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OSPF: Anatomy of an Internet Routing Protocol

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OSPF: Anatomy of an Internet Routing Protocol


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  • Copyright 1998
  • Dimensions: 7-1/2" x 9-1/2"
  • Pages: 360
  • Edition: 1st
  • Book
  • ISBN-10: 0-201-63472-4
  • ISBN-13: 978-0-201-63472-3

Written for TCP/IP network administrators, protocol designers, and network application developers, this book gives the most complete and practical view ever into the inner workings of Internet routing. The book focuses on OSPF (Open Shortest Path First), 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, the book provides not only a theoretical description of Internet routing, but also a real-world look into how theory translates into practice. It shows how algorithms are implemented, and how the routing protocols function in a working network where transmission lines and routers routinely break down.

You will find clear explanations of such routing fundamentals 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 it has evolved to keep pace with the rapidly changing Internet environment. OSPF topics covered by the book include the following:

  • 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, including a catalog of available debugging tools. The book also offers side-by-side comparisons of all the unicast and multicast routing protocols currently in use in the Internet.

You will come away from this book with 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 deeper understanding to work in your network environment.


Sample Content


Introduction The Internet is a global communications network. With connections in more than 100 countries, tens of millions of people use the Internet for business, education, and recreation. Electronic commerce is beginning on the Internet as businesses connect to sell their products and services. Academics collaborate over the Internet by exchanging electronic mail. People can converse using Internet phones, send faxes, participate in online chats and bulletin boards, play multiuser games, and experiment with virtual environments. Special-purpose computers called routers connect the Internet together. As data is forwarded from one place in the Internet to another, it is the routers that make the decisions as to where and how the data is forwarded. The protocols that dynamically inform the routers of the paths that the data should take are called routing protocols. It is the job of these protocols to react quickly to changes in the Internet's infrastructure, such as transmission lines going in and out of service, routers crashing, changes in network policies, and so on. Routing is what makes the Internet tick. Although many users of the Internet and the World Wide Web are unaware of the machinery underlying the network applications, routing is an interesting but complicated subject. Routing protocols are sophisticated distributed algorithms that must also be extremely robust to keep a large, decentralized network like the Internet running smoothly. Audience This book is for students of data communications, TCP/IP network administrators, protocol designers, developers of routing protocol software, and other professionals involved in the design, development, and management of TCP/IP networks. This book is a practical, hands-on description of Internet routing rather than a theoretical treatment. Although we describe how the various protocols were intended to work, we also describe how well the design has translated into practice. Internet protocol design is a practical undertaking itself, with efficiency of implementation often dictating design choices. For this reason, this book gives an in-depth treatment of how a router really works. Instead of just describing the algorithms, this book goes beyond to show how the algorithms are implemented. We often present ideas in a historical context, showing how Internet protocols have evolved. This is done for two reasons. First, you can learn a lot from the mistakes (and successes) of the past. Second, in order to participate in Internet discussion groups, many of which are dominated by old-timers, it is good to have some context. This book is not an elementary introduction to TCP/IP and its routing. Instead we assume that you have some familiarity with the TCP/IP protocol suite and some exposure to the basic concepts of routing. These assumptions allow us to explore many of the facets of Internet routing in greater detail than possible in an introductory text. Organization of This Book

This book is organized into five parts. Part I sets the groundwork for a discussion of Internet routing. After a brief description of how routing fits together with the rest of the Internet's protocols, Chapter 1 describes in depth how a router forwards packets. This discussion naturally leads to an explanation of IP addressing and CIDR, as well as of the interaction of hosts and routers. Internet routing protocols are introduced in Chapter 2, beginning with a treatment of the end product of all routing protocols: the router's routing table. Chapter 2 ends with an overview of the Internet's routing architecture and the two main routing technologies in use in today's Internet: Distance Vector and link-state algorithms.

Part II describes the Internet's OSPF routing protocol. We start in Chapter 3 with an explanation of why the OSPF protocol was developed in the first place. Chapter 4 discusses the basics of link-state routing; Chapter 5, how OSPF behaves over various subnet technologies; Chapter 6, its use of hierarchical routing; and Chapter 7, extensions to OSPF. Each chapter not only describes how OSPF works but also explains why it works that way. We explore the reasons behind OSPF's design decisions and how the OSPF protocol has evolved to keep pace with the rapidly changing Internet environment. Part II concludes with an OSPF FAQ (Chapter 8).

