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Cisco IP Routing: Packet Forwarding and Intra-domain Routing Protocols

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Cisco IP Routing: Packet Forwarding and Intra-domain Routing Protocols


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  • Copyright 2002
  • Dimensions: 7-3/8" x 9-1/4"
  • Edition: 1st
  • Book
  • ISBN-10: 0-201-60473-6
  • ISBN-13: 978-0-201-60473-3

In this book, a leading expert on Cisco routing offers in-depth coverage of four key intra-domain protocols -- RIP, IGRP, OSPF, and EIGRP. Unlike other books on Cisco protocols, Alex Zinin shows you exactly what's happening inside your routers when you use these protocols -- so you can maximize your control over them, and leverage their full power. Cisco IP Routing demystifies even the most complex internals of Cisco IP routing with clear explanations, extensive visuals, and many real-world examples, configurations, and network designs. The heart of the book is its coverage of dynamic routing, starting with theory and then moving to the practical details of effective configuration. Alex Zinin also presents in-depth coverage of controlling routing by altering update flow, redistribution, and policy routing. For all network administrators, other Cisco networking professionals, and anyone preparing for Cisco's top-of-the-line CCIE exam.

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Table of Contents


1. Overview of Cisco Routers.

Description of a Cisco Router.

Interfacing With Routers.

Router Configuration.

Monitoring and Troubleshooting Tools.

The Test Lab.


2. Review of IP Addressing.

IP Addressing.


Special Addressing Rules.

Classful and Classless Addressing.

Variable Length Subnet Masks.

IP Addressing Details In Cisco Routers.

Frequently Asked Questions.


3. Routing and Forwarding Processes.

Packet-Switched Technologies.

Router Operation Overview.

Routing Information Sources.

Static Routing.

Dynamic Routing.

Default Routing.

Basic Forwarding Algorithm.

Classful Routing Operations.

Classless Routing Operations.

4. Routing Table Maintenance.

Routing Information Sources.

Routing Table Maintenance.


Frequently Asked Questions.

5. Packet Forwarding.

Overview of IP Forwarding.

Packet Input.

Forwarding Engine.

Packet Delivery.

Forwarding Methods in Cisco IOS.

Load Sharing in Cisco IOS.


Frequently Asked Questions.


6. Static Routes.

Static Routes in Cisco Routers.

Backup Static Routes.

Using Static Routes in NBMA and Dialup Environments.

Default Routes.

Routing Loops and Discard Routes.

Implementation Scenarios.

Frequently Asked Questions.

7. Dynamic Routing Protocols in Cisco IOS.

Common Functionality of Dynamic Routing Protocols.

Routing Protocol Configuration.

Routing Protocol Data Structures.

Route Redistribution.

Events Processed by Routing Protocols.


Frequently Asked Questions.


8. Distance-Vector Routing Protocols.

Distance-Vector Principles.

Routing Information Protocol.

Interior Gateway Routing Protocol.


9. Link-State Routing Protocols.

Introduction to Link-State Routing.

Open Shortest Path First (OSPF).


10. Enhanced IGRP.

Basic Description and History.

Theoretical Basis.

Subsystems and Data Structures.

Message Format.

Input Processing.

Internal Event Processing.

Sending EIGRP Packets.

Shortest Path Calculation.

Default Route Support.

Route Aggregation.

EIGRP Stub Router Extension.

EIGRP Configuration.

Configuration Examples.


Index. 0201604736T10152001


The role of the Internet today cannot be overestimated. It has become a part of our culture. Children learn how to use the Internet at school. Millions of people start their day by checking the e-mail messages. Internet connectivity is not considered as something extraordinary anymore. People use the Internet every day, as they have been using cars or TV sets for decades. More and more devices become connected to the Internet. These are not limited to servers and personal computers anymore. Electronic organizers, cellular phones, TV sets, and many other types of equipment come with the Internet applications. Digital video cameras and phones are connected to the Internet to instantly cast data to remote users around the globe.

The value of the Internet is definitely not in its just being a global network, but in the resources available through this network. Resources are provided and used by the end devices (for example, servers and personal computers)--the largest part of the Internet. The other part of it is the intermediate network devices or routers. The role of the routers is to provide connectivity between the end devices by properly forwarding the packets of the Internet Protocol (IP) that end devices send to each other in order to exchange data such as e-mail messages or the contents of the web pages. In order to deliver IP packets to their destinations successfully, routers communicate the network reachability information to each other using routing protocols. The information provided by the routing protocols allows the routers to calculate the paths to remote networks. All routing protocols are divided into two groups--intra-domain and inter-domain. Intra-domain routing protocols (for exampl! e, RIP or OSPF) are used for routing within an elementary block of the Internet architecture (an autonomous system or domain), while inter-domain protocols (the Internet is using BGPv4 today) are used to exchange routing information among these blocks.

Cisco routers are the most widely spread network devices in the Internet today. They are used to provide connectivity to the end users, aggregate traffic from multiple access devices, and perform routing in the Internet backbone. This book describes very deep details of Cisco routers functionality with the emphasis on packet forwarding and intra-domain dynamic routing protocols.


