Troubleshooting BGP A Practical Guide to Understanding and Troubleshooting BGP Networking Technology Series

The definitive guide to troubleshooting today?s complex BGP networks

This is today?s best single source for the techniques you need to troubleshoot BGP issues in modern Cisco IOS, IOS XR, and NxOS environments. BGP has expanded from being an Internet routing protocol and provides a scalable control plane for a variety of technologies, including MPLS VPNs and VXLAN. Bringing together content previously spread across multiple sources, Troubleshooting BGP describes BGP functions in today?s blended service provider and enterprise environments.

Two expert authors emphasize the BGP-related issues you?re most likely to encounter in real-world deployments, including problems that have caused massive network outages. They fully address convergence and scalability, as well as common concerns such as BGP slow peer, RT constraint filtering, and missing BGP routes. For each issue, key concepts are presented, along with basic configuration, detailed troubleshooting methods, and clear illustrations. Wherever appropriate, OS-specific behaviors are described and analyzed.

Troubleshooting BGP is an indispensable technical resource for all consultants, system/support engineers, and operations professionals working with BGP in even the largest, most complex environments.

· Quickly review the BGP protocol, configuration, and commonly used features

· Master generic troubleshooting methodologies that are relevant to BGP networks

· Troubleshoot BGP peering issues, flapping peers, and dynamic BGP peering

· Resolve issues related to BGP route installation, path selection, or route policies

· Avoid and fix convergence problems

· Address platform issues such as high CPU or memory usage

· Scale BGP using route reflectors, diverse paths, and other advanced features

· Solve problems with BGP edge architectures, multihoming, and load balancing

· Secure BGP inter-domain routing with RPKI

· Mitigate DDoS attacks with RTBH and BGP Flowspec

· Understand common BGP problems with MPLS Layer 3 or Layer 2 VPN services

· Troubleshoot IPv6 BGP for service providers, including 6PE and 6VPE

· Overcome problems with VXLAN BGP EVPN data center deployments

· Fully leverage BGP High Availability features, including GR, NSR, and BFD

· Use new BGP enhancements for link-state distribution or tunnel setup

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.

