Cisco Express Forwarding

Understanding and troubleshooting CEF in Cisco routers and switches

Nakia Stringfield, CCIE® No. 13451/Russ White, CCIE No. 2635/Stacia McKee

How does a router switch a packet? What is the difference between routing a packet, switching a frame, and packet switching? What is the Cisco® Express Forwarding (CEF) feature referred to in Cisco documentation and commonly found in Cisco IOS® ...

See more details below
Cisco Express Forwarding

Available on NOOK devices and apps  
  • NOOK Devices
  • Samsung Galaxy Tab 4 NOOK
  • NOOK HD/HD+ Tablet
  • NOOK
  • NOOK Color
  • NOOK Tablet
  • Tablet/Phone
  • NOOK for Windows 8 Tablet
  • NOOK for iOS
  • NOOK for Android
  • NOOK Kids for iPad
  • PC/Mac
  • NOOK for Windows 8
  • NOOK for PC
  • NOOK for Mac

Want a NOOK? Explore Now

NOOK Book (eBook)
BN.com price
(Save 42%)$55.99 List Price


Cisco Express Forwarding

Understanding and troubleshooting CEF in Cisco routers and switches

Nakia Stringfield, CCIE® No. 13451/Russ White, CCIE No. 2635/Stacia McKee

How does a router switch a packet? What is the difference between routing a packet, switching a frame, and packet switching? What is the Cisco® Express Forwarding (CEF) feature referred to in Cisco documentation and commonly found in Cisco IOS® commands? CEF is a general term that describes the mechanism by which Cisco routers and Catalyst® switches packet-switch (route) frames. CEF is found in almost all Cisco routers and Catalyst switches, and understanding how CEF operates can improve the performance, scalability, and efficiency of your network.

Cisco Express Forwarding demystifies the internal workings of Cisco routers and switches, making it easier for you to optimize performance and troubleshoot issues that arise in Cisco network environments. This book addresses common misconceptions about CEF and packet switching across various platforms, helping you to improve your troubleshooting skills for CEF- and non-CEF-related problems.

The first part of the book provides an overview of packet-switching architectures and CEF operation and advanced features. It also covers the enhanced CEF structure and general troubleshooting. The second part of the book provides case studies that focus on the common topics that have been problematic for customers and those supporting Cisco networks.

Full of practical examples and configurations, this book draws on years of experience to help you keep your Cisco networks running efficiently.

  • Learn the key features of packet-switching architectures
  • Understand the basics of the CEF architecture and operation
  • Examine the enhanced CEF structure, which improves scalability
  • Learn how to troubleshoot in software-switching environments
  • Understand the effect of CEF on a Cisco Catalyst 6500 Supervisor 720
  • Configure and troubleshoot load sharing with CEF
  • Evaluate the effect of CEF in an MPLS VPN environment
  • Review CEF design considerations that impact scalability

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: Routing and Switching

Read More Show Less

Product Details

  • ISBN-13: 9780132796873
  • Publisher: Pearson Education
  • Publication date: 5/8/2007
  • Series: Networking Technology
  • Sold by: Barnes & Noble
  • Format: eBook
  • Edition number: 1
  • Pages: 288
  • File size: 21 MB
  • Note: This product may take a few minutes to download.

Meet the Author

Nakia Stringfield, CCIE No. 13451, is a network consulting engineer for Advanced Services at Cisco in Research Triangle Park, North Carolina, supporting top financial customers with network design and applying best practices. She was formerly a senior customer support engineer for the Routing Protocols Technical Assistance Center (TAC) team, troubleshooting issues related to CEF and routing protocols. Nakia has been with Cisco for more than six years, previously serving as a technical leader for the Architecture TAC team. She has given training courses on CEF operation and troubleshooting for internal

employees. Nakia also worked for a year with IBM Global Services LAN Support in Research Triangle Park, North Carolina. Nakia attended North Carolina State University and completed her bachelor of science degree in electrical engineering in 1996. She also earned a master of science in computer networking

and computer engineering from North Carolina State University in 2000.

Russ White, CCIE No. 2635, is a member of the Routing Protocol Design and Architecture Team at Cisco, Research Triangle Park, North Carolina. He is a member of the Internet Engineering Task Force (IETF) Routing Area Directorate, a cochair of the Routing Protocols Security Working Group in the IETF, a regular speaker at Networkers, a member of the Cisco Certified Internetwork Expert (CCIE) Content Advisory Group, a member of the core team developing the new Cisco Design certification, a regular contributor to the Internet Protocol Journal, and the coauthor of six other books about routing and routing protocols, including Optimal Routing Design, from Cisco Press. Russ primarily works in the development of new features and design architectures for routing protocols.

