Download CE 4226 Network Systems Analysis and Design

Network Analysis and
Design
Introduction to Network Design
Network Design
A network design is a blueprint for
building a network
 The designer has to create the
structure of the network [and] decide
how to allocate resources and spend
money

2
Elements of Good Network
Design
Deliver the services requested by
users
 Deliver acceptable throughput and
response times
 Cost efficiency
 Reliable
 Expandable
 Manageable
 Well-documented

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Network Design Issues

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User requirements
Locations of devices
Characteristics of applications
Types of traffic
Topologies
Routing protocols
Budget
Performance
Etc.
4
Classifications of Network
Design
Build a new network
 Expand or upgrade the existing
network
 Create the overlay network

 Virtual
Private Network (VPN)
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Types of Networks

Access network:
 The
ends or tails of networks that
connect the small sites into the network
 LAN, campus network

Backbone network:
 The
network that connects major sites
 Corporate WAN
6
Objectives
How to design a network using the
correct techniques?
 Some common guidelines applicable
for all types of network design

7
Top-Down Network Design
Methodology
 A complete process that matches
business needs to available technology
to deliver a system that will maximize an
organization’s success
 Don’t just start connecting the dots


In the LAN, it is more than just buying a
few devices
In the WAN, it is more than just calling the
phone company
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Top-Down Network Design
Methodology (Contd.)
 Analyze business and technical goals
first
 Explore divisional and group structures
to find out who the network serves and
where they reside
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Top-Down Network Design
Methodology (Contd.)
 Determine what applications will run on
the network and how those applications
behave on a network
 Focus on applications, sessions, and
data transport before the selection of
routers, switches, and media that
operate at the lower layers
10
Network Design Phases
 Requirement analysis
 Logical network design
 Physical network design
11
Phase I - Requirement
Analysis Phase
 Analyze goals and constraints
 Characterize the existing network
 Characterize network traffic
12
Phase II - Logical Network
Design Phase
 Map the requirements into the
conceptual design
 Design a network topology
 Node locations
 Capacity assignment
13
Phase III - Physical Network
Design Phase
 Select technologies and devices for your
design
 Implementation
14
Business Goals
 Increase revenue
 Reduce operating costs
 Improve communications
 Shorten product development cycle
 Expand into worldwide markets
 Build partnerships with other companies
 Offer better customer support or new
customer services
15
Recent Business Priorities
 Mobility
 Security
 Resiliency (fault tolerance)
 Business continuity after a disaster
 Networks must offer the low delay
required for real-time applications such
as VoIP
16
Business Constraints
 Budget
 Staffing
 Schedule
 Politics and policies
17
Information
 Goals of the project
 What problem are they trying to solve?
 How will new technology help them be more
successful in their business?
 Scope of the project
 Small in scope: Allow sales people to access
network via a VPN
 Large in scope: An entire redesign of an enterprise
network
 Does the scope fit the budget, capabilities of
staff and consultants, schedule?
18
Information (Contd.)
 Applications, protocols, and services
 Current logical and physical architecture
 Current performance
19
Technical Goals
 Scalability
 Availability
 Performance
 Security
 Manageability
 Usability
 Adaptability
 Affordability
20
Scalability
 Scalability refers to the ability to grow
 Network must adapt to increases in
network usage and scope in the future
 Flat network designs don’t scale well
 Broadcast traffic affects the scalability of
a network
21
Availability
 Availability is the amount of time a
network is available to users
 Availability can be expressed as a
percent up time per year, month, week,
day, or hour, compared to the total time
in that period


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24/7 operation
Network is up for 165 hours in the 168hour week
Availability is 98.21%
22
Availability (Contd.)
 Different applications may require
different levels
 Some enterprises may want 99.999% or
“Five Nines” availability
23
Availability (Contd.)
 An uptime of 99.70 %