Foreword xxii

Introduction xxiii

Part I BGP Fundamentals

Chapter 1 BGP Fundamentals 1

Border Gateway Protocol 1

Autonomous System Numbers 2

Path Attributes 3

Loop Prevention 3

Address Families 3

BGP Sessions 4

Inter-Router Communication 5

BGP Messages 6

OPEN 6

Hold Time 6

BGP Identifier 7

KEEPALIVE 7

UPDATE 7

NOTIFICATION Message 8

BGP Neighbor States 8

Idle 9

Connect 9

Active 10

OpenSent 10

OpenConfirm 10

Established 10

Basic BGP Configuration 11

IOS 11

IOS XR 12

NX-OS 13

Verification of BGP Sessions 14

Prefix Advertisement 17

BGP Best-Path Calculation 20

Route Filtering and Manipulation 21

IBGP 22

IBGP Full Mesh Requirement 24

Peering via Loopback Addresses 25

EBGP 26

EBGP and IBGP Topologies 28

Next-Hop Manipulation 30

IBGP Scalability 31

Route Reflectors 31

Loop Prevention in Route Reflectors 33

Out-of-Band Route Reflectors 33

Confederations 34

BGP Communities 37

Route Summarization 38

Aggregate-Address 39

Flexible Route Suppression 40

Selective Prefix Suppression 40

Leaking Suppressed Routes 40

Atomic Aggregate 40

Route Aggregation with AS_SET 42

Route Aggregation with Selective Advertisement of AS-SET 42

Default Route Advertisement 42

Default Route Advertisement per Neighbor 42

Remove Private AS 43

Allow AS 43

LocalAS 43

Summary 44

References 45

Part II Common BGP Troubleshooting

Chapter 2 Generic Troubleshooting Methodologies 47

Identifying the Problem 47

Understanding Variables 48

Reproducing the Problem 49

Setting Up the Lab 49

Configuring Lab Devices 52

Triggering Events 56

Sniffer-Packet Capture 57

SPAN on Cisco IOS 58

SPAN on Cisco IOS XR 60

SPAN on Cisco NX-OS 62

Remote SPAN 63

Platform-Specific Packet Capture Tools 65

Netdr Capture 66

Embedded Packet Capture 68

Ethanalyzer 70

Logging 74

Event Monitoring/Tracing 77

Summary 81

Reference 81

Chapter 3 Troubleshooting Peering Issues 83

BGP Peering Down Issues 83

Verifying Configuration 84

Verifying Reachability 87

Find the Location and Direction of Packet Loss 88

Verify Whether Packets Are Being Transmitted 89

Use Access Control Lists to Verify Whether Packets Are Received 90

Check ACLs and Firewalls in Path 91

Verify TCP Sessions 94

Simulate a BGP Session 95

Demystifying BGP Notifications 96

Decode BGP Messages 99

Troubleshoot Blocked Process in IOS XR 103

Verify BGP and BPM Process State 104

Verify Blocked Processes 105

Restarting a Process 106

BGP Traces in IOS XR 106

BGP Traces in NX-OS 108

Debugs for BGP 110

Troubleshooting IPv6 Peers 112

Case Study–Single Session Versus Multisession 113

Multisession Capability 114

Single-Session Capability 115

BGP Peer Flapping Issues 115

Bad BGP Update 115

Hold Timer Expired 116

Interface Issues 116

Physical Connectivity 117

Physical Interface 117

Input Hold Queue 117

TCP Receive Queue 119

MTU Mismatch Issues 120

High CPU Causing Control-Plane Flaps 125

Control Plane Policing 127

CoPP on NX-OS 129

Local Packet Transport Services 134

Dynamic BGP Peering 138

Dynamic BGP Peer Configuration 139

Dynamic BGP Challenges 142

Misconfigured MD5 Password 142

Resource Issues in a Scaled Environment 142

TCP Starvation 142

Summary 143

References 143

Chapter 4 Troubleshooting Route Advertisement and BGP Policies 145

Troubleshooting BGP Route Advertisement 145

Local Route Advertisement Issues 145

Route Aggregation Issues 147

Route Redistribution Issues 150

BGP Tables 152

Receiving and Viewing Routes 154

Troubleshooting Missing BGP Routes 156

Next-Hop Check Failures 157

Bad Network Design 160

Validity Check Failure 162

AS-Path 162

Originator-ID/Cluster-ID 165

BGP Communities 167

BGP Communities: No-Advertise 167

BGP Communities: No-Export 169

BGP Communities: Local-AS (No Export SubConfed) 170

Mandatory EBGP Route Policy for IOS XR 172

Filtering of Prefixes by Route Policy 173

Conditional Matching 174

Access Control Lists (ACL) 174

Prefix Matching 175

Regular Expressions (Regex) 177

UnderScore _ 179

Caret ^ 180

Dollar Sign $ 181

Brackets [ ] 181

Hyphen - 182

Caret in Brackets [^] 182

Parentheses ( ) and Pipe | 183

Period . 183

Plus Sign + 183

Question Mark ? 184

Asterisk * 184

Looking Glass and Route Servers 185

Conditionally Matching BGP Communities 185

Troubleshooting BGP Router Policies 185

IOS and NX-OS Prefix-Lists 186

IOS and NX-OS AS-Path ACLs 188

Route-Map Processing 191

IOS and NX-OS Route-Maps 192

IOS XR Route-Policy Language 196

Incomplete Configuration of Routing Policies 198

Conditional BGP Debugs 199

Summary 203

Further Reading 204

References in This Chapter 204

