CIS 460 – Network Analysis and Design

Chapter 3 – Characterizing the Existing Internetwork

Characterizing the Existing Network

• Examine the customers existing network to better judge how to meet expectations for network scalability, performance and availability

• Understanding the existing network’s structure, uses, and behavior you get a better feel if the design goals are realistic

• Most designers design network enhancements to existing networks

Characterizing the Network Infrastructure

• Develop a network map• Learning location of major internetworking

devices and network segments• Documenting the names and addresses of major

devices and segments• Documenting the types and lengths of physical

cabling• Investigating architectural and environmental

constraints

Developing a Network Map

• Location of major hosts, interconnection devices and network segments

• Help understand traffic flow• Data on performance characteristics of network

segments coupled with location information gives insight to where users are concentrated and the level of traffic to be supported

• Goal is to obtain a map of the already-implemented network

Tools for Developing Network Maps

• Invest in a good network-diagramming tool

• Visio is one example

• Some companies offer diagramming and network documentation tools that automatically discover existing networks.

What Should a Network Map Include?

• Geographical – countries, states, provinces, cities and campuses

• Wan connections between countries, states, and cities

• Buildings and floors and rooms or cubicles if possible

• WAN and LAN connections between buildings and campuses

What should a network map include? (Cont’d)

• Indication of the data-link technology for WANs and LANs

• Service provider for WANs• Location of routers and switches• Virtual Private Networks• Major servers or server farms• Location of major network-management stations• Location and reach of any virtual LANs

What should a network map include? (Cont’d)

• Topology of any firewall security system

• Location of any dial-in and dial out systems

• Indication of where workstations reside

• Depiction of the logical topology or architecture of the network

Characterizing Network Addressing and Naming

• Documenting any strategies customer has for networking addressing and naming

• On detailed network maps include the names of major sites, routers, network segments and servers

• Investigate the network-layer addresses your customer uses

• A customer goal might be to use route summarization

• Existing addressing scheme might affect the routing protocols you can select

Characterizing Wiring and Media

• Document existing cabling design to help plan for enhancements and identify any potential problems

• Assess who well equipment and cables are labeled• Document connections between buildings

(number of pairs of wire and type)• Locate telecommunications wiring closets, cross-

connect rooms and any lab or computer rooms• Use the chart in Table 3-1 on page 61.

Checking Architectural and Environmental Constraints

• Pay attention to environmental during cabling investigation

• Flooding, right of way issues, heavy equipment usage

• Building issues

Checking the Health of the Existing Internetwork

• Knowing baseline of existing system give a standard to measure new system against

• Existing segments will effect overall network performance

• Segments that will interoperate with new segments, backbone networks and networks that connect old and new areas

• Legacy systems may have to be included

The Challenges of Developing a Baseline of Network Performance

• Not an easy task– Time selection– Time allocation– Typical time period– Periods of normal traffic laod– Customer may not recognize need

• Need good understanding of customers technical and business goals

Analyzing Network Availability

• Gather statistics on MTBF and MTTR

• Learn about causes of the most recent and most disruptive periods of downtime

• Document availability in a Table such as Table 3-2 on page 65

Analyzing Network Utilization

• A measure of how much bandwidth is in use during a specific time interval

• Usually a percentage of capacity• Different tools use different averaging windows• Use appropriate division of time• Averaged over too short or long a period can be

misleading• It is always better to error on the side of gathering

too much data

Bandwidth Utilization by Protocol

• Measure by broadcast versus unicast traffic

• By each major protocol

• Some protocols send excessive broadcast traffic

• Use a protocol analyzer on each major network segment

• Relative usage specifies how much bandwidth is used by the protocol in comparison to total bandwidth currently in use by the segment.

