Cisco express forwarding

How does a router switch a packet? What is the difference between routing a packet and switching a packet? What is this CEF feature that is referred to in Cisco documentation and commonly found in Cisco IOS commands? Cisco Express Forwarding addresses these questions through comprehensive discussions of Cisco Express Forwarding (CEF). CEF is a term used to describe one of the mechanisms used by Cisco IOS routers and Cisco Catalyst switches to forward packets. CEF offers the benefits of improved performance, scalability, and resilience. CEF is found in almost all Cisco IOS routers and Catalyst switches, however documentation of the topic is very scarce. This book addresses common misconceptions about CEF and packet switching across various platforms. It helps readers understand CEF and how to troubleshoot whether there is a CEF or another problem occurring in the network. The book guides readers through CEF basics, supplying exercises and troubleshooting scenarios that enhance their ability to recognize common mistakes, as well as to provide them with some models for configurations that they can follow in their own networks.

Author Notes

Nakia Stringfield, CCIE[Registered] No. 13451, is a network consulting engineer for Advanced Services at Cisco
Russ White, CCIE No. 2635, is a Principle Engineer in the Routing Protocol Design and Architecture team at Cisco
Stacie McKee is a customer support engineer and technical leader of the Routing Protocols Technical Assistance Center (TAC) team