Quite a few very high quality books on IP, IP routing, IP network design, and configuration of Cisco routers have recently become available. However, it is still hard to find detailed explanation of the processes happening inside the routers. Nevertheless, this knowledge is vital for network professionals to be able to properly configure and efficiently troubleshoot Cisco routers.

This book does not teach how to build networks. It also does not teach the basics of the IP protocol or Cisco routers. Instead, it is written to thoroughly explain a very limited set of questions--IP routing and IP forwarding inside Cisco routers. The goal of this book is to give the reader a very deep understanding of the routing and forwarding technologies in general and in connection with their implementation in Cisco routers. The contents are intended to make sure the reader feels the details instead of just memorizing them. The book also clarifies a great number of very common misunderstandings about IP routing technologies and Cisco routers in particular.


An important condition that applies to the readers of this book is the presence of at least basic knowledge of IP and Cisco routers. Even though the book provides some introduction to these topics, the reader is assumed to have spent some time in the lab with Cisco routers and be familiar with such notions as TCP/IP, ARP, or Ethernet.

Except for this reservation, the book is addressed to a wide range of readers. This includes IP network architects, engineers and operators that would like to know more details on IP routing technologies and processes inside Cisco routers, network engineers preparing for the Cisco certified internetworking engineer (CCIE) examination, network support and consulting engineers, and so on.


The book consists of ten chapters. The first three chapters refresh the knowledge of Cisco routers, IP addressing and routing technologies that is required for the rest of the book. Remain chapters guide the reader through a thorough explanation of the generic routing and forwarding mechanisms inside Cisco routers and intra-domain IP routing protocols. Brief description of each chapter of the book follows.

Chapter 1--Introduction. Chapter 1 provides brief overview of Cisco routers including basic information on routers' hardware and software, configuration and monitoring. This chapter clarifies some points important for other chapters and describes the lab network used in the illustrations.

Chapter 2--IP addressing review. This part of the book explains topics that are important for the routing discussion--classful IP addressing and subnetting, classless IP addressing, and variable length subnet masks.

Chapter 3--Routing and forwarding processes. This chapter sets required background for a detailed discussion on specific routing technologies. The reader is familiarized with the fundamental networking concepts and router operation algorithms. The difference between routing and forwarding processes is explained. It is followed by a description of the sources of routing information and more detailed discussions on each source and the principles of default routing. The chapter also explains the basic forwarding algorithm and the differences between classful and classless routing operations.

Chapter 4--Routing table maintenance. Chapter 4 provides a very detailed discussion on how routing information is organized and maintained in Cisco routers. The chapter starts with a thorough explanation of each source of routing information. The second part of the chapter is dedicated to the routing table maintenance process. This includes processing of connected, static and dynamic routes, as well as the default route selection algorithm.

Chapter 5--Packet forwarding. Chapter 5 describes in detail the IP packet forwarding process--the module that actually uses information in the routing table to move packets from one interface to another. The chapter starts with the forwarding process overview, followed by a systematic explanation of each stage. Packet switching mechanisms available in Cisco IOS, including fast, optimum, distributed, NetFlow switching and Cisco Express Forwarding are described in the following section. The chapter ends with a discussion on the load sharing techniques available in Cisco routers.

Chapter 6--Static routes. This chapter provides a complete description of static routes in Cisco IOS. This includes the details on recursive static routes, static routes via interfaces, backup (floating), default and discard static routes. The chapter also includes implementation scenarios that help the reader understand the issues and tradeoffs involved into managing of a network using static routes.

Chapter 7--Dynamic routing protocols in Cisco IOS. Chapter 7 provides information about generic mechanisms used by the dynamic routing protocols. This includes basic routing protocol configuration, data structures, routes redistribution and event processing.

Chapter 8--Distance vector routing protocols. This chapter begins with a discussion on principles of distance-vector routing and, specifically, the Bellman-Ford algorithm. Very detailed descriptions of two distance-vector IP routing protocols--Routing Information Protocol (RIP) and Interior Gateway Routing Protocol (IGRP)-are provided. Each protocol description includes information on protocol packet formats, input message and event processing, outbound message generation, as well as the configuration commands and examples.

Chapter 9--Link-state routing protocols. Chapter 9 describes the mechanisms of link-state routing in general and a link-state routing protocol-Open Shortest Path First (OSPF). The first part of the chapter gives a very detailed explanation of the link-state routing concepts, including the Dijkstra algorithm, link-state database synchronization and flooding. An overview of the two link-state routing protocols (integrated IS-IS, and OSPF) currently used in IP world is given as well. The second part of the chapter provides a complete guide to the OSPF protocol and details of Cisco's OSPF implementation. Practically every aspect of the protocol is explained very thoroughly in this section. The chapter is completed with information on OSPF configuration commands and configuration examples.

