A guide to building and modifying Tcl scripts to automate network administration tasks

Streamline Cisco network administration and save time with Tcl scripting

Cisco networking professionals are under relentless pressure to accomplish more, faster, and with fewer resources. The best way to meet this challenge is to automate mundane or repetitive tasks wherever possible. In this book, three Cisco experts show you how to use Tcl scripting for Cisco IOS devices to do just that.

You’ll learn easy techniques for creating, using, and modifying Tcl scripts that run directly on Cisco network devices from the Cisco IOS command line. The authors first teach basic Tcl commands and concepts for capturing and manipulating data and for querying or controlling Cisco equipment. Building on these core skills, they show you how to write scripts that automate and streamline many common IOS configuration, monitoring, and problem-solving tasks.

The authors walk through the entire script development process, including planning and flowcharting what you want to accomplish, formatting your code, adding comments, and troubleshooting script errors. They also present many downloadable sample scripts, along with practical guidance for adapting them to your own environment.

Whatever your role in managing, monitoring, or securing Cisco IOS networks and equipment, this book will help you get the job done more rapidly and efficiently.

Ray Blair, CCIE No. 7050, is a Cisco vertical solutions architect specializing in large network designs. He has more than 20 years of experience in designing, implementing, and maintaining networks, and maintains three CCIE certifications.

Arvind Durai, CCIE No. 7016, Advanced Services Technical Leader for Cisco, specializes in supporting major Cisco enterprise customers in finance, manufacturing, e-commerce, government, and healthcare. He holds CCIEs in Routing and Switching, and in Security.

John Lautmann, Cisco Software Engineer, has developed and enhanced network management software for twelve years, and holds six patents. He has helped develop new Cisco IOS features ranging from data link switching to IOS Tcl Interpeter and digitally signed Tcl scripts.

• Automate routine administration tasks you’ve always performed manually
• Instantly collect and modify IOS router configurations and other data
• Write Syslog scripts to document failures, monitor network health, collect statistics, and send alarm messages
• Implement automated network performance measurement using IP SLA
• Use the Embedded Event Manager’s event detectors, server, and policies to customize device operation
• Trigger preplanned actions to correct problems as they arise
• Simplify policy management using the Tcl script refresh feature
• Protect Tcl script security with digital signatures and PKI
• Understand how Tcl functions within the Cisco IOS environment
• Master Tcl syntax and commands through hands-on practice
• Learn best scripting practices through expert examples
• Quickly modify this book’s examples for your own environment

This book is part of the Networking Technology Series from Cisco Press®, which offers networking professionals valuable information for constructing efficient networks, understanding new technologies, and building successful careers.

