Characterizing the Existing Internetwork.pdf

Page 1Copyright 2004 Cisco Press & Priscilla Oppenheimer

Top-Down Network Design, Ch. 3: Characterizing the Existing Internetwork

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Top-Down Network Design

Chapter Three

Characterizing the Existing Internetwork

Copyright 2010 Cisco Press & Priscilla Oppenheimer

What’s the Starting Point?

� According to Abraham Lincoln:

◦ “If we could first know where we are and whither (to what place) we are tending, we could better judge what to do and how to do it.”



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Where Are We?

� Learning about the topology and physical structure and assessing the network's performance.

� Characterize the exiting internetwork in terms of:

◦ Its infrastructure

� Logical structure (modularity, hierarchy, topology)

� Physical structure

◦ Addressing and naming

◦ Wiring and media

◦ Architectural and environmental constraints

◦ Health

Modular Blocks (Cisco)



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Cisco Hierarchical Network Design Model

Get a Network MapThe drawing shows a physical topology

The logical topology is a

hub&spoke shape with 3 layers

Learning the location

of major hosts,

interconnection devices,

and network segments



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Characterize Addressing and Naming

� IP addressing for major devices, client networks, server networks, and so on

� Any addressing oddities, such as discontiguous subnets?

� Any strategies for addressing and naming?

◦ For example, sites may be named using airport codes

� San Francisco = SFO, Oakland = OAK

Discontiguous SubnetsSome routing protocols do not

support classless addressing,

variable-length subnet masking

(VLSM), or discontiguous subnets.

Subnet 108 of network 10 is

divided into two areas that are

separated by network 192.168.49.0

Classless addressing uses a variable number of bits for the network and

host portions of the address.

CIDR (classless interdomain routing) allows routers to group routes

together to cut down on the quantity of routing information carried by the

core routers.

VLSM (variable-length subnet mask) Ability to specify a different subnet

mask for the same network number on different subnets. VLSM can help

optimize available address space.



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Characterize the Wiring and Media

� Single-mode fiber

� Multi-mode fiber

� Shielded twisted pair (STP) copper

� Unshielded-twisted-pair (UTP) copper

� Coaxial cable

� Microwave

� Laser

� Radio

� Infra-red

Campus Network Wiring

Vertical wiring

runs between

floors.

Horizontal wiring

runs from tele-

communications

closets to

wallplates in

cubicles or

offices.

Work-area wiring

runs from the

wallplate to a

wks in a cubicle

or office



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Document

building

wiring

information

Architectural Constraints� Make sure the following are sufficient

◦ Air conditioning

◦ Heating

◦ Ventilation

◦ Power

◦ Protection from electromagnetic interference

◦ Doors that can lock

� Make sure there’s space for:

◦ Cabling conduits

◦ Patch panels

◦ Equipment racks

◦ Work areas for technicians installing and troubleshooting



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Issues for Wireless Installations� Reflection causes the signal to bounce back on itself. The signal can

interfere with itself in the air and affect the receiver's ability to discriminate between the signal and noise in the environment. Reflection is caused by metal surfaces.

� Absorption. Some of the electromagnetic energy of the signal can be absorbed by the material in objects through which it passes, resulting in a reduced signal level. Water has significant absorption properties.

� Refraction.When an RF signal passes from a medium with one density into a medium with another density, the signal can be bent. The signal changes direction and may interfere with the nonrefracted signal.

� Diffraction is similar to refraction. the RF signal is bent around the edge of the diffractive region and can then interfere with that part of the RF signal that is not bent.

Check the Health of the Existing Internetwork

� Performance

� Availability

� Bandwidth utilization

� Accuracy

� Efficiency

� Response time

� Status of major routers, switches, and firewalls



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Baseline of Network Performance

Baseline: Characterization of the normal traffic flow and performance of a network, used as input to a new or enhanced design for the network.

A baseline of normal performance should not include nontypical problems caused by exceptionally large traffic loads.

In general, errors, packet/cell loss, and latency increase with load. To get a meaningful measurement of typical accuracy and delay, try to do your baseline analysis during periods of normal traffic load.