Downtime = 0.003 x 60 x 24 x 7
30.24 mins per week
 An uptime of 99.95 %
 Downtime = 0.0005 x 60 x 24 x 7
 5.04 mins per week
 An uptime of 99.999 %
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
Downtime = 0.00001 x 60 x 24 x 365
5.256 mins per year
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Availability (Contd.)
 System availability (R) is calculated from
the component availability (Ri)
 Series:
 R =  Ri
 Parallel:
 R = 1 – (1 – Ri)
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Availability (Contd.)
 R1 = 99.95%, R2 = 99.5%
 Series:


R = 0.9995 x 0.995 = 99.45%
Decreases system availability
 Parallel:


R = 1 – [(1 – 0.9995) x (1 – 0.995)] =
99.99975%
Increases system availability
26
Availability (Contd.)
 99.999% may require high redundancy
(and cost)
ISP 1
ISP 2
ISP 3
Enterprise
27
Availability (Contd.)
 Availability can also be expressed as a
mean time between failure (MTBF), and
mean time to repair (MTTR)
 Availability = MTBF / (MTBF + MTTR)
 A typical MTBF goal for a network that is
highly relied upon is 4000 hours. A
typical MTTR goal is 1 hour.
 4000 / 4001 = 99.98% availability
28
Network Performance
 Common performance factors include

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Bandwidth
Throughput
Bandwidth utilization
Offered load
Accuracy
Efficiency
Delay (latency) and delay variation
Response time
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Bandwidth Vs. Throughput
 They are not the same thing
 Bandwidth is the data carrying capacity
of a circuit
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Usually specified in bits per second
Fixed
 Throughput is the quantity of error free
data transmitted per unit of time
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Measured in bps, Bps, or packets per
second (pps)
Varied
30
Other Factors that Affect
Throughput
 The size of packets
 Inter-frame gaps between packets
 Packets-per-second ratings of devices that forward
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packets
Client speed (CPU, memory, and HD access speeds)
Server speed (CPU, memory, and HD access speeds)
Network design
Protocols
Distance
Errors
Time of day
etc.
31
Throughput of Devices
 The maximum PPS rate at which the
device can forward packets without
dropping any packets
 Theoretical maximum is calculated by
dividing bandwidth by frame size,
including any headers, preambles, and
interframe gaps
Bandwidth
PPS 
Frame Size  Header Size
32
Throughput of Devices
(Contd.)
Frame Size
(Bytes)
64
128
256
512
768
1024
1280
1518
Theoretical Max PPS
(100-Mbps Ethernet)
148,800
84,450
45,280
23,490
15,860
11,970
9,610
8,120
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Bandwidth, Throughput, Load
100 % of Capacity
T
h
r
o
u
g
h
p
u
t
Actual
100 % of Capacity
Offered Load
34
Throughput Vs. Goodput
 Most end users are concerned about the
throughput for applications
 Goodput is a measurement of good and
relevant application layer data
transmitted per unit of time
 In that case, you have to consider that
bandwidth is being “wasted” by the
headers in every packet
35
Utilization
 The percent of total available capacity in
use
 For WANs, optimum average network
utilization is about 70%
 For hub-based Ethernet LANs, utilization
should not exceed 37%, beyond this
limit, collision becomes excessive
36
Utilization (Contd.)
 For full-duplex Ethernet LANs, a point-to-point
Ethernet link supports simultaneous
transmitting and receiving
 Theoretically,


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Fast Ethernet means 200 Mbps available
Gigabit Ethernet means 2 Gbps available
100% of this bandwidth can be utilized
 Full-duplex Ethernet is becoming the standard
method for connecting servers, switches, and
even end users' machines
37
Efficiency