Introduction	p. xvi
Part I Understanding, Configuring, and Troubleshooting CEF	p. 3
Chapter 1 Introduction to Packet-Switching Architectures	p. 5
Routing and Switching	p. 5
Understanding Broadcast and Collision Domains	p. 5
Broadcast and Collision Domains	p. 6
Broadcast and Collision Domains in Routing	p. 7
Layer 3 Switching	p. 8
Understanding Router Pieces and Parts	p. 9
Interface Processors	p. 10
Central Processing Unit	p. 11
Memory	p. 11
Backplanes and Switching Fabrics	p. 11
Shared Memory	p. 11
Crossbar Switching Fabric	p. 13
Bus Backplanes	p. 14
Cisco IOS Software: The Brains	p. 17
Memory Management	p. 17
Memory Pools	p. 17
Memory Regions	p. 18
Packet Buffers	p. 20
Interaction with Interface Processors	p. 28
Processes and Scheduling	p. 28
Process Memory	p. 28
Process Scheduling	p. 29
Understanding the Scheduler	p. 29
Process Life Cycle	p. 29
Process Priorities	p. 32
Scheduling Processes	p. 32
Process Watchdog	p. 34
Special Processes	p. 35
Putting the Pieces Together: Switching a Packet	p. 35
Getting the Packet off the Network Media	p. 35
Inbound Packets on Shared Media Platforms	p. 36
Inbound Packets on Centralized Switching Routers with Line Cards	p. 37
Inbound Packet Handling on Distributed Switching Platforms	p. 38
Switching the Packet	p. 39
Switching the Packet Quickly During the Receive Interrupt	p. 39
Process-Switching the Packet	p. 41
Transmitting the Packet	p. 44
Hardware and Software show Commands	p. 45
Summary	p. 48
Chapter 2 Understanding Cisco Express Forwarding	p. 51
Evolving Packet-Switching Methods	p. 51
Process Switching	p. 51
Fast Switching	p. 52
What Is CEF?	p. 53
CEF Tables	p. 54
Forwarding Information Base (FIB)	p. 54
The Adjacency Table	p. 60
Relating the CEF Tables	p. 61
CEF Table Entries	p. 62
FIB Entries	p. 62
Attached FIB Entry	p. 63
Connected FIB Entry	p. 63
Rcceive FIB Entry	p. 63
Recursive FIB Entry	p. 64
Default Route Handler FIB Entry	p. 66
ADJFIB FIB Entry	p. 66
Learned from IGPs	p. 67
Generic FIB Entries	p. 67
Interface-Specific FIB Entries	p. 68
FIB Entries Built for a Multiaccess Network Interface	p. 68
FIB Entries Built on a Point-to-Point Network Interface	p. 69
FIB Entries Built on a 31-Bit Prefix Network Interface	p. 69
Special Adjacencies	p. 69
Auto Adjacencies	p. 70
Punt Adjacency	p. 70
Glean Adjacency	p. 71
Drop Adjacency	p. 72
Discard Adjacency	p. 73
Null Adjacency	p. 73
No Route Adjacencies	p. 74
Cached and Uncached Adjacencies	p. 74
Unresolved Adjacency	p. 75
Switching a Packet with CEF	p. 75
The CEF Epoch	p. 77
Configuring CEF/dCEF	p. 77
Summary	p. 78
References	p. 79
Chapter 3 CEF Enhanced Scalability	p. 81
Fundamental Changes to CEF for CSSR	p. 82
Data Structures	p. 82
Switching Path Changes	p. 84
Changes to show Commands	p. 86
Show ip cef	p. 86
Show ip cef interface	p. 86
Show ip cef summary	p. 87
Show cef state capabilities	p. 88
New show ip cef Commands	p. 89
Show ip cef tree	p. 89
Show ip cef internal	p. 90
Show ip cef switching statistics	p. 91
New show cef Commands	p. 91
CEF Event Logger	p. 94
CEF Consistency Checker	p. 97
Passive Checkers	p. 97
Active Checkers	p. 97
Consistency-Checking Process	p. 98
New CEF Processes	p. 100
FIB Manager	p. 100
Adjacency Manager	p. 100
Update Manager	p. 100
Summary	p. 101
Chapter 4 Basic IP Connectivity and CEF Troubleshooting	p. 103
Troubleshooting IP Connectivity	p. 103
Accurately Describe the Problem	p. 104
Scoping the Network Topology	p. 105
Reviewing the OSI Model for Troubleshooting	p. 106
Troubleshooting Physical Connectivity	p. 106
Troubleshooting Layer 2 Issues	p. 107
Verifying the ARP Table	p. 108
Verifying the Routing Table	p. 111
Using IOS Ping with the Record Option to Rule Out CEF	p. 115
Troubleshooting the CEF FIB Table	p. 116
Verifying the CEF Configuration	p. 117
Confirming the IP CEF Switching Path	p. 119
Using CEF Accounting Counters to Confirm the Switching Path	p. 123
Verifying the CEF Switching Details	p. 123
Verifying the Adjacency Table	p. 126
Hardware-Specific Troubleshooting	p. 128
Troubleshooting Punt Adjacencies	p. 129
Understanding CEF Error Messages	p. 131
Troubleshooting Commands Reference	p. 131
Summary	p. 133
References	p. 133
Part II CEF Case Studies	p. 135
Chapter 5 Understanding Packet Switching on the Cisco Catalyst 6500 Supervisor 720	p. 137
CEF Switching Architecture on the Cisco Catalyst 6500	p. 137
Understanding Software-Based CEF and Hardware-Based CEF	p. 137
Centralized and Distributed Switching	p. 138
Troubleshooting CEF on the Catalyst 6500 SUP720 Platforms	p. 139
Simple Checking of Connectivity	p. 139
Systematic Checking of Connectivity	p. 141
Troubleshooting Load Sharing	p. 148
Summary	p. 149
References	p. 149
Chapter 6 Load Sharing with CEF	p. 153
Benefits of Load Sharing	p. 153
Load Sharing with Process Switching and Fast Switching	p. 154
Comparing CEF Per-Packet and CEF Per-Destination Load Sharing	p. 158
Understanding Per-Destination Load Sharing	p. 158
Understanding Per-Packet Load Sharing	p. 159
Minimizing Out-of-Order Packets	p. 159
Configuring CEF Per-Packet Load Sharing	p. 160
CEF Architecture and Load Sharing	p. 161
CEF Load Sharing Across Parallel Paths	p. 163
CEF Per-Destination Example	p. 163
CEF Per-Packet Example	p. 170
Per-Packet Load Sharing on Hardware-Based Platforms	p. 174
CEF Per-Packet Load Sharing on the Cisco GSR Platform	p. 175
CEF Load-Sharing Troubleshooting Examples	p. 176
CEF Per-Destination Load Sharing Overloading One Link	p. 176
CEF Per-Packet Load Sharing Causing Performance Issues	p. 188
Troubleshooting a Single-Path Failure with CEF Load Sharing	p. 190
CEF Traffic-Share Allocation	p. 192
CEF Polarization and Load-Sharing Algorithms	p. 200
Original Algorithm	p. 202
Universal Algorithm	p. 202
Tunnel Algorithm	p. 203
Hardware Platform Implementations	p. 203
Summary	p. 204
References	p. 205
Chapter 7 Understanding CEF in an MPLS VPN Environment	p. 207
An Internet Service Provider's Simple MPLS VPN Design	p. 207
Understanding the CEF and MPLS VPN Relationship	p. 209
Case 1 Label Disposition	p. 211
Case 2 Label Imposition	p. 212
Case 3 Label Swapping	p. 214
Troubleshooting an MPLS VPN	p. 214
CEF Considerations When Troubleshooting MPLS VPN Across Various Platforms	p. 215
Cisco 7200 Router with an NPE-G2	p. 216
Cisco 7500 Router	p. 216
Cisco Catalyst 6500 with a Supervisor 2	p. 217
Catalyst 6500 with a Supervisor 720 3BXL	p. 218
Cisco 12000 Series Router	p. 221
Cisco 10000 Series Router	p. 226
CEF and MPLS VPN Load-Sharing Considerations	p. 227
PE-CE Load Sharing: CE Multihomed to Same PE	p. 227
PE-CE Load Sharing: Site Multihomed to Different PEs	p. 233
Load Sharing Between P and P Devices	p. 242
CEF and MPLS VPN Load-Sharing Platform Dependencies	p. 243
Summary	p. 243
References	p. 244
Part III Appendix	p. 247
Appendix A Scalability	p. 249
Index	p. 255

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Table of Contents