Chapter 10--Enhanced IGRP. This chapter gives a detailed explanation of enhanced IGRP (EIGRP)--a Cisco's proprietary routing protocol, based on the diffusing update calculation algorithm. The chapter introduces the concepts of EIGRP by showing how the protocol solves real-life problems in the distance-vector protocols. As in the previous chapters, full description of EIGRP packets, message and event processing, as well as update generation is included. The chapter also discusses the topics of the default route support and route aggregation in EIGRP, followed by a description of the EIGRP stub router extension. Sections on EIGRP configuration and configuration examples conclude the chapter.


When this book was being written, the emphasis was made on explaining why certain things work in a specific way, not just how they work. This is why practically every chapter in the book contains a theoretical introduction to the topic. The core of every chapter is the actual explanation of specific mechanisms inside the router. In most of the cases, discussions on specific topics are given in the context of Cisco's implementation of the algorithms and mechanisms, illustrated with logs from real Cisco routers.

The book contains description of many algorithms. All of them are described in human language, and for some of them a pseudocode definition is given for better understanding.

Chapters 4 through 10 also contain sections that answer the most frequently asked questions about a specific technology.


The author would like to note that pseudocode used in this book is not a recompilation of the Cisco IOS source code and any similarity between the two is accidental.



ABRs see OSPF/routing domain structure
Access control list (ACL), 168-169
table, 223
two-way connectivity check, 481
types, 224
see also OSPF
Administrative distance (AD), 100, 110, 275-276, 291, 374
as additional level of route selection, 101-103, 102f
comparison, 103-104
see also CompareRoutes() function
defaults, 100, 100t
static route examples, 106-109, 159
see also Proto_ADForRoute function
configuration of tunnel interfaces, 202-203
implicit control, 3
interface with routers see Command line interface
ip subnet-zero command, 28, 29l
manual routing table clearance, 144-147
map table configuration, 194-196, 286-289
policy routing, 177-182
route cache control, 212-214
routing table initialization, 110-111
setting router configuration, 6-11
dynamic changes, 6, 6l, 78
shutdown interface, 28
and static routing, 41, 44, 241
adding/deleting static route, 126-127
AFI (address family identifier), 317
AppleTalk (network-layer protocol), 1, 166
ARP (Address Resolution Protocol), 35, 189-190
ARPLookUp() function, 190
inverse, 193
mechanism, 190l-191l, 191-192
ATM, using static routes, 247
Autonomous system (AS), 44, 45f
vs. routing domain, 45
AUX (auxiliary asynchronous serial port), 2
line interface with router, 3-4, 3f, 4f
Bellman-Ford algorithm, 295, 307-310
BGP (routing protocol), 1
routes, 80
Broadcast types and addressing conventions, 182, 439-440
all subnets, 21
directed, 20-21
frame transmission, 182f
ip broadcast-address, 29, 29l-30l
ip directed-broadcast command, 30
local, 20
and packet delivery, 188-192
CIDR (classless interdomain routing), 22
and route aggregation, 60, 61f, 61t, 62, 62f
Cisco router
architecture, 1
configuration, 5
interface with, 3-5
monitoring/troubleshooting tools, 11-14, 13l
see also debug commands; ping command; show commands; Traceroute tool
unicast Reverse-Path Forwarding (RPF) check, 35
see also IP addressing details (Cisco routers); Routing table structure
Classful environment
classful addressing, 22, 53
and hierarchical routing table structure, 81
and default route treatment, 56-58, 57f, 62
potential problems, 59
route types, 53-54, 54f, 58
router operation summary, 58-59, 353
routing table lookup algorithm, 54-55, 139, 142-143
example, 55-56, 140-142
and supernet routes, 58, 62, 63t
Classless environment
and automatic route summarization, 59
classless addressing, 22
see also VLSM
and default routes, 62-63
example, 63-64, 64f
routing table lookup algorithm, 64, 237-238
clear ip route command, 144-147
Command line interface (CLI), 1
EXEC process, 3-4, 3f, 4f, 4l
parser, 6
CompareRoutes() function, 103-104l, 104
CON (console port), 2
line interface with router, 3-4, 3f, 4f
configure command, 6, 6l-7l
configure network command, 7, 8l
configure overwrite-network command, 8
configure terminal command, 7, 110
Connected field flags, 85
Connected routes see Directly attached networks
copy command, 8, 8l-9l
copy running-config startup-config command, 9
copy running tftp command, 9
misconceptions, 10-11
Counting, and routing loop, 302
Data link layer protocols, 1