Characterize Availability

Availability characteristics of the current network



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Network Utilization in Minute Intervals

Network utilization

is a measurement

of how much

bandwidth is in use

during a specific

time interval

Network Utilization in Hour Intervals

changing to a long interval can be misleading

because peaks in traffic get averaged out



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Bandwidth Utilization by Protocol

Place a protocol analyzer or remote monitoring (RMON) probe

on each major network segment and fill out the chart

Relative usage specifies how much bandwidth is used by the

protocol in comparison to the total bandwidth currently in use

on the segment.

Absolute usage specifies how much bandwidth is used by the

protocol in comparison to the total capacity of the segment.

Characterize Packet Sizes

Many of the frames were 64-byte acknowledgments.

HTTP used 1500-byte packets.

The other traffic consisted of DNS lookups and replies, Simple Mail Transfer Protocol

(SMTP), Post Office Protocol (POP), and Address Resolution Protocol (ARP) packets.



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Characterize Response Time

the term Node to mean router, server, client, or mainframe.

Check the Status of Major Routers, Switches, and Firewalls

� show buffers

� show environment

� show interfaces

� show memory

� show processes

� show running-config

� show version



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Tools for Characterizing the Existing Internetwork

� Protocol analyzer is a tool that captures network traffic, decodes the protocols in the captured packets, and provides statistics to characterize load, errors, and response time.

� Network-Monitoring and Management Tools: Multi Router Traffic Grapher (MRTG) is a tool for monitoring network traffic load and other performance characteristics on routed and switched internetworks.

� Remote monitoring (RMON) probe facilitates gathering statistics on the following data link layer performance factors: CRC errors, Ethernet collisions, Frame sizes, The number of packets in and out of a device, The rate of broadcast packets

Cisco Tools for Characterizingthe Existing Internetwork

� Cisco Discovery Protocol (CDP) specifies a method for Cisco routers and switches to send configuration information to each other on a regular basis.

� Cisco IOS NetFlow technology collects and measures data as it enters router or switch interfaces.

� CiscoWorks enables assess and manage the capabilities of Cisco-based LANs, WANs, VPNs, and voice networks. The tools centrally manage critical network-performance characteristics such as availability, responsiveness, resilience, and security.

� Cisco IOS Service Assurance Agent (SAA) collects network performance information in real time including response times, latency, jitter, packet loss, and other statistics.

� Cisco Internetwork Performance Monitor (IPM) measures network response time and availability.



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Network Health Checklist� The network topology and physical infrastructure are well

documented.

� Network addresses and names are assigned in a structured manner and are well documented.

� Network wiring has been tested and certified.

� Network wiring between telecommunication closets and end stations is no more than 100 meters.

� Network availability meets current customer goals.

� Network security meets current customer goals.

� No LAN or WAN segments are becoming saturated. (70% in a 10-minute window)

� On Ethernet half-duplex segments, less than 0.1% of frames are collisions.

Network Health Checklist.2� There are no collisions on Ethernet full-duplex links.

� Broadcast traffic is less than 20% of all traffic on each network segment.

� Frame sizes have been optimized to be as large as possible for the data link layer in use.

� No routers are overutilized (5-minute CPU utilization is under 75%)

� On an average, routers are not dropping more than 1% of packets.

� Up-to-date router, switch, and other device configurations have been collected, archived, and analyzed as part of the design study.

� The response time between clients and hosts is less than 100ms.



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Summary

� Characterize the exiting internetwork before designing enhancements

� Helps you verify that a customer’s design goals are realistic

� Helps you locate where new equipment will go

� Helps you cover yourself if the new network has problems due to unresolved problems in the old network

Review Questions

� What factors will help you decide if the existing internetwork is in good enough shape to support new enhancements?

� When considering protocol behavior, what is the difference between relative network utilization and absolute network utilization?

� Why should you characterize the logical structure of an internetwork and not just the physical structure?

� What architectural and environmental factors should you consider for a new wireless installation?

Documents

Characterizing the Existing Internetwork.pdf