Part III (Chapters 9 and 10) describes TCP/IP multicast routing, including broadcast and multicast forwarding, the MBONE, and the two distinct types of multicast routing protocols: source-based trees and shared-tree algorithms. As we did with unicast routing, we go further into the subject of multicast routing through the examination of a particular multicast routing protocol: the Multicast Extensions to OSPF (MOSPF).

Part IV covers the configuration and management of Internet routing. The configuration and management of OSPF is explained in detail in Chapter 11. Chapter 12 describes the tools used to monitor and debug routing in a TCP/IP network. For each tool, we describe its use, how it works, and its advantages and drawbacks.

Part V is a comparison of Internet routing protocols. Chapter 13 compares and contrasts the routing protocols in use in the Internet: RIP, OSPF, BGP, IGRP, and IS-IS. In Chapter 14, we examine the available multicast protocols: DVMRP, MOSPF, PIM Dense and Sparse, and CBT.

Following Chapter 14 is an extensive bibliography arranged and numbered in alphabetical order. Within the text, the citation 85, for example, refers to item 85 in the bibliography.

Companion Book: OSPF Complete Implementation

The companion book OSPF Complete Implementation, in keeping with the Internet tradition that reveres "working code" over all else, explores even further the mechanics of Internet routing through examination of a real, working OSPF implementation. The book contains a complete implementation of OSPF on CD. Written in C++, the OSPF implementation is intended to be portable to a wide range of environments. Two sample ports are included: an OSPF routing daemon (called ospfd) for FreeBSD 2.1 and an OSPF routing simulator that can be run on Windows 95. The OSPF implementation has been developed using publicly available tools.


I would like to thank the technical reviewers who improved this book through their thoughtful and timely reviews: Ran Atkinson, Eural Authement, Fred Baker, Howard Berkowitz, Jeffrey Burgan, Joel Halpern, Mukesh Kacker, Robert Minnear, Jim Reid, and W. Richard Stevens. Thanks also to Tim Stoddard and the Arkansas Public School Computer Network (APSCN) for letting me collect OSPF statistics on the APSCN network and use that network as an example of OSPF configuration in Chapter 11, Configuration and Management. Thanks to S. Randall McLamb for drawing the figures.

I would also like to acknowledge the help of my editors at Addison Wesley Longman over the long life of this project: Carol Long, Karen Gettman, and Mary Harrington.

And special thanks to my wife, Sonya Keene, who designed the book, edited rough drafts, created the index, and gave encouragement while this book was being written.

October, 1997



A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

ABRs (area border routers), 122-123
definition of, 158-159
in MOSPF, 201
configuration of, 226
Abstraction, 55, 102
of broadcast subnets, 108-110
of NBMA subnets, 113
of Point-to-MultiPoint subnets, 116
Adapters in promiscuous mode, 189, 261
Add-ons for IP datagram forwarding, 13-14
add route command, 30
Address prefixes, 16-18
Address scoping, 23
IP, 8, 16-22
lending, 18
multicast, 175
Adjacent neighbors, 90
Administrative scoping, 175
Advanced Peer-to-Peer Networking (APPN) protocol, 49
Advanced Research Projects Agency network (ARPANET), 3, 33
routing algorithm in, 48-50
sequence bug in, 56, 77
synchronization in, 89
ADVERTISER attribute, 296
Advertising routers, 75-76, 156
Aggregation, 17-18
in BGP, 290
in IGRP, 299
in IS-IS, 302
in MOSPF, 199-203
in NSSA, 144
in OSPF, 157, 283
in OSPF MIB, 218
in RIP, 278-279
All-subnets-broadcast addresses, 20
AllDRouters address, 95
AllSPFRouters address, 95
for debugging, 259-262
for interoperability testing, 58
integrated with IP datagram forwarding, 14
Application layer, 9
Application protocols, 5
APPN (Advanced Peer-to-Peer Networking) protocol, 49
APSCN (Arkansas Public School Computer Network) example, 228-230
Area border routers (ABRs), 122-123
definition of, 158-159
in MOSPF, 201
configuration of, 226
Area flooding scope, 132
Area IDs, 86
Areas. See also OSPF areas
in MOSPF, 199-203
in IS-IS, 301
Arkansas Public School Computer Network example, 228-230
ARPANET (Advanced Research Projects Agency network), 3, 33
routing algorithm in, 48-50
sequence bug in, 56, 77
synchronization in, 89
AS-external-LSAs, 75, 128-131, 284
in Database Overflow extensions, 145
with external-attributes-LSAs, 147-149
flooding, 131-132
with IBGP, 235
vs. Type-7-LSAs, 143
AS flooding scope, 131
AS numbers, 34
AS_PATH attribute, 295
AS paths in BGP, 286, 295
AS sequences, 290
AS sets, 235, 290
ASBR-summary-LSAs, 75, 131
ASBRs (Autonomous System boundary routers), 127-128, 133, 144
ASN.1 format, 215
ASs (Autonomous Systems), 32-34
in BGP, 147-149, 296-297
configuring, 213
routing protocols with, 146
ATOMIC_AGGREGATE attribute, 296
cryptographic, 237-240
header fields for, 86
Automatic neighbor discovery, 106
Autonomous System boundary routers (ASBRs), 127-128, 133, 144
Autonomous Systems (ASs), 32-34
in BGP, 147-149, 296-297
configuring, 213
routing protocols with, 146
Backbone areas, 55, 125
Backup Designated Router field, 87
Backup Designated Routers
in database synchronization, 106-110
design rationale, 53-54
function of, 156-157
in NBMA subnets, 111-113
Bellman-Ford Vector algorithm, 36, 39
Best matches in routing tables, 28
BGP (Border Gateway Protocol), 34, 39, 284-292
with ASs, 147-149, 296-297
IBGP, 292-294
interactions with, 233-235
MIB for, 254
operational issues, 296-297
path attributes, 286, 294-296
Binary trees for routing tables, 30-31
Border Gateway Protocol. See BGP (Border Gateway Protocol)
Boundaries of OSPF areas, 157-158
Broadcast-and-prune multicast routing protocols, 310-312
Broadcast forwarding, 178-184
Broadcast IP addresses, 16
Broadcast storms, 10
Broadcast subnets, 104-105
abstraction of, 108-110
database synchronization in, 106-108
neighbor discovery and maintenance in, 105-106
problems in, 110-111
Burgan, Jeff, 63