debug commands, 11
debug eigrp fsm, 593, 593l-594l
debug ip commands, 12, 13l, 89l, 145l
debug ip rip command, 366l-367l
Default routes
candidate, 147-148
in classful environment, 56-58, 57f, 62
in classless environment, 62-64, 64f
configuration using static routes, 248-250
dynamic propagation, 151
selection algorithm, 151-152
examples, 152-155
types of addresses
major network, 149-150
subnet, 148-149
DelSubnet() function, 99
Demand circuits see OSPF; Routing Information Protocol
dialer in-band interface command, 196
DialerProcess() function, 196-198, 197l-198l
interesting flag, 198-199
queue checking, 199-200
Dijkstra algorithm, 408, 410-411, 411l-413l
intra-area route calculation (OSPF), 479-480
topological information input, 466
underlying concept, 409-410
Directly attached networks, 66
and interface routes, 67, 67l-68l, 68, 79
Ethernet segment connection example, 69, 69f, 70l
example of IOS route install, 68l-69l
Directly connected routes, 65
Distance-vector routing protocols
Bellman-Ford algorithm, 295, 307-310
counting to infinity problem, 301-302, 301f
solutions, 302-303, 307
DV_Holddown() function, 330-331
holddown timers, 305-306
neighbor and network discovery, 270
path characteristics augmentation, 298
principles, 295
route-refresh requirements, 350
routing algorithm, 299-301
snapshot routing, 402
split-horizon technique, 303-305, 304f
topology discovery
full-mesh, 296, 296f, 297l
partial mesh, 297, 297f, 298f
triggered updates, 306-307
see also IGRP
Distribute lists, 274-275
Distributed Services method see 7500-series router/(VIPs)
Dotted-decimal notation, 16
DUAL (diffusing update algorithm), 551, 552-553, 558
DV_Holddown() function, 330-331
Dynamic routes
common functionality, 269-271
cooperation with static routes, 143-144
dynamic routing protocol, 41-42, 44-46, 44f, 45f, 78
configuration commands, 76-78
and metrics, 85
no packet forwarding, 66
events processed, 290-291
processing, 124-126
EGPs (exterior gateway protocols), 45
BGP; and inter-domain routing, 45
EGP, 45
EIGRP (enhanced IGRP), 45, 151, 612
adjacency database, 554, 561, 562, 563l
fields, 563
classful route summarization, 582
configuration, 581-586
configuration examples
basic commands, 587-592
in CIDR environment, 592-596
manual route summarization, 605-607
route redistribution/filtering/and route maps, 607-611, 612l
topology, 586-587, 586f
variance parameters, 596-597
default route support, 578
examples, 597-604
DUAL algorithm/module, 551, 552-553
events, 558, 559t
route states, 559t
and update processing, 573-574
feasible successor concept, 554
FSM (finite state machine), 558, 560f
parameters/combinations, 558, 559t
general characteristics, 552t
Hello protocol-independent module, 560, 585
and IGRP, 402, 551
input processing; generic packet processing, 571-573
internal event processing, 574-575
loop-free routing techniques, 271
message format, 566f, 567l
body see TLVs
header fields, 565-566
multiple routes/metrics, 85, 292, 554
and administrative control, 581
example, 555-558
and neighbors, 572, 613
and network command, 77
packet-spoofing attack prevention, 572
path attributes
FD (feasible distance), 554
RD (reported distance), 554
protocol-specific modules, 560
cooperation, 561f
queries/replies, 574
route aggregation, 579, 605-607
routing table interface module/route redistribution, 560, 562, 584
RTP (reliable transport protocol), 553, 560, 562
sending packets, 575-576
Hellos, 576
replies, 577
updates/queries, 576-577
shortest-path calculation, 577-578
stub router extension, 579, 580f
modifications to regular EIGRP behavior, 580-581
timers, 582
SIA (stuck-in-active), 557, 558l, 579
topology database, 554, 561, 564l, 613
fields/information, 564-565, 589l
transport module, 560, 562
enable command, 6, 6l
EvXXX (Routing Information Protocol/state machine events), 333-335
EXEC process see Command line interface
Flash memory, 2
Flooding algorithm, 270-271, 481
assumptions, 414
goals, 414
and LSDB maintenance procedures, 473
link-state update package and processing steps, 473-476, 474f
LSA distribution, 477-478
LSA installation and SPF scheduling, 476-477
new information sources, 473
special cases, 478-479
requirements, 415
FlowCollector, 228
Forwarding, 33, 155, 157
engine algorithm, 170l-171l, 172-173
packet forwarding actions, 161, 237
and router components, 162f
virtual routing and (VRF), 238
see also Switching technologies
Forwarding algorithm (basic), 47
data structures (interface)
address mask, 49, 157
IP address, 49
IP status, 49
IP unnumbered flag, 49
state, 48
type, 48
data structures (packet), 48
data structures (routing table entries)
default candidate, 49
network prefix, 49
paths (intermediate address), 49
paths (outbound interface), 49