Chapter 5 Troubleshooting BGP Convergence 205

Understanding BGP Route Convergence 205

BGP Update Groups 207

BGP Update Generation 212

Troubleshooting Convergence Issues 216

Faster Detection of Failures 218

Jumbo MTU for Faster Convergence 219

Slow Convergence due to Periodic BGP Scan 219

Slow Convergence due to Default Route in RIB 222

BGP Next-Hop Tracking 223

Selective Next-Hop Tracking 225

Slow Convergence due to Advertisement Interval 226

Computing and Installing New Path 226

Troubleshooting BGP Convergence on IOS XR 227

Verifying Convergence During Initial Bring Up 227

Verifying BGP Reconvergence in Steady State Network 228

Troubleshooting BGP Convergence on NX-OS 234

BGP Slow Peer 237

BGP Slow Peer Symptoms 238

High CPU due to BGP Router Process 238

Traffic Black Hole and Missing Prefixes in BGP table 238

BGP Slow Peer Detection 239

Verifying OutQ value 240

Verifying SndWnd 240

Verifying Cache Size and Pending Replication Messages 241

Workaround 242

Changing Outbound Policy 242

Advertisement Interval 243

BGP Slow Peer Feature 245

Static Slow Peer 245

Dynamic Slow Peer Detection 245

Slow Peer Protection 246

Slow Peer Show Commands 246

Troubleshooting BGP Route Flapping 246

Summary 250

Reference 250

Part III BGP Scalability Issues

Chapter 6 Troubleshooting Platform Issues Due to BGP 251

Troubleshooting High CPU Utilization due to BGP 251

Troubleshooting High CPU due to BGP on Cisco IOS 252

High CPU due to BGP Scanner Process 253

High CPU due to BGP Router Process 255

High CPU Utilization due to BGP I/O Process 256

Troubleshooting High CPU due to BGP on IOS XR 258

Troubleshooting High CPU due to BGP on NX-OS 262

Capturing CPU History 265

Troubleshooting Sporadic High CPU Condition 265

Troubleshooting Memory Issues due to BGP 267

TCAM Memory 269

Troubleshooting Memory Issues on Cisco IOS Software 269

Troubleshooting Memory Issues on IOS XR 274

Troubleshooting Memory Issues on NX-OS 278

Restarting Process 281

Summary 281

References 282

Chapter 7 Scaling BGP 283

The Impact of Growing Internet Routing Tables 283

Scaling Internet Table on Various Cisco Platforms 285

Scaling BGP Functions 288

Tuning BGP Memory 290

Prefixes 290

Managing the Internet Routing Table 290

Paths 292

Attributes 293

Tuning BGP CPU 295

IOS Peer-Groups 295

IOS XR BGP Templates 295

NX-OS BGP Peer Templates 296

BGP Peer Templates on Cisco IOS 297

Soft Reconfiguration Inbound Versus Route Refresh 298

Dynamic Refresh Update Group 302

Enhanced Route Refresh Capability 305

Outbound Route Filtering (ORF) 309

Prefix-Based ORF 309

Extended Community—Based ORF 309

BGP ORF Format 310

BGP ORF Configuration Example 312

Maximum Prefixes 316

BGP Max AS 318

BGP Maximum Neighbors 322

Scaling BGP with Route Reflectors 322

BGP Route Reflector Clusters 324

Hierarchical Route Reflectors 331

Partitioned Route Reflectors 332

BGP Selective Route Download 339

Virtual Route Reflectors 342

BGP Diverse Path 346

Shadow Route Reflectors 349

Shadow Sessions 355

Route Servers 357

Summary 364

References 365

Chapter 8 Troubleshooting BGP Edge Architectures 367

BGP Multihoming and Multipath 367

Resiliency in Service Providers 370

EBGP and IBGP Multipath Configuration 370

EIBGP Multipath 372

R1 373

R2 374

R3 374

R4 375

R5 376

AS-Path Relax 377

Understanding BGP Path Selection 377

Routing Path Selection Longest Match 377

BGP Best-Path Overview 379

Weight 380

Local Preference 380

Locally Originated via Network or Aggregate Advertisement 380

Accumulated Interior Gateway Protocol (AIGP) 381

Shortest AS-Path 383

Origin Type 383

Multi-Exit Discriminator (MED) 384

EBGP over IBGP 386

Lowest IGP Metric 386

Prefer the Oldest EBGP Path 387

Router ID 387

Minimum Cluster List Length 388

Lowest Neighbor Address 388

Troubleshooting BGP Best Path 389

Visualizing the Topology 390

Phase I–Initial BGP Edge Route Processing 391

Phase II–BGP Edge Evaluation of Multiple Paths 392

Phase III–Final BGP Processing State 394

Path Selection for the Routing Table 394

Common Issues with BGP Multihoming 395

Transit Routing 395

Problems with Race Conditions 397

Peering on Cross-Link 402

Expected Behavior 403

Unexpected Behavior 406

Secondary Verification Methods of a Routing Loop 409

Design Enhancements 411

Full Mesh with IBGP 412

Problems with Redistributing BGP into an IGP 413

Summary 417

References 418

Part IV Securing BGP

Chapter 9 Securing BGP 419

The Need for Securing BGP 419

Securing BGP Sessions 420

Explicitly Configured Peers 421

IPv6 BGP Peering Using Link-Local Address 421

BGP Session Authentication 424

