OSPF Anatomy of an Internet Routing Protocol

348 Pages·2003·2.64 MB·English

by  John T. Moy

OSPF; Anatomy of an Internet Routing Protocol
Written for TCP/IP network administrators, protocol designers, and network application developers,
OSPF: Anatomy of an Internet Routing Protocol gives the most complete and practical view of the
inner workings of Internet routing. OSPF (Open Shortest Path First) is a common TCP/IP routing
protocol that provides robust and efficient routing support in the most demanding Internet environments.
A methodical and detailed description of the protocol is offered and OSPF's role within the wider context
of a TCP/IP network is demonstrated.
Practical throughout, this book provides not only a theoretical description of Internet routing, but
also a real-world look at theory translated into practice. For example, Moy describes how algorithms are
implemented, and shows how the routing protocols function in a working network where transmission
lines and routers routinely break down.
Readers will find clear explanations of routing fundamentals, such as how a router forwards packets,
IP addressing, CIDR (Classless Inter-Domain Routing), the routing table, Internet routing architecture, and
the two main routing technologies—Distance Vector and link-state algorithms. OSPF is discussed in depth,
with an examination of the rationale behind OSPF's design decisions and how OSPF has evolved to keep
pace with the rapidly changing Internet environment. Topics covered by the book include:
• OSPF areas and virtual links
• NBMA (Nonbroadcast multi-access) and Point-to-MultiPoint network segments
• OSPF configuration and management
• Interaction with other routing protocols
• OSPF cryptographic authentication
• OSPF protocol extensions, including the Demand Circuit extensions
and the multicast extensions to OSPF (MOSPF)
IP multicast and multicast routing are also discussed. Methods for debugging routing problems are
explained, and are supplemented with a catalog of available debugging tools. An OSPF FAQ answers
some of the most frequently asked questions about the OSPF protocol. The book also offers side-by-side
comparisons of all the unicast and multicast routing protocols currently in use in the Internet.
Readers will gain a sophisticated understanding of Internet routing and of the OSPF protocol in
particular. Moreover, the book's practical focus will enable you to put this knowledge to work in your
network environment.
John T. Moy is a Senior Consulting Engineer at Ascend Communications. He is the author of the
OSPF and MOSPF protocol specifications and currently chairs the OSPF and MOSPF Working Groups in
the Internet Engineering Task Force. Mr. Moy has been involved in the design and development of router
software for fifteen years, currently at Ascend, and previously at Proteon and at Bolt Beranek and Newman.
Mr. Moy holds a master of arts in mathematics from Princeton University and a bachelor of engineering in
mathematics from the University of Minnesota.
Many of the designations used by manufacturers and sellers to distinguish their products are claimed as
trademarks. Where those designations appear in this book, and we were aware of a trademark claim, the
designations have been printed in initial capital letters or in all capitals.
The author and publisher have taken care in the preparation of this book, but make no expressed or
implied warranty of any kind and assume no responsibility for errors or omissions. No liability is
assumed for incidental or consequential damages in connection with or arising out of the use of the
information or programs contained herein.
The publisher offers discounts on this book when ordered in quantity for special sales. For more informa-
tion, please contact:
Pearson Education Corporate Sales Division
One Lake Street
Upper Saddle River, NJ 07458
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[email protected]
Visit AW on the Web: www.awl.com/cseng/
Library of Congress Cataloging-in-Publication Data
Moy, John T.
OSPF : anatomy of an Internet routing protocol / John T. Moy.
p. cm.
Includes bibliographical references and index.
ISBN 0-201-63472-4
1. Internet (Computer network) 2. Computer network protocols. 3. Computer network
architectures. I. Title.
TK5105.875.I57M69 1998
004.6'6-dc21 97-39463
CIP
Copyright © 1998 by Addison-Wesley
All rights reserved. No part of this publication may be reproduced, stored in a retrieval system, or
transmitted, in any form, or by any means, electronic, mechanical, photocopying, recording, or other-
wise, without the prior consent of the publisher. Printed in the United States of America. Published
simultaneously in Canada.
Cover art: Turner & Devries, The Image Bank