initiation of packet-delivery procedure, 50
see also Packet-delivery process
outline of router function, 49-50
Frame check sequence (FCS), 2, 165-166
Frame Relay
and Data Link Connection Identifier (DLCI), 35, 185
data link protocol, 1
frame-relay map commands; configuration example, 273-274
and packet-delivery process, 51-52
as point-to-multipoint, 183
cloud, 183
static routes, 247-248
error, 242-243
Garcia Luna Aceves, J., 271, 551
GetNextHop() function, 176, 176l, 177
Gigabit Switch Router (GSR), 2
and CEF, 224
HDLC (data link protocol), 1, 70
on serial interface, 158
Hedrick, Charles, and RIP, 310
protocol-independent module (EIGRP), 560, 562, 576
subprotocol, 270, 420, 437
and dynamic neighbor discovery, 449
incoming (processing), 451-452
packet formats, 448-449, 448f
rules per media, 450-451
tasks, 447
hostname global configuration command, 8
Hub-and-spoke topology, 42, 43f
ICMP (Internet Control Message Protocol), addressing, 20
IDB see IOS/interface descriptor block
IGPs (interior gateway protocols)
and explicit neighbor configuration, 272-273
hybrids, 551
interface dependency, 79
intra-domain routing functions, 45
see also EIGRP; IGRP; Link-state routing protocols; Routing Information
Protocol (RIP)
IGRP (interior gateway routing protocol), 45, 374, 401
adjacencies maintenance, 271
boundary routers, 380, 381f
characteristics, 375
configuration, 158, 388-389
configuration examples
basic configuration, 389l-390l
basic configuration/route details, 392l
basic configuration/routing tables, 391l
default routing, 395, 396l-397l
redistribution, 397l-398l
selecting routes with more available bandwidth, 392l-393l
subnet mask change, 399l-401l
unequal cost load-sharing, 393-395
considerations, 402-403
and default routing information, 380, 382
development of/requirements, 375-376, 402
and EIGRP, 402
and external routes, 383
input processing, 386-388
message format
header, 383-385, 383l, 384f
source and destination address setting, 383
message types
request, 384-385
update, 385
multiple routes/metrics, 85, 379, 380, 382
neighbor and network discovery, 270
and network command, 77
protocol parameters, 380-383
route attributes alteration, 386-387, 387f
route poisoning, 376, 387-388
routing information entries format, 385, 385l, 386f
special timers, 125, 305-306, 376, 381
and subnet routes, 83
see also Path calculations
Inter-domain routing, and BGP, 45
Interface data structures, 279
see also TRPInterfaceInfo
interface loopback 0 command, 28, 86l-87l
Interface routes see Directly attached networks
interface tunnel command, 202-203
as example of dynamic routing, 44
impact on router development, 220
IOS (Internetwork Operating System), 1, 3
interface descriptor block (IDB), 81
old vs. new command versions, 9, 10t
unnumbered point-to-point interfaces, 27
alternatives, 31
see also Command line interface; Load sharing
IP addressing details (Cisco routers), 25
assigning IP address to an interface, 25l-26l
assigning secondary IP address, 27l
and OSPF, 481-482
root masks and routes provided, 52-53
IP commands
ip address, 25, 26
ip broadcast-address, 29, 29l-30l
ip classless, 25, 174, 175, 253
ip default-network, 150-151, 249, 249l-250l, 597, 598l, 601, 601l-602l
ip directed-broadcast, 30
ip route cache, 209-212
ip route-cache flow, 227
ip subnet-zero, 28, 29l, 31
ip unnumbered, 27-28, 28l, 70l-71l
ip verify unicast reverse-path interface, 169-170
IP (Internet Protocol)
datagram, 15
header, 15-16, 16f
address class/ranges, 17-18, 17f, 18t
address mask, 16-17, 17l
addressing conventions, 20-21
destination address, 15, 48
source address, 15
TOS byte, 225
see also Classful addressing; Classless addressing
logical addressing, 15
network layer of OSI reference model, 1, 15
packet vs. datagram, 41
packet fragmentation, 40, 52
subnetting procedure, 18-19, 19l, 19t
address ranges, 19-20, 20t-21t
see also VLSM
IP networks, as datagram networks, 39
IpForward() function, 170l-171l
goal, 172-173
process, 172
see also Policy routing; Process switching; Route/lookup
IpInput() function, 167-168
IPinterfaceDown() function, 119-120, 120l-121l, 128
IPinterfaceUp() function, 118-119, 119l
IpOutput() function, 187, 187l-188l, 188
for point-to-multipoint interfaces, 193-194
for point-to-point interfaces, 192
IpPolicyRoute() function, 181
IPRIP functions
IPRIPAddMsgEntry(), 346-347, 346l
IPRIPAnnounceDefault(), 344l-345l
conditions for default-route announcement, 345
IPRIPComposeRoute(), 325-326, 325l
IPRIPGetMask(), 324l-325l, 325
IPRIPIfDown(), 337, 337l
IPRIPIfUp(), 336l-338l
IPRIPIntEventHandler(), 335l
IPRIPReceive(), 318l-319l
IPRIPSanityEntry(), 323l-324l, 324
IPRIPShouldAnnounce(), 343l, 344
IPRIPUpdate(), 329, 338, 338l-340l
ForIf/interface message creation steps, 341, 342l-343l
