge.
A bridge eliminates unnecessary traffic and
minimizes the chances of collisionsoccurring on a network by dividing it into
segments
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Bridges
A network bridge connects multiple network segments at
the data link layer (layer 2) of the OSI model.
Bridges are similar to repeaters or network hubs, devices that
connect network segments at the physical layer
However a bridge works by using bridging where trafficfrom one network is managed rather than simply rebroadcast
to adjacent network segments
In Ethernet networks, the term "bridge" formally means a
device that behaves according to theIEEE
802.1D standard -this is most often referred to as a network switch in
marketing literature.
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At what layer of the OSI model
do bridges operate? Because bridges operate at the data link layer, layer 2,they are not required to examine upper-layer
information.
A bridge works at the data-link (physical network) level of a
network, copying a data frame from one network to the next
network along the communications path.
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How are bridge data-
forwarding decisions limited?
Although bridges use tables todetermine whether or not toforward data to other segmentsof the network, the types of
comparisons and decisions theymake are relatively low level,simple ones
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What types of network traffic
problems is a bridge incapableof solving?
Bridges work best where traffic from one
segment of a network to other segments is
not too great.
However, when traffic between network
segments becomes too heavy, the bridge canbecome a bottleneck and actually slow
down communication
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Transparent and Source Route
BridgingTransparent Bridging
This method uses a forwardingdatabase to send frames acrossnetwork segments. The
forwarding database is initiallyempty and entries in the databaseare built as the bridge receivesframes. If an address entry is notfound in the forwardingdatabase, the frame isrebroadcast to all ports of thebridge, forwarding the frame toall segments except the sourceaddress.
Also known as adaptive bridges.
Route Source Bridging
With source route bridging twoframe types are used in order tofind the route to the destination
network segment. Single-Route(SR) frames comprise most of thenetwork traffic and have setdestinations, while All-Route(AR) frames are used tofind routes. Bridges send ARframes by broadcasting on allnetwork branches; each step ofthe followed route is registeredby the bridge performing it.
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ADVANTAGES
Self configuring
Primitive bridges are often inexpensive
Reduce size of collision domain
Transparent to protocols above the MAC layer
Allows the introduction of management -performance information and access control
LANs interconnected are separate and physicalconstraints such as number of stations,repeaters and segment length don't apply
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How many addressing schemes
are there in networking?
One of the addressing schemes is the
MAC address
The second addressing scheme in
networking makes use of what is calledthe IP address
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How do IP addresses differ
from MAC addresses? Like MAC addresses, every IP address is
unique. No two IP addresses are ever alike.
However, while MAC addresses are
physical addresses that are actually hard-
coded into the NIC card and occur at the
data link layer IP addresses are implemented in software
and occur at the network layer of the OSI
model.
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Switches
A switch is a multi-port bridge.
It operates at OSI data link layer 2.
It stores MAC addresses in an internal lookup table.
Temporary switched paths are created between the
frames source destination.
Some Switches have limited layer 3 IP routing
capabilities. Switches can be configured to use VLANS.
Switches support spanning tree protocol to create
resilient networks.
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What are routers?
Routers are another type of internetworking
device.
These devices pass data packets between
networks based on network protocol or
layer 3 information.
Routers have the ability to make intelligentdecisions as to the best path for delivery of
data on the network.
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Routers Routers are OSI network layer 3 devices
Using interface modules can connect different layer
2 technologies e.g. Ethernet, FDDI, token ring etc
Routers have the capability to interconnect network
segments or entire networks (WANS/MANS).
These devices examine incoming packets to
determine the destination address of the data. It
then examines its internal routing table to choose
the best path for the packet through the network,
and switches them to the proper outgoing port.
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What network problems can
routers help resolve?
The problem of excessive broadcast traffic
can be solved by using a router
Routers are able to do this, because they do
not forward broadcast frames unlessspecifically told to do so
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How do routers differ from
bridges?
First, bridging occurs at the data link layer
or layer 2,while routing occurs at thenetwork layer or layer 3 of the OSI model.
Second, bridges use physical orMAC
addresses to make data forwarding
decisions. Routers use a different
addressing scheme that occurs at layer three
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