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Large headers are one cause for
inefficiency
How much overhead is required to deliver
an amount of data?
How large can packets be?
 Larger
better for efficiency (and goodput)
 But too large means too much data is lost if a
packet is damaged
 How many packets can be sent in one bunch
without an acknowledgment?
38
Efficiency (Contd.)
Small Frames (Less Efficient)
Large Frames (More Efficient)
39
Delay from the User’s Point of
View
 Response Time
 The time between a request for some service and a
response to the request
 The network performance goal that users care
about most
 A function of the application and the equipment the
application is running on, not just the network
 Most users expect to see something on the screen
in 100 to 200 ms
 The 100-ms threshold is often used as a timer
value for protocols that offer reliable transport of
data
40
Delay from the Engineer’s
Point of View
 Propagation delay
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Signal travels in a cable at about 2/3 the speed of
light in a vacuum
Relevant for all data transmission technologies, but
especially for satellite links and long terrestrial
cables
Geostationary satellites: propagation delay is
about 270 ms for an intercontinental satellite hop
Terrestrial cables: propagation delay is about 1 ms
for every 200 km
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Delay from the Engineer’s
Point of View (Contd.)
 Transmission delay



Also known as serialization delay
Time to put digital data onto a transmission
line
Depends on the data volume and the data
rate of the line
 It
takes about 5 ms to output a 1,024 byte
packet on a 1.544 Mbps T1 line
42
Delay from the Engineer’s
Point of View (Contd.)
 Packet-switching delay

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

The latency accrued when switches and routers
forward data
The latency depends on
 the speed of the internal circuitry and CPU
 the switching architecture of the internetworking
device
 the type of RAM that the device uses
Routers tend to introduce more latency than
switches
QoS, NAT, filtering, and policies introduce delay
43
Delay from the Engineer’s
Point of View (Contd.)
 Queueing delay

The average number of packets in a queue
on a packet-switching device increases
exponentially as utilization increases
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Average Queue Depth
Queuing Delay and Bandwidth
Utilization
15
12
9
6
3
0
0.5
0.6
0.7
0.8
0.9
1
Average Utilization
Number of packets in a queue increases exponentially
as utilization increases
45
Delay Variation (Jitter)
 The amount of time average delay varies
 Users of interactive applications expect minimal
delay in receiving feedback from the network
 Users of multimedia applications require a
minimal variation in the amount of delay
 Delay must be constant for voice and video
applications
 Variations in delay cause disruptions in voice
quality and jumpiness in video streams
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Delay Variation (Jitter)
(Contd.)
 Short fixed-length cells, for example
ATM 53-byte cells, are inherently better
for meeting delay and delay-variance
goals
 Packet size tradeoffs

Efficiency for high-volume applications
versus low and non-varying delay for
multimedia
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Delay Variation (Jitter)
(Contd.)
 Audio/video applications minimize jitter
by providing a buffer that the network
puts data into
 Display software or hardware pulls data
from the buffer
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Accuracy
 Data received at the destination must be the
same as the data sent by the source
 Error fames must be retransmitted, which has a
negative effect on throughput
 In IP networks, TCP provides retransmission of
data
 For WAN links, accuracy goals can be specified
as a bit error rate (BER) threshold


Fiber-optic links: about 1 in 1011
Copper links: about 1 in 106
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Accuracy (Contd.)
 On shared Ethernet, errors often result
from collisions

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Collisions happen in the 8-byte preamble
of the frames (not counted)
Collisions happen past the preamble and
somewhere in the first 64 bytes of the data
frame (legal collision)
Collisions happen beyond the first 64 bytes
of a frame (late collision)
50
Accuracy (Contd.)
 Late collisions are illegal and should never
happen (too large network)
 A goal for Ethernet collisions: less than 0.1%
affected by a legal collision
 Collisions should never occur on full-duplex
Ethernet links
 In wireless LAN 802.11 CSMA/CA, collisions
can still occur
51
Security
 Security design is one of the most
important aspects of enterprise network
design
 Security problems should not disrupt the
company's ability to conduct business
 The cost to implement security should
not exceed the cost to recover from
security incidents
52
Security (Contd.)
 Network Assets