IPRoute functions
IPRouteGarbageTimer(), 332-333
IPRouteHolddownTimer(), 332
IPRouteInvalidTimer(), 330
IpRTLookup(), 173, 173l-174l, 176
and ip classless flag, 174
NextHop argument, 174
IPX (network-layer protocol), 1, 166
alternative to secondary addresses, 157
and MAC address of next-hop router, 188-189
IS-IS, 45
Hello subprotocol, 270
ISDN (integrated services digital network)
interfaces and dialer, 196
using static routes, 247
Lab see Test lab
LAN interfaces, 182
LastUpdated field, 85
LAT (local-area transport) services, interface with router, 3-5, 3f, 4f
Link-state routing protocols, 419
data unit (LSPDU), 413
age-check algorithm, 416-417
asynchronous distribution, 415
see also Flooding algorithm
Dijkstra algorithm, 408-413
and LSDB synchronization, 417-418, 473
steps, 463
request packet format, 466f
routing information calculation, 418-419
theoretical basis, 405-408, 406f, 407f
topology broadcasting, 406-407, 407f
leaf entries, 408, 408f
shortest-path tree (SPT), 407-408
see also IS-IS; LSAs; OSPF
Load sharing, 229
for CEF, 234-237
for dynamic routes, 230
for legacy process switching, 230-232
per destination (fast switching code), 232-233
per packet, 232
for static route configuration, 229, 229l, 231l, 268
techniques in Cisco IOS, 229t
unequal cost, 379-380, 393-395
LookUp() function, 175
LSAs, 466
Age field, 528
and external routing information
(AS-external-LSAs), 493-495
formats, 495-496, 496f, 502f
rules, 496-497
group pacing, 508-509
network-LSA, 470, 471f
origination, 472-473
origination of summary-LSAs, 487-488
router-LSA, 468-469, 469f
and link records, 469-470
link records, 470-471
router steps, 471-472
standard header fields, 466-468, 468f
types, 480, 485, 486f, 504
see also Flooding algorithm
MAC (Media Access Control) address, 35
Major network, vs. subnet, 19
Malkin, Gary, 311
address, 49, 157
route, 157
subnet, 157
maximum-paths router configuration command, 80
Metric field, 85
Minnear, Robert, 311
MOSPF (Multicast OSPF), 513
neighbor router configuration command, 273
Network Address Translation (NAT), 35
network command, 77, 271, 291
address specifications, 292
Network interface controllers (NICs), 2
Network interface types, 182
dialer, 184
lists, 198
packet delivery, 196-201
sample configuration, 200
loopback, 184
packet delivery, 203-204
multilink, 184
null, 184
packet delivery, 204-205
subinterface, 184-185, 185l, 186f
tunnel, 184
configuration, 202-203
packet delivery, 201-202
virtual-access, 184
see also Broadcast; OSPF; Point-to-multipoint; Point-to-point
Network layer
load sharing in Cisco IOS, 229
protocols, 1, 166
Next field, 86
NHRP (Next-Hop Resolution Protocol), 201
NMBA (non-broadcast multi-access)
and static routes, 247-248
see also ATM; Frame Relay; X.25
no peer neighbor-route interface configuration command, 70
NVRAM (nonvolatile random-access) memory, 2
saving active configuration, 8, 8l-9l
and show configuration command, 9
offset-list configuration command, 389
Open Systems Interconnection (OSI) reference model, 1, 15
Originator field, 85
OSPF (Open Shortest Path First) routing protocol, 1, 45, 419-420, 547-548
adjacencies, 458, 460
comparison with IS-IS, 421t-422t
convergence characteristics, 426-428
DBD packet, 461, 463-465, 464f
demand circuit extensions, 506-507
designated router election algorithm, 454, 456
procedures, 456-458
external routing information areas, 493-495, 504
normal, 423
not-so-stubby (NSSA), 424, 500, 501f, 502-504, 503f
stub, 423, 499-500
fragmentation issues, 445
global data structure, 428
area data structure, 429-430
information block, 428-429
implementation details, 508
LSA group pacing, 508-509
packet pacing, 508
interface data structure, 431-433
interface states, 452-453
events, 453-454, 455t, 456f
intra-area functionality, 436-437
subprocesses, 437, 438f
see also LSAs
loopback states, 443
Multicast, 513
NBMA mode, 440-441, 517-519
alternatives, 441
and Hello subprotocol, 449
neighbor data structures, 433
fields, 434-435
neighbor FSM, 461, 462t-463t, 465, 467f
neighbor states, 458-459
events, 460-461
network types, 439
Options field, 446-447, 447f
packet processing, 445-446
periodic updates, 480-481
point-to multipoint networks, 441-442, 442f
redistribute RIP route (query), 293
route aggregation techniques, 425-426
route calculation, 505-506
distance-vector approach, 492
external, 498-499
inter-area, 486-487
preference rules, 506
via network topology, 46, 427-428, 470-480
route calculation/partial calculation, 509
route types, 425
external, 425
inter-area, 425, 427, 536l
intra-area, 425, 436
Router-ID, 482
router types (internal/backbone), 424-425, 538-540
routing domain structure, 424f, 483
area border routers (ABRs), 423, 490-493, 492f, 504, 536l-537l