BGP Pass Through 426

EBGP-Multihop 427

BGP TTL Security 428

Filtering 429

Protecting BGP Traffic Using IPsec 431

Securing Interdomain Routing 431

BGP Prefix Hijacking 432

S-BGP 439

IPsec 439

Public Key Infrastructure 439

Attestations 441

soBGP 442

Entity Certificate 442

Authorization Certificate 443

Policy Certificate 443

BGP SECURITY Message 443

BGP Origin AS Validation 443

Route Origination Authorization (ROA) 445

RPKI Prefix Validation Process 446

Configuring and Verifying RPKI 449

RPKI Best-Path Calculation 460

BGP Remote Triggered Black-Hole Filtering 463

BGP Flowspec 467

Configuring BGP Flowspec 469

Summary 479

References 480

Part V Multiprotocol BGP

Chapter 10 MPLS Layer 3 VPN (L3VPN) 481

MPLS VPNs 481

MPLS Layer 3 VPN (L3VPN) Overview 483

Virtual Routing and Forwarding 483

Route Distinguisher 485

Route Target 485

Multi-Protocol BGP (MP-BGP) 486

Network Advertisement Between PE and CE Routers 487

MPLS Layer 3 VPN Configuration 487

VRF Creation and Association 488

IOS VRF Creation 488

IOS XR VRF Creation 489

NX-OS VRF Creation 490

Verification of VRF Settings and Connectivity 492

Viewing VRF Settings and Interface IP Addresses 492

Viewing the VRF Routing Table 494

VRF Connectivity Testing Tools 495

MPLS Forwarding 495

BGP Configuration for VPNv4 and PE-CE Prefixes 497

IOS BGP Configuration for MPLS L3VPN 497

IOS XR BGP Configuration for MPLS L3VPN 499

NX-OS BGP Configuration for MPLS L3VPN 500

Verification of BGP Sessions and Routes 502

Troubleshooting MPLS L3VPN 506

Default Route Advertisement Between PE-CE Routers 508

Problems with AS-PATH 509

Suboptimal Routing with VPNv4 Route Reflectors 514

Troubleshooting Problems with Route Targets 520

MPLS L3VPN Services 524

RT Constraints 534

MPLS VPN Label Exchange 538

MPLS Forwarding 541

Summary 542

References 542

Chapter 11 BGP for MPLS L2VPN Services 543

L2VPN Services 543

Terminologies 545

Virtual Private Wire Service 548

Interworking 549

Configuration and Verification 550

VPWS BGP Signaling 558

Configuration 560

Virtual Private LAN Service 561

Configuration 562

Verification 564

VPLS Autodiscovery Using BGP 569

VPLS BGP Signaling 580

Troubleshooting 586

Summary 588

References 589

Chapter 12 IPv6 BGP for Service Providers 591

IPv6 BGP Features and Concepts 591

IPv6 BGP Next-Hop 591

IPv6 Reachability over IPv4 Transport 596

IPv4 Routes over IPv6 Next-Hop 601

IPv6 BGP Policy Accounting 604

IPv6 Provider Edge Routers (6PE) over MPLS 607

6PE Configuration 611

6PE Verification and Troubleshooting 615

IPv6 VPN Provider Edge (6VPE) 620

IPv6-Aware VRF 622

6VPE Next-Hop 623

Route Target 624

6VPE Control Plane 624

6VPE Data Plane 626

6VPE Configuration 627

6VPE Control-Plane Verification 629

6VPE Data Plane Verification 633

Summary 639

References 639

Chapter 13 VxLAN BGP EVPN 641</p

Vinit Jain, CCIE No. 22854 (R&S, SP, Security & DC), is a High Touch Technical Support (HTTS) engineer with Cisco providing support to premium customers of Cisco on complex routing technologies. Before joining Cisco, Vinit worked as a CCIE trainer and a network consultant. In addition to his expertise in networks, he has experience with software development, with which he began his career.

Vinit holds certifications for multiple vendors, such as Cisco, Microsoft, Sun Microsystems, VMware, and Oracle, and also is a Certified Ethical Hacker. Vinit is a speaker at Cisco Live and various other forums, including NANOG. Vinit pursued his graduation from Delhi University in Mathematics and earned his Masters in Information Technology from Kuvempu University in India. Vinit is married and is presently based out of RTP, North Carolina. Vinit can be found on Twitter @vinugenie.

Brad Edgeworth, CCIE No. 31574 (R&S & SP), has been with Cisco working as a systems engineer and a technical leader. Brad is a distinguished speaker at Cisco Live, where he has presented on multiple topics. Before joining Cisco, Brad worked as a network architect and consulted for various Fortune 500 companies. Brad’s other certifications include Cisco Certified Design Professional (CCDP) and Microsoft Certified Systems Engineer (MCSE). Brad has been working in the IT field with an emphasis on enterprise and service provider environments from an architectural and operational perspective. Brad holds a Bachelor of Arts degree in Computer Systems Management from St. Edward’s University in Austin, Texas. Brad can be found on Twitter @BradEdgeworth.

The definitive, up-to-date, and comprehensive guide to troubleshooting real world BGP deployments