A | B | C | D | E | F | G | H | I | J | K | L | M | N | O | P | Q | R | S | T | U | V | W | X | Y | Z

Cache hits with hash functions, 31
in multicast forwarding, 183
in MOSPF, 195-197
for routing tables, 31-32
Candidate lists, 97-98
CBQ (Class Based Queuing) algorithm, 13
CBT (Core Based Trees) routing protocol, 180, 315
CheckAge constant, 80
in database synchronization, 94
in IP headers, 24-25
for LSAs, 78, 84, 159-160
in OSPF headers, 86
CIDR (Classless Inter-Domain Routing), 17, 67
for hierarchical routing, 121
for RIP, 277
CIDR blocks, 33
CIDR notation, 17
Cisco routers, 223-224
CIXs (Commercial Internet Exchanges), 34
Class Based Queuing (CBQ) algorithm, 13
Classes of IP addresses, 17, 19-21, 172-174
Classless Inter-Domain Routing (CIDR), 17, 67
for hierarchical routing, 121
for RIP, 277
Clients of Route Reflectors, 293
Clocks for OSPF aging, 160
Cloud subnets, 110
CLUSTER_LIST attribute, 293, 296
Clusters in BGP, 294
Coltun, Rob, 56, 60, 151, 263
Commercial Internet Exchanges (CIXs), 34
COMMUNITY attribute, 296
of Database Overflow, 146
of Demand Circuit, 142
when extending OSPF, 136
of external-attributes-LSA, 149
in MOSPF, 203-205
of NSSA areas, 145
of stub areas, 140
of TOS-based routing, 139
Complete Sequence Number PDUs (CSNPs), 302
Confederations in BGP, 292-294
Conferencing applications, 174
Configuration, OSPF, 222-225
difficulties in, 155-156
link costs, 225
OSPF areas, 226-227
timers, 227-228
Distance Vector, 35-39
as OSPF design issue, 45
in routing, 27
Convergence time, 27
Core Based Trees (CBT) routing protocol, 180, 315
Core routers, 180. See also Rendezvous Points
link, 152-153, 225
in MOSPF, 163-166
Counting to infinity, 37-39, 298
Cryptographic authentication, 237-240
CSNPs (Comple



The following corrections were made for the second printing of this title.

  • Page xvii: The word forthcoming was added (midpage) to indicate that the second volume of this book is not yet available.
Chapter 4
  • Page 84: In line 4, the word column was changed to row.
  • Page 93: In Figure 4.8 the arrow next to the label Time T3+ (at the bottom left of the figure) was changed so that the arrowhead points toward instead of away from
Chapter 6
  • Page 125: Five lines from the bottom, the word best was changed to better.

The index was also revised for this reprint.

The following corrections will be made in the third printing of this title.

Chapter 9
  • Page 172: In the second sentence of the last paragraph,the network segment 195.5.2/24 should be 192.5.2/24.

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