autonomous system boundary routers (ASBRs), 423, 495, 497-498
flow of information, 483-485, 484f
session stages, 461
subprotocols, 420
adjacency establishment, 420
LSA flooding and LSDB maintenance, 423, 437-438
LSA origination and flushing, 420
routing table calculation, 423
see also Hello subprotocol
transport protocol and packet formats, 443
header fields, 443-444, 443f
virtual links, 442, 485, 488-490, 490f, 491f, 493
OSPF (Open Shortest Path First) routing protocol configuration, 509
basic, 510
basic ABR configuration, 519, 519l-520l
hub and spoke, 532-533
multiple areas, 533-538
NSSA, 544-547
over Frame Relay Cloud, 529f, 530-531
route redistribution and stub areas, 540-544
single-area configuration, 525-528
virtual links/backbone interface, 538-540
general parameters, 510-514
interface-specific parameters, 514-517
and NBMA interface techniques, 517-519
NSSA, 522-523
route aggregation, 520
route redistribution (ASBR), 521-522
stub areas, 522
timers, 523-524
virtual links, 520-521
Packet-delivery process
on broadcast interfaces, 188-192
decapsulation, 165
encapsulation, 155f, 165-166, 201f
general functionality, 187-188
initiation by forwarding algorithm, 50
packet flows in Cisco routers, 162, 163f
packet input, 165-170
on point-to-point interfaces, 192-193
steps, 51
see also Forwarding; IpOutput() function; Network interface types
passive-interface router configuration command, 271-272, 272l, 292-293
Path calculations (IGRP), 376, 378
considerations, 403
effective bandwidth, 376-377
path delay, 376, 378
path load, 376, 377-378
path reliability, 376, 378
Path descriptor
level in routing table hierarchy, 80, 81
Metric field, 85
and parallel routes, 89l-90l
structured type definition, 84l-85l
view via show ip route, 90-91, 90l
PathValid() function, 116-117, 116l, 160
Ping, 13-14, 13l
connectivity test in static route, 74l
Point-to-multipoint interfaces, 182, 186f
and OSPF, 441-442, 442f
and packet-delivery process, 193-196
Point-to-point interfaces, 182, 186f, 439, 440f
and IP address (OSPF), 481-482
and packet-delivery process, 51-52, 192-193
unnumbered, 27
Policy routing, 173-178, 178l
and route maps, 178-179
examples, 179-181
see also IpPolicyRoute() function
PPP (data link protocol), 1, 70
example of reconfiguration, 71, 71l-72l
on serial interface, 158
Prefix descriptor see Routing table structure
Proto_ADForRoute function, 280-281, 325
Protocol descriptor, 100, 100l, 291
fields, 277-278
PSTN environment, using static routes, 247-248
QOS (quality-of-service) techniques, 482
Radix tree, and IP route cache, 209
RAM (random-access memory), 2
redistribute command, 609, 609l-610l
Reverse Path Forwarding (RPF) verification, 170
RIP (routing protocol), 1, 45, 292
and dynamic routing protocol, 78
hops for metric calculations, 46
and network command, 77
special timers, 125
RISC (reduced instruction set computer), 2
ROM (read-only memory), 2
Route, 158, 176, 578
aggregation, 60, 61f, 61t, 62, 62f, 425-426, 579
cache entries see Switching technologies
expiration technique, 270
filtering, 274-275
lookup, 173-174
manipulation, 99, 110
see also Administrative distance; Route source selection
parallel, 89l-90l, 159
poisoning, 376
redistribution, 281, 282, 292
algorithm, 285
case example, 281-282
paths for directly connected routes, 285-286
redistribute router configuration command, 282, 282l-283l, 283
see also Route maps
resolvability, 115-118
types (Cisco), 65
Route maps, 286, 286l-287l
match clauses; 287; parameters, 287-288
redistribution examples, 289
set clauses, 288
see also Policy routing
Route source selection, 110, 291
assigned AD values, 100, 101t
see also IGPs; Route/redistribution
RouteDelete() function, 114-115
RouteInstall() function, 91, 91l-92l
AddSubnet() field, 93
AddToChain() field, 93
algorithm, 93
CheckPath() field, 93, 94-95, 94l
CreateNetwork() field, 93
FindInChain() field, 92
FindRoute() field, 92
FreeRoute() field, 92
GetEntryType() field, 92
Major() field, 92
operation/management examples, 95-97
UnlinkFromChain () field, 93
RouteMetricUnreachable() function, 330-331
router command, 76, 271, 291
router id, 511
router igrp, 381, 388
Router operation, 162f
complex routed network example, 37-39, 38f
in datagram networks, 39
datagram networks rules of thumb, 39-40
default gateway, 35-36, 36f
dropped packets, 40, 164
maximum transmission unit (MTU), 40, 52
metrics calculation, 46, 85
output queuing, 165
route example, 36-37, 37f
see also Forwarding algorithm (basic)
RouteRemove() function, 97-98, 97l-98l
see also DelSubnet() function; RouteDelete() function
RouteUpdate() function, 104, 104l-105l, 105, 105l-106l, 114-115, 326l-327l
Routing, 33
decision (forwarding decision), 34
default, 46-47, 47f
dynamic propagation, 46, 48f