Stacia McKee is a customer support engineer and technical leader of the Routing Protocols (RP) Technical Assistance Center (TAC) team at Cisco in research Triangle Park, North Carolina. This team focuses on providing postsales support of IP routing protocols, Multiprotocol Label Switching (MPLS),

quality of service (QoS), IP multicast, and many other Layer 3 technologies. Stacia has been with Cisco for more than six years, previously serving as a technical leader of the Architecture TAC team and member of the WAN/Access TAC team. She has created and presented training on packet switching, router architecture, and troubleshooting for internal employees. Stacia has also been a technical editor and reviewer of Cisco.com technical documentation, mainly in router and IOS architecture and IP routing protocols technologies. She works closely with the IP Routing and IP Services groups within the Cisco

Network Software and Systems Technology Group (NSSTG) on customer problems and early field trials. In 2000, Stacia completed her bachelor of science degree in computer information systems, bachelor of science degree in business administration, and bachelor of arts degree in computer science at the

College of Charleston in Charleston, South Carolina.

Read More Show Less

Table of Contents

Part I Understanding, Configuring, and Troubleshooting CEF 3

Chapter 1 Introduction to Packet-Switching Architectures 5

Routing and Switching 5

Understanding Broadcast and Collision Domains 5

Layer 3 Switching 8

Understanding Router Pieces and Parts 9

Interface Processors 10

Central Processing Unit 11

Memory 11

Backplanes and Switching Fabrics 11

Cisco IOS Software: The Brains 17

Memory Management 17

Interaction with Interface Processors 28

Processes and Scheduling 28

Process Memory 28

Process Scheduling 29

Putting the Pieces Together: Switching a Packet 35

Getting the Packet off the Network Media 35

Switching the Packet 39

Transmitting the Packet 44

Hardware and Software show Commands 45

Summary 48

Chapter 2 Understanding Cisco Express Forwarding 51

Evolving Packet-Switching Methods 51

Process Switching 51

Fast Switching 52

What Is CEF? 53

CEF Tables 54

Forwarding Information Base (FIB) 54

The Adjacency Table 60

Relating the CEF Tables 61

CEF Table Entries 62

FIB Entries 62

Generic FIB Entries 67

Interface-Specific FIB Entries 68

FIB Entries Built for a Multiaccess Network Interface 68

FIB Entries Built on a Point-to-Point Network Interface 69

FIB Entries Built on a 31-Bit Prefix Network Interface 69

Special Adjacencies 69

Auto Adjacencies 70

Punt Adjacency 70

Glean Adjacency 71

Drop Adjacency 72

Discard Adjacency 73

Null Adjacency 73

No Route Adjacencies 74

Cached and Uncached Adjacencies 74

Unresolved Adjacency 75

Switching a Packet with CEF 75

The CEF Epoch 77

Configuring CEF/dCEF 77

Summary 78

References 79

Chapter 3 CEF Enhanced Scalability 81

Fundamental Changes to CEF for CSSR 82

Data Structures 82

Switching Path Changes 84

Changes to show Commands 86

show ip cef 86

show ip cef interface 86

show ip cef summary 87

show cef state capabilities 88

New show ip cef Commands 89

show ip cef tree 89

show ip cef internal 90

show ip cef switching statistics 91

New show cef Commands 91

CEF Event Logger 94

CEF Consistency Checker 97

Passive Checkers 97

Active Checkers 97

Consistency-Checking Process 98

New CEF Processes 100

FIB Manager 100

Adjacency Manager 100

Update Manager 100

Summary 101

Chapter 4 Basic IP Connectivity and CEF Troubleshooting 103

Troubleshooting IP Connectivity 103

Accurately Describe the Problem 104

Scoping the Network Topology 105

Reviewing the OSI Model for Troubleshooting 106

Verifying the ARP Table 108

Verifying the Routing Table 111

Troubleshooting the CEF FIB Table 116

Troubleshooting Punt Adjacencies 129

Understanding CEF Error Messages 131

Troubleshooting Commands Reference 131

Summary 133

References 133

Part II CEF Case Studies 135

Chapter 5 Understanding Packet Switching on the Cisco Catalyst 6500 Supervisor 720 137