ISBN 0-201-63472-4
Text printed on recycled paper
56789 10-MA-0403020100
5th printing, May 2000
OSPF
Anatomy of an
Internet Routing Protocol
John T. Moy
TT
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Contents
List of Tables IX
List of Figures xi
Preface xv
Part I. Internet Routing Overview 1
Chapter 1. Role of Routers in the Internet 3
1.1 The Internet Protocol Suite 4
1.2 Forwarding IP Datagrams 9
1.3 IPv6 22
.3
Chapter 2. Internet Routing Protocols 27
2.1 Routing Tables 27
2.2 Internet Routing Architecture 32
2.3 Distance Vector Algorithms 35
2.4 Link-State Algorithms 39
Part II. The OSPF Protocol 41
Chapter 3. Developing the OSPF Protocol 43
3.1 Functional Requirements 43
3.2 Design Decisions 47
vi OSPF: Anatomy of an Internet Routing Protocol Contents
3.3 OSPFvl: A False Start 56
3.4 Interoperability Testing 57
3.5 Field Trials 61
3.6 On Becoming a Standard 66
3.7 The Internet Evolves 67
Chapter 4. OSPF Basics 71
4.1 An OSPF Example 72
4.2 Link State Advertisements (LSAs) 74
4.3 A Sample LSA: The Router-LSA 81
4.4 The Link-State Database 83
4.5 Communicating between OSPF Routers: OSPF Packets 85
4.6 Neighbor Discovery and Maintenance 86
4.7 Database Synchronization 87
4.8 Routing Calculations 95
Chapter 5. OSPF Network Types 101
5.1 The IP Subnet Model 102
5.2 Broadcast Subnets 104
5.3 NBMA Subnets 111
5.4 Point-to-MultiPoint Subnets 114
Chapter 6. Hierarchical Routing in OSPF 119
6.1 OSPF Areas 122
6.2 Incorporating External Routing Information 127
6.3 OSPF Area Types 132
Chapter 7. OSPF Extensions 135
7.1 TOS-Based Routing 137
7.2 Stub Areas 139
7.3 Demand Circuit Extensions 140
7.4 NSSA Areas 143
7.5 Database Overflow Support 145
7.6 The External-Attributes-LSA 146
Chapter 8. An OSPF FAQ 151
Part III. Internet Multicast Routing 169
Chapter 9. Internet Multicast Routing 171
9.1 Internet Multicast Model 171
9.2 The Multicast Protocol Stack 173
Contents OSPF: Anatomy of an Internet Routing Protocol vii
9.3 Broadcast Forwarding 178
9.4 MBONE 184
Chapter 10. MOSPF 187
10.1 An Extended Example 188
10.2 Group-Membership-LSAs 192
10.3 MOSPF Routing Calculations 194
10.4 Hierarchical Multicast in MOSPF 199
10.5 Backward Compatibility: Mixing with Nonmulticast Routers 203
10.6 MOSPF in the MBONE 206
Part IV. Configuration and Management 211
Chapter 11. OSPF Management 213
11.1 SNMP 214
11.2 OSPF MIB 218
11.3 Configuring OSPF 222
11.4 An Example: The Arkansas Public School Computer Network 228
11.5 Monitoring the OSPF Protocol 230
11.6 Interactions with Other Routing Protocols 233
11.7 OSPF Security 236
Chapter 12. Debugging Routing Problems 243
12.1 War Stories 244
12.2 Finding Tools for Debugging Routing Problems 245
12.3 Tool Interpretation 245
12.4 The ping Tool 247
12.5 The traceroute Tool 250
12.6 SNMP MIBs 254
12.7 MIB-Based Tools 257
12.8 Network Analyzers 259
12.9 Protocol-Specific Tools 263
12.10 Product-Specific Monitoring and Tracing 265
12.11 Multicast Traceroute 266
Part V. Routing Protocol Comparisons 273
Chapter 13. Unicast Routing Protocols 275
13.1 RIP 277
13.2 OSPF 281
13.3 BGP 284
13.4 IGRP 297
viii OSPF: Anatomy of an Internet Routing Protocol Contents
13.5 Integrated IS-IS 300
13.6 Historical Protocols 303
13.7 Interaction among Routing Protocols 305
Chapter 14. Multicast Routing Protocols 309
14.1 Broadcast-and-Prune Protocols 310
14.2 DVMRP 312
14.3 MOSPF 313
14.4 PIM Dense 314
14.5 PIM Sparse 314
14.6 CBT 315
14.7 Interaction among Multicast Routing Protocols 315
Bibliography 317
Index 331
List of Tables
1.1 Division of IP Address Space by Function 16
1.2 Special-Purpose IP Unicast Addresses 18
1.3 Historical Class Division of IP Address Space 19
2.1 The Routing Table of Figure 1.5's Router C 28
2.2 RIP Convergence When Subnet 192.1.4/24 Is Added to Figure 2.5 36
2.3 Distance Vector Convergence When Subnet 192.1.4/24 Is Deleted from
Figure 2.5 37
4.1 Actions Taken by OSPF Router, Based on LS Age Fields 80
4.2 Link-State Database for Sample Network 84
4.3 Dijkstra Calculation Performed by Router 10.1.1.3 97
4.4 Router 10.1.1.3's IP Routing Table 98
6.1 Distribution of Area Routing Information, Using Distance Vector
Mechanisms 126
6.2 OSPF's Four-Level Routing Hierarchy 129
7.1 Summary of OSPF Extensions 137
8.1 Summary-LSAs Generated for Segment 10.15.6.0/24 166
9.1 IP Multicast Address Assignments 175
9.2 IGMP Packet Types 177
9.3 Default TTL Values for MBONE Applications 186
11.1 Default Timer Values in OSPF 227
12.1 Common Failure Indications of traceroute 251
IX
x OSPF: Anatomy of an Internet Routing Protocol List of Tables
13.1 Classification of TCP/IP Routing Protocols 275
13.2 Aggregation Example Requiring AS Sets 291
13.3 BGP Path Attributes 295
14.1 Categorization of Internet Multicast Protocols 309