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Hardware
Software
Applications
Data
Intellectual property
Trade secrets
Company’s reputation
53
Affordability
 Affordability is sometimes called cost



effectiveness
A network should carry the maximum amount of
traffic for a given financial cost
Financial costs include nonrecurring equipment
costs and recurring network operation costs
Campus networks: low cost is often more
important than availability and performance.
Enterprise networks: availability is usually more
important than low cost
54
Affordability (Contd.)
 Monthly charges for WAN circuits are the most
expensive aspect of running a large network
 How to save

Use a routing protocol that minimizes WAN traffic

Improve efficiency on WAN circuits by using such
features as compression
Eliminate underutilized trunks
Use technologies that support oversubscription


55
Adaptability
 Avoid incorporating any design elements
that would make it hard to implement
new technologies in the future
 Change can come in the form of new
protocols, new business practices, new
traffic patterns
56
Usability
 The ease of use with which network
users can access the network and
services
 Usability might also include a need for
mobility
 Some design decisions will have a
negative affect on usability:

Strict security, for example
57
Characterizing a Network
(Why?)
 Verify that a customer's technical design
goals are realistic
 Understand the current topology
 Locate existing network segments and
equipment
 Locate where new equipment will go
 Develop a baseline of current
performance
58
Characterizing a Network
(What?)
 Infrastructure
 Addressing and naming
 Wiring and media
 Architectural and environmental
constraints
 Health
59
Infrastructure
 Develop a set of network maps
 Learn the location of major
internetworking devices and network
segments
60
Infrastructure (Contd.)
 Information to collect

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Geographical locations
LAN, WAN connections
Buildings and floors, and possibly rooms
Location of major servers or server farms
Location of routers and switches
Location of mainframes
Location of major network-management stations
Location and reach of virtual LANs (VLANs)
Etc.
61
Medford
Fast Ethernet
50 users
Infrastructure (Contd.)
Frame Relay
CIR = 56 Kbps
DLCI = 5
Roseburg
Fast Ethernet
30 users
Frame Relay
CIR = 56 Kbps
DLCI = 4
Grants Pass
HQ
16 Mbps
Token Ring
Gigabit
Ethernet
Grants Pass
HQ
Fast Ethernet
75 users
FEP
(Front End
Processor)
IBM
Mainframe
T1
Web/FTP server
Eugene
Ethernet
20 users
T1
Internet
62
Addressing and Naming
 IP addressing for major devices, client
networks, server networks
 What to consider?




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Private/public address
Classless/classful addressing
Variable-length subnet mask (VLSM)
Route aggregation or supernetting
Discontiguous subnets
63
Discontiguous Subnets
Area 0
Network
192.168.49.0
Router A
Area 1
Subnets 10.108.16.0 10.108.31.0
Router
B
Area 2
Subnets 10.108.32.0 10.108.47.0
64
Wiring and Media
 Document the types of cabling in use as
well as cable distances
 Distance information is useful when
selecting data link layer technologies
based on distance restrictions
65
Wiring and Media (Contd.)
 Single-mode (SM) fiber
 Multi-mode (MM) fiber
 Shielded twisted pair (STP) copper
 Unshielded-twisted-pair (UTP) copper
 Coaxial cable
 Microwave
 Laser
 Radio
 Infra-red
66
Architectural Constraints
 Make sure the following are sufficient





Air conditioning
Heating
Ventilation
Power
Protection from electromagnetic
interference
67
Architectural Constraints
(Contd.)
 Make sure there’s space for:




Cabling conduits
Patch panels
Equipment racks
Work areas for installing and
troubleshooting equipment
68
Wireless Installations
 Reflection



Signal bounces back and interferes with
itself
Metal surfaces such as steel girders,
scaffolding, shelving units, steel pillars,
and metal doors
Implementing a WLAN across a parking lot
can be tricky because of metal cars that
come and go
69
Wireless Installations (Contd.)
 Absorption