CEF Switching Architecture on the Cisco Catalyst 6500 137

Understanding Software-Based CEF and Hardware-Based CEF 137

Centralized and Distributed Switching 138

Troubleshooting CEF on the Catalyst 6500 SUP720 Platforms 139

Simple Checking of Connectivity 139

Systematic Checking of Connectivity 141

Troubleshooting Load Sharing 148

Summary 149

References 149

Chapter 6 Load Sharing with CEF 163

Benefits of Load Sharing 163

Load Sharing with Process Switching and Fast Switching 164

Comparing CEF Per-Packet and CEF Per-Destination Load Sharing 168

Understanding Per-Destination Load Sharing 168

Understanding Per-Packet Load Sharing 169

CEF Architecture and Load Sharing 171

CEF Load Sharing Across Parallel Paths 173

CEF Per-Destination Example 173

CEF Per-Packet Example 180

Per-Packet Load Sharing on Hardware-Based Platforms 184

CEF Per-Packet Load Sharing on the Cisco GSR Platform 185

CEF Load-Sharing Troubleshooting Examples 186

CEF Per-Destination Load Sharing Overloading One Link 186

CEF Per-Packet Load Sharing Causing Performance Issues 198

Troubleshooting a Single-Path Failure with CEF Load Sharing 200

CEF Traffic-Share Allocation 202

CEF Polarization and Load-Sharing Algorithms 210

Summary 214

References 215

Chapter 7 Understanding CEF in an MPLS VPN Environment 217

An Internet Service Provider’s Simple MPLS VPN Design 217

Understanding the CEF and MPLS VPN Relationship 219

Case 1: Label Disposition 221

Case 2: Label Imposition 222

Case 3: Label Swapping 224

Troubleshooting an MPLS VPN 224

CEF Considerations When Troubleshooting MPLS VPN Across Various

Platforms 225

Cisco 7200 Router with an NPE-G2 226

Cisco 7500 Router 226

Cisco Catalyst 6500 with a Supervisor 2 227

Catalyst 6500 with a Supervisor 720 3BXL 228

Cisco 12000 Series Router 231

Cisco 10000 Series Router 236

CEF and MPLS VPN Load-Sharing Considerations 237

PE-CE Load Sharing: CE Multihomed to Same PE 237

PE-CE Load Sharing: Site Multihomed to Different PEs 243

Load Sharing Between P and P Devices 252

CEF and MPLS VPN Load-Sharing Platform Dependencies 253

Summary 253

References 254

Part III Appendix 257

Appendix A Scalability 259

Read More Show Less

Customer Reviews

Be the first to write a review
( 0 )
Rating Distribution

5 Star


4 Star


3 Star


2 Star


1 Star


Your Rating:

Your Name: Create a Pen Name or

Barnes & Noble.com Review Rules

Our reader reviews allow you to share your comments on titles you liked, or didn't, with others. By submitting an online review, you are representing to Barnes & Noble.com that all information contained in your review is original and accurate in all respects, and that the submission of such content by you and the posting of such content by Barnes & Noble.com does not and will not violate the rights of any third party. Please follow the rules below to help ensure that your review can be posted.

Reviews by Our Customers Under the Age of 13

We highly value and respect everyone's opinion concerning the titles we offer. However, we cannot allow persons under the age of 13 to have accounts at BN.com or to post customer reviews. Please see our Terms of Use for more details.

What to exclude from your review:

Please do not write about reviews, commentary, or information posted on the product page. If you see any errors in the information on the product page, please send us an email.

Reviews should not contain any of the following:

  • - HTML tags, profanity, obscenities, vulgarities, or comments that defame anyone
  • - Time-sensitive information such as tour dates, signings, lectures, etc.
  • - Single-word reviews. Other people will read your review to discover why you liked or didn't like the title. Be descriptive.
  • - Comments focusing on the author or that may ruin the ending for others
  • - Phone numbers, addresses, URLs
  • - Pricing and availability information or alternative ordering information
  • - Advertisements or commercial solicitation


  • - By submitting a review, you grant to Barnes & Noble.com and its sublicensees the royalty-free, perpetual, irrevocable right and license to use the review in accordance with the Barnes & Noble.com Terms of Use.
  • - Barnes & Noble.com reserves the right not to post any review -- particularly those that do not follow the terms and conditions of these Rules. Barnes & Noble.com also reserves the right to remove any review at any time without notice.
  • - See Terms of Use for other conditions and disclaimers.
Search for Products You'd Like to Recommend

Recommend other products that relate to your review. Just search for them below and share!

Create a Pen Name

Your Pen Name is your unique identity on BN.com. It will appear on the reviews you write and other website activities. Your Pen Name cannot be edited, changed or deleted once submitted.

Your Pen Name can be any combination of alphanumeric characters (plus - and _), and must be at least two characters long.

Continue Anonymously

    If you find inappropriate content, please report it to Barnes & Noble
    Why is this product inappropriate?
    Comments (optional)