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Opis 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. Spis treści Cisco Express Forwarding: Part I Understanding, Configuring, and Troubleshooting CEF 3
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
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
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
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
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
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
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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