Energy of the signal can be absorbed by the
material in objects through which it passes
Reduces signal level
Water has significant absorption properties, and
objects such as trees or thick wooden structures
can have a high water content
Implementing a WLAN in a coffee shop can be
tricky if there are large canisters of liquid coffee
70
Wireless Installations (Contd.)
 Refraction



RF signal is bent when it passes from a
medium with one density into a medium
with another density
The signal changes direction and may
interfere with the nonrefracted signal
It can take a different path and encounter
other, unexpected obstructions, and arrive
at recipients damaged or later than
expected
71
Wireless Installations (Contd.)
 Diffraction


Similar to refraction
Like refraction, the signal is bent around
the edge of the diffractive region and can
then interfere with that part of the signal
that is not bent
72
Wireless Installations (Contd.)
 Boost the power level to compensate for
variable environmental factors
 The additional power added to a
transmission is called the fade margin
73
Health
 Performance
 Availability
 Bandwidth utilization
 Accuracy
 Efficiency
 Response time
 Status of major routers, switches, and
firewalls
74
Develop a Performance
Baseline
 How much better the new internetwork
performs once your design is
implemented
 Baseline of normal performance should
not include nontypical problems caused
by exceptionally large traffic loads
 The decision whether to measure normal
performance, performance during peak
load, or both, depends on the goals of
the network design
75
Characterize Availability
MTBF
MTTR
Date and Duration of
Last Major
Downtime
Cause of Last
Major
Downtime
Enterprise
Segment 1
Segment 2
Segment n
76
Utilization
 Measurement of how much bandwidth is
in use during a specific time interval
 Different tools use different averaging
windows for computing network
utilization
 Trade-off between amount of statistical
data that must be analyzed and
granularity
77
Utilization in Minute Intervals
Network Utilization
16:40:00
16:43:00
16:46:00
16:49:00
Time
16:52:00
16:55:00
16:58:00
17:01:00
17:04:00
17:07:00
17:10:00
0
1
2
3
4
5
6
7
Utilization (%)
78
Utilization in Hour Intervals
Network Utilization
13:00:00
Time
14:00:00
15:00:00
16:00:00
17:00:00
0
0.5
1
1.5
2
2.5
Utilization (%)
3
3.5
4
4.5
79
Utilization (Contd.)
 The size of the averaging window
depends on your goals



When troubleshooting network problems,
keep the interval very small, either minutes
or seconds
For performance analysis and baselining
purposes, use an interval of 1 to 5 minutes
For long-term load analysis, to determine
peak hours, days, or months, set the
interval to 10 minutes
80
Bandwidth Utilization by
Protocol
Relative
Network
Utilization
Absolute
Network
Utilization
Broadcast
Rate
Multicast
Rate
Protocol 1
Protocol 2
Protocol 3
Protocol n
81
Accuracy
 Bit error rate (BER)
 Frame error rate (FER)
 Packet loss
 Collision
 Runt (partial) frame
 Healthy network should not have more
than one bad frame per megabyte of
data
82
Characterize Packet Sizes
 Increasing the maximum transmission
unit (MTU) on router interfaces can also
improve efficiency
 Increasing MTU can increase
serialization delay
83
Characterize Packet Sizes
(Contd.)
84
Characterize Packet Sizes
(Contd.)
 Small frames consist of control
information and acknowledgments
 Data frames fall into the large frame-size
categories
 Frame sizes typically fall into what is
called a bimodal distribution
85
Characterize Response Time
 A more common way to measure
response time is to send ping packets
and measure the round-trip time (RTT)
 Variance measurements are important
for applications that cannot tolerate
much jitter
 You can also document any loss of
packets
86
Characterize Response Time
(Contd.)
Node A
Node A
Node B
Node C
Node B
Node C
Node D
X
X
X
X
Node D
node = router, server, client, or mainframe
87
Checking Status of Major
Devices
 CPU utilization
 How many packets it has processed
 How many packets it has dropped
 Status of buffers and queues
 You can use SNMP or commands in the
devices
88
Characterizing Network Traffic
(Why?)
 Analyze network traffic patterns to help
you select appropriate logical and
physical network design solutions to
meet a customer's goals
89
Network Traffic Factors
 Location of traffic sources and sinks
 Traffic load
 Traffic behavior
90
Traffic Flow
 Information transmitted between
communicating entities during a single
session
 Flow attributes:





addresses for each end of the flow
direction
symmetry
path
number of packets or bytes
91
Traffic Flow Types
 Terminal/host
 Client/server
 Peer-to-peer
 Server/server
 Voice over IP
92
Terminal / Host
 Examples: Telnet, ssh
 Usually asymmetric: terminal sends a few
characters and the host sends many characters
 In some full-screen terminal applications, the
terminal sends characters typed by the user
and the host returns data to repaint the screen
 The screen is usually 80 characters wide by 24
lines long, which equals 1920 characters
 The full transfer is a few thousand bytes
93
Client / Server
 Examples: FTP, HTTP
 Usually bidirectional and asymmetric
 Requests are typically small frames
except when writing data to the server
 Responses range from 64 bytes to 1500
bytes or more, depending on the MTU of
the data link layer
94
Peer-to-Peer
 Examples: Workgroup,
videoconferencing, P2Ps
 No hierarchy and no dedicated server
 Usually bidirectional and symmetrical
 Another example is a meeting between
business people at remote sites using
videoconferencing equipment
 Information dissemination in a class is a
client/server model
95
Server / Server
 To implement directory services, to cache
heavily used data, to mirror data for load
balancing and redundancy, to back up data,
and to broadcast service availability
 Generally bidirectional
 With most server/server applications, the flow is
symmetrical, but in some cases there is a
hierarchy of servers, with some servers sending
and storing more data than others
96
VoIP
 The flow associated with transmitting the
audio voice is separate from the flows
associated with call control


The voice flow for transmitting the digital
voice is essentially peer-to-peer
The call control flow for call setup and
teardown is a client/server flow
97
Traffic Load
 Network capacity is sufficient to avoid
bottleneck
 Key parameters:




Number of stations
Average time that a station is idle between
sending frames
Time required to transmit a message once
medium access is gained
Application usage patterns
98
Traffic Load (Contd.)
 Traffic load caused by applications
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Terminal screen: 4 Kbytes
Simple e-mail: 10 Kbytes
Simple web page: 50 Kbytes
High-quality image: 50,000 Kbytes
Database backup: 1,000,000 Kbytes or
more
99
Traffic Load (Contd.)
 Protocol overhead
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IPX: 30 bytes
TCP: 20 bytes
IP: 20 bytes
Ethernet: 18 + 8-byte preamble + 12-byte
interframe gap (IFG)
HDLC: 10 bytes
100
Traffic Behavior
 Broadcast
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Goes to all network stations on a LAN
All ones data-link layer destination address
 FF: FF: FF: FF: FF: FF
Doesn’t necessarily use huge amounts of
bandwidth
But does disturb every CPU in the
broadcast domain
101
Traffic Behavior (Contd.)
 Multicast
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Goes to a subset of stations
01:00:0C:CC:CC:CC (Cisco Discovery
Protocol)
Should just disturb NICs that registered to
receive it
Requires multicast routing protocol on
internetworks
102
Traffic Behavior (Contd.)
 Broadcast/multicast traffic is necessary
and unavoidable
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share topology information
advertise services
locate services
addresses and names
 No more than 20% of the network traffic,
otherwise segment the network using
routers or VLANs
103
Traffic Behavior (Contd.)
 Layer 2 devices, such as switches and
bridges, forward broadcast and multicast
frames out all ports
 Router does not forward broadcasts or
multicasts
 All devices on one side of a router are
considered part of a broadcast domain
 VLANs can also limit the size of a
broadcast domain based on membership
104