Overview

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 ...

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Cisco Express Forwarding

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Overview

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

 

 

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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.

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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

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