• Absolute usage specifies how much bandwidth is used by the protocol in comparison to the total capacity of the segment

• Document using a Table such as 3-3 on page 69

Analyzing Network Efficiency

• Use a bit error rate tester on serial lines to test number of damaged bits compared to total bits

• On packet-switched networks measure frame errors because a whole frame is bad if a single bit is changed or dropped. Utilizes cyclic redundancy check (CRC) based on bits in a frame

• A protocol analyzer can check the CRC on received frames

Analyzing Network Efficiency (Cont’d)

• Errors increase as utilization increases so document errors as function of the number of bytes seen by the monitoring tool

• Some monitors allow you to print the top 10 stations sending frames

• Baseline analysis should also include upper-layer problems

• Accuracy should also include a measurement of lost packets

Analyzing ATM Errors• Accuracy is measure in terms of cell error ration

(CER), cell loss ration (CLR), cell misinsertion rate (CMR) and severely errored cell block ration (SECBR)

• CER is number of errored cells divided by total number of successfully transferred cells plus errored cells

• CMR is caused by an undetected error in the header of a cell

• If no tool, check the performance by analyzing the level of frame errors and upper-level problems

Analyzing Network Efficiency

• Bandwidth utilization is optimized when applications and protocols are configured to send large amounts of data per frame.

• The goal is to maximize the number of data bytes compared to the number of bytes in headers and in acknowledgment packets

• Use a protocol analyzer to examine the current frame sizes on the network

• Analyzing frame sizes can help understand health of a network, not just the efficiency

Analyzing Delay and Response Time

• Measure response time between significant network devices before and after a new network design is implemented

• Send ping packets and measure time to send and respond

• Document using a Table such as 3-4 on page 75• Measure response time from a user point of view• Test user applications and system protocols• Do some testing when the system is experiencing

problems or change

Checking the Status of Major Routers

• Includes determining how busy the router is, how many packets the router has processed, how many packets the router has dropped and the status of buffers and queues.

• You can use the Simple Network Management Protocol (SNMP) to check the health of a router

• To check the health of a router you need to check the router variable on a regular basis over a few days

Tools for Characterizing the Existing Internetwork

• Protocol Analyzers• Remote Monitoring Tools• Cisco Tools for characterizing an Existing

Internetwork• Cisco Discovery Protocol• Enterprise Accounting for NetFlow• Netsys Service-Level Management Suite• Cisco Works• Other Tools

Protocol Analyzers

• A fault-and-performance-management tool that captures network traffic, decodes the protocols in the captured packets and provides statistics to characterize load, errors, and response time.– Sniffer Network Analyzer– EtherPeek

Remote Monitoring Tools

• Remote Monitoring (RMON) MIB developed to overcome shortcomings in the standard SNMP MIB for gathering statistics on data-link and physical-layer parameters

• Gathers statistics on CRC errors, Ethernet collisions, Token-ring soft errors, frame sizes, number of packets in and out of a device, and the rate of broadcast packets

• The RMON MIB alarm group allows a network manager to set thresholds for network parameters and automatically delivers alerts to management

Cisco Tools for Characterizing an Existing Internetwork

• Cisco has a complete range of tools for characterizing an existing internetwork, ranging from the Cisco Discovery Protocol to sophisticated Netsys tools

Cisco Discovery Protocol

• Specifies a method for Cisco routers and switches to send configuration information to each other on a regular basis

• Look at information about neighboring routers:– Protocols enabled

– Network address for enabled protocols

– Number and types of interfaces

– Type of platform and its capabilities

– Version of Cisco IOS software

Enterprise Accounting for NetFlow

• Can help understand bandwidth usage and allocation, quality of service levels, ro8uter usage, and router port usage

• Recognizes network flows and characterizes network and router usage by user (IP address), application, and department

Netsys Service-Level Management Suite

• Enables defining, monitoring, and assessing network connectivity, security, and performance.

• Particularly useful for characterizing an existing network as part of a network design proposal

Cisco Works

• A series of SNMP-based internetworking management software application for device monitoring, configuration maintenance, and troubleshooting of Cisco devices

Other Tools for Characterizing an Existing Internetwork

• Many other tools out there. Visit various web sites identified in the book to look at some of the tools.

Conclusion

• Use a network health checklist like the one on page 81 to assist in verifying the health of an existing network.