Cisco Express Forwarding
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Cisco Express Forwarding

by Nakia Stringfield, Russ White, Stacia McKee

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ISBN-10: 1587052369

ISBN-13: 9781587052361

Pub. Date: 05/08/2007

Publisher: Cisco Press

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


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.

Nakia Stringfield, CCIE® No. 13451, is a network consulting engineer for Advanced Services at Cisco, 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.

Russ White, CCIE No. 2635, is a Principle Engineer in the Routing Protocol Design and Architecture team at Cisco. He is a member of the IETF Routing Area Directorate, co-chair of the Routing Protocols Security Working Group in the IETF, a regular speaker at Cisco Networkers, a member of the CCIE Content Advisory Group, 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 Technical Assistance Center (TAC) team. This team focuses on providing post-sales support of IP routing protocols, MPLS, 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 a member of the WAN/Access TAC team.

  • 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

Part I Understanding, Configuring, and Troubleshooting CEF

Chapter 1 Introduction to Packet-Switching Architectures

Chapter 2 Understanding Cisco Express Forwarding Chapter 3 CEF Enhanced Scalability Chapter 4 Basic IP Connectivity and CEF Troubleshooting

Part II CEF Case Studies

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

Chapter 6 Load Sharing with CEF

Chapter 7 Understanding CEF in an MPLS VPN Environment

Part III Appendix

Appendix A 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


Product Details

Cisco Press
Publication date:
Networking Technology Series
Edition description:
New Edition
Product dimensions:
7.40(w) x 9.20(h) x 0.90(d)

Table of Contents

Introduction     xvi
Understanding, Configuring, and Troubleshooting CEF     3
Introduction to Packet-Switching Architectures     5
Routing and Switching     5
Understanding Broadcast and Collision Domains     5
Broadcast and Collision Domains     6
Broadcast and Collision Domains in Routing     7
Layer 3 Switching     8
Understanding Router Pieces and Parts     9
Interface Processors     10
Central Processing Unit     11
Memory     11
Backplanes and Switching Fabrics     11
Shared Memory     11
Crossbar Switching Fabric     13
Bus Backplanes     14
Cisco IOS Software: The Brains     17
Memory Management     17
Memory Pools     17
Memory Regions     18
Packet Buffers     20
Interaction with Interface Processors     28
Processes and Scheduling     28
Process Memory     28
Process Scheduling     29
Understanding the Scheduler     29
Process Life Cycle     29
Process Priorities     32
Scheduling Processes     32
Process Watchdog     34
Special Processes     35
Putting the Pieces Together: Switching a Packet     35
Getting the Packet off the Network Media     35
Inbound Packets on Shared Media Platforms     36
Inbound Packets on Centralized Switching Routers with Line Cards     37
Inbound Packet Handling on Distributed Switching Platforms     38
Switching the Packet     39
Switching the Packet Quickly During the Receive Interrupt     39
Process-Switching the Packet     41
Transmitting the Packet     44
Hardware and Software show Commands     45
Summary     48
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
Attached FIB Entry     63
Connected FIB Entry     63
Rcceive FIB Entry      63
Recursive FIB Entry     64
Default Route Handler FIB Entry     66
ADJFIB FIB Entry     66
Learned from IGPs     67
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
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
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
Troubleshooting Physical Connectivity     106
Troubleshooting Layer 2 Issues     107
Verifying the ARP Table     108
Verifying the Routing Table     111
Using IOS Ping with the Record Option to Rule Out CEF      115
Troubleshooting the CEF FIB Table     116
Verifying the CEF Configuration     117
Confirming the IP CEF Switching Path     119
Using CEF Accounting Counters to Confirm the Switching Path     123
Verifying the CEF Switching Details     123
Verifying the Adjacency Table     126
Hardware-Specific Troubleshooting     128
Troubleshooting Punt Adjacencies     129
Understanding CEF Error Messages     131
Troubleshooting Commands Reference     131
Summary     133
References     133
CEF Case Studies     135
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
Load Sharing with CEF     153
Benefits of Load Sharing     153
Load Sharing with Process Switching and Fast Switching     154
Comparing CEF Per-Packet and CEF Per-Destination Load Sharing     158
Understanding Per-Destination Load Sharing     158
Understanding Per-Packet Load Sharing     159
Minimizing Out-of-Order Packets     159
Configuring CEF Per-Packet Load Sharing     160
CEF Architecture and Load Sharing     161
CEF Load Sharing Across Parallel Paths     163
CEF Per-Destination Example     163
CEF Per-Packet Example     170
Per-Packet Load Sharing on Hardware-Based Platforms     174
CEF Per-Packet Load Sharing on the Cisco GSR Platform     175
CEF Load-Sharing Troubleshooting Examples     176
CEF Per-Destination Load Sharing Overloading One Link     176
CEF Per-Packet Load Sharing Causing Performance Issues     188
Troubleshooting a Single-Path Failure with CEF Load Sharing     190
CEF Traffic-Share Allocation     192
CEF Polarization and Load-Sharing Algorithms     200
Original Algorithm     202
Universal Algorithm     202
Tunnel Algorithm     203
Hardware Platform Implementations      203
Summary     204
References     205
Understanding CEF in an MPLS VPN Environment     207
An Internet Service Provider's Simple MPLS VPN Design     207
Understanding the CEF and MPLS VPN Relationship     209
Label Disposition     211
Label Imposition     212
Label Swapping     214
Troubleshooting an MPLS VPN     214
CEF Considerations When Troubleshooting MPLS VPN Across Various Platforms     215
Cisco 7200 Router with an NPE-G2     216
Cisco 7500 Router     216
Cisco Catalyst 6500 with a Supervisor 2     217
Catalyst 6500 with a Supervisor 720 3BXL     218
Cisco 12000 Series Router     221
Cisco 10000 Series Router     226
CEF and MPLS VPN Load-Sharing Considerations     227
PE-CE Load Sharing: CE Multihomed to Same PE     227
PE-CE Load Sharing: Site Multihomed to Different PEs     233
Load Sharing Between P and P Devices     242
CEF and MPLS VPN Load-Sharing Platform Dependencies     243
Summary     243
References     244
Appendix     247
Scalability     249
Index     255

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