Category: Networking

Covers: Network Administration

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Introduction xiv

Chapter 1 The Origin of Tcl 1

Tcl and Cisco IOS Software 3

Embedded Event Manager and Tcl 4

Restriction of Tcl in IOS 4

Tcl with EEM Support in IOS 5

Using Tcl Scripts in the Network 8

Troubleshooting Problems 8

Monitoring the Network 8

Adding Intelligence to Cisco IOS Protocols 9

Summary 9

References 9

Chapter 2 Tcl Interpreter and Language Basics 11

Simple Variables in Tcl 12

Storing Variables 12

Viewing Variables 13

The append Command 13

The incr Command 13

Representation of Variables in Tcl 14

Command Substitution 14

Variable Substitution 15

Lists 17

lappend 18

lindex 18

linsert 18

llength 19

lsearch 19

lreplace 20

lrange 20

lsort 20

Procedures 21

for Command 22

foreach Command 23

while Command 23

Arrays 24

if Command 26

switch Command 27

Files 28

Summary 31

References 31

Chapter 3 Tcl Functioning in Cisco IOS 33

Understanding the Tcl Interpreter in Cisco IOS 33

Using Cisco IOS Exec-Mode Parser in the Tcl Shell 34

Entering an IOS Command into the Tcl Command Interpreter 35

Using Tcl to Enter Commands 36

Copying a Tcl Script to a Cisco IOS Device 38

Fetching a Cisco IOS Tcl Script from a Remote Device 41

Using Tcl to Examine the Cisco IOS Device Configuration 41

Using Tcl to Modify the Router Configuration 43

Using Tcl with SNMP to Check MIB Variables 44

Other Uses of SNMP 44

Enabling SNMP on a Cisco IOS Device 47

Querying the Configuration of a Cisco IOS Device Using SNMP 48

Modifying the Configuration of a Cisco IOS Device Using SNMP 51

Summary 53

References 53

Chapter 4 Embedded Event Manager (EEM) 55

EEM Architecture 55

Policies 56

EEM Server 56

Event Detectors 57

Software Release Support for EEM 60

Platform and IOS Considerations for EEM 65

Writing an EEM Applet 66

Practical Example of an Event Trigger 68

Using Object Tracking as an Event Trigger 69

Creating Applet Actions 70

Examples of EEM Applets 70

Configuring the IP SLA Sender and Responder 72

Applet and IP SLA Route Failover Example 74

Applet That Monitors the Default Route 83

Applet and Application Failover with a Network Address Translation Example 88

Using EEM and Tcl Scripts 96

Programming Policies with Tcl 97

Tcl Example Used to Check for Interface Errors 98

Tcl Example Used to Check the CPU Utilization 104

Summary 110

References 110

Chapter 5 Advanced Tcl Operation in Cisco IOS 111

Introduction to the Syslog Protocol 112

Configuring Syslog Server Parameters in Cisco IOS 113

Syslog Tcl Script Example 116

Syslog Tcl Script Sample Output 118

Sending Syslog Messages to a File 121

Syslog Server Script Procedures 124

Syslog Server Script Body 127

Putting the Syslog Script into Operation 129

Introduction to Embedded Syslog Manager 130

Filtering Syslog Messages 130

ESM Global Variables 134

Rebuilding a Syslog Message from Its Components 136

Displaying/Adding ESM Tcl Script Filters 137

Introduction to Embedded Menu Manager 139

Using Tcl as a Web Server 144

Obtaining a Free Web Server Application 147

Reverse Engineering the Web Server 149

Creating Your Own Simple Web Page 152

Creating a Web Page Using IOS show Commands 154

Adding User Input to the Web Page 157

Introduction to IP SLA 160

Adding the IP SLA Measurement to the Web Page 162

Modifying the Button and Label for User Input 162

Creating a Tcl Script to Display IP SLA Measurement Results 163

Putting the New Tcl Scripts into Operation 165

Reformatting the IP SLA Output for Readability 167

Automatic Removal and Creation of IP SLA Entries 170

Displaying the Results of the IP SLA Measurement with Auto-Refresh 174

Tcl Script Refresh Policy 177

SNMP Proxy Event Detector 178

Remote-Procedure Call Requests 179

Multiple-Event Support for Event Correlation 180

Using the clear Command 181

Summary 182

References 182

Chapter 6 Tcl Script Examples 183

Creating an Application from Start to Finish 183

Determine What You Want to Accomplish 183

Creating a Flowchart 184

Deciding What the User Interface Should Look Like 185

Write the Code in Pseudo-Code 187

Before You Begin 188

Starting to Program the Application 190

Configuring the Web Server 190

Writing Code for the MPLS VPN Script 191

Configuring HTML 209

Writing Code for the MPLS CFG Script 216

Troubleshooting as You Go 228

Using Tcl to Troubleshoot Network Problems 230

Monitoring the Console for Events 233

Creating a Web Application for Remote SNMP Graphing 236

Summary 241

References 241

Chapter 7 Security in Tcl Scripts 243

Introduction to PKI Infrastructure 243

PKI Prerequisite 244

Confidentiality with PKI 244

Digital Signatures with PKI 245

Using Digital Signatures to Sign a Tcl Script 247

Step 1: Decide on the Final Tcl Script Contents (Myscript) 248

Step 2: Generate a Public/Private Key Pair 248

Step 3: Generate a Certificate with the Key Pair 250

Step 4: Generate a Detached S/MIME pkcs7 Signature for Myscript Using the Private Key 250

Step 5: Modify the Format of the Signature to Match the Cisco Style for Signed Tcl Scripts and Append

It to the End of Myscript 251

Tcl Script-Failure Scenario 256

Scaling Tcl Script Distribution 257

Summary 258

References 258

Appendix A Cisco IOS Tcl Commands Quick Reference 259

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