domain, 45
snapshot, 402
summary, 47, 53
tables, 34
sources, 38f, 41-42, 65
see also Routing table structure (Cisco routers)
updates, 45
virtual and forwarding (VRF), 238
Routing Information Protocol (RIP), 310-311, 371-372
configuration examples
basic configuration, 357l-358l
basic configuration/routing table entries, 358, 358l-360l, 360
route redistribution from dynamic routing protocols, 367, 367f, 368l-371l
static routing connection, 365l-366l
test lab, 356-357, 356f
uniform offset list for every serial interface/all routers, 362, 362l-365l
version 2 routing tables, 360l-362l
configuration parameters, 311-315, 352
information database, 315
interface information block parameters, 311-312
route redistribution, 355-356
setting split-horizon flag, 312, 312l, 353
timers, 315, 354-355
version, 353
database, 352
default-route announcement conditions, 345, 354
demand circuit extensions, 350
alternative routes, 351-352
presumption of reachability, 351
event processing (external), 335
interface state change (up/down), 336-337
IPRIPIntEventHandler(), 335l-336l
redistributed route, 337-338
event processing (internal)
garbage collection route timer expiration, 332-333
holddown route timer expiration, 331-332
invalid route timer expiration, 330
IPRIPUpdate() function, 329, 338, 338l-340l
logic, 336t
periodic/triggered update or general request response, 329, 335
general characteristics, 313t
hops for metric calculations/controls, 46, 312, 314, 372-373
input processing
algorithm function, 318-319
reply processing routine, 320, 321l
request processing routine, 319-320
sanity check, 319
message format, 315, 315l-316l
address family identifier (AFI), 317
general vs. specific request, 317-318
header, 317
v1, 316f
message processing, 322-323, 373f
adding an entry, 346-347
IPRIPComposeRoute() function, 325-326, 325l
IPRIPGetMask() function, 324l-325l, 325
IPRIPSanityEntry() function, 323l-324l, 324
timers, 328-329
update steps, 327-328
message types, 317
for demand circuits, 351
neighbor and network discovery, 270, 374
offset lists, 362
port 520, 315, 322
sending updates
logic, 340-341
using IPRIPUpdate() function, 338, 338l-340l
state machine, 333, 334f
events, 333-335
v1/v2, 45, 310
v2, 347-348
authentication subheader, 349f
input processing changes, 348-349
message entry format, 348f
message sending changes, 349-350
Routing protocols, 1, 45-46
and classful vs. classless addressing, 25
configuration, 271-276
data structures, 276, 276l-277l
usage, 279
see also Interface data structures; Protocol descriptors
events processed, 290-291
and forwarding engine, 66
link-state protocols, 270
and route filtering, 274-275
see also Distance-vector routing protocols; EGPs; IGPs; Static routes
Routing table structure (Cisco routers), 65, 87
composition process example, 89, 89l
configuration file commands/changes to routing table, 111
hierarchical database, 81, 82f, 158
interface descriptor level, 81
prefix descriptors level, 80, 81, 83l-84l
subnet descriptor level, 81
see also path descriptor/level
linkages example, 87, 88f, 89
location of route in table, 96
and routing source and forwarding engine, 66, 67f
RT (maintenance) process, 85, 113-114, 156t
algorithm, 113
deletion consequences, 113
interface going up/down, 111, 118-121, 119l, 120l, 121l
IP processing enabled/disabled on an interface, 112, 121-122
Route installation/deletion requested, 112, 124, 124l-125l
Route marked as default candidate/deleted, 112-113
route selection, 101-103, 102f
Secondary IP address added/deleted to an interface, 112, 123-124
sample, 65l-66l
size constraints, 158
sources supported, 78-79, 79l
see also Dynamic routes; HDLC; PPP; Route/redistribution; RouteInstall()
function; RouteRemove() function; Static routes
RTP (reliable transport protocol/EIGRP), 553
service timestamps command, 12, 12l
7000-series router, switching methods, 214-215, 215f
7500-series router, 2, 162
architecture, 216f
distributed switching, 217, 218f
flow, 218-219
Route Switch Processors (RSPs), 216, 219
Versatile Interface Processors (VIPs), 217
Distributed Services method, 219
ShareCount field, 85
show commands, 11
show adjacency, 223, 223l-224l
show configuration, 9
show interface, 81, 164, 164l
show interface serial, 0, 26
show ip cache flow, 227, 227l-228l
show ip cef, 222l-223l, 234l-235l
show ip eigrp topology, 554
show ip ospf, 430l-431l, 525l, 535l
show ip ospf interface, 433, 433l
show ip ospf neighbor, 435l-436l, 530l-531l
show ip protocol, 587, 588l, 589, 590l
show ip route set of commands, 11, 11l-12l, 65, 65l-70l
and contents of TNetworkInfo record, 86l
EIGRP route examples, 588l, 599l-600l, 611l
and path descriptors, 90-91, 90l
and routing table hierarchy, 82, 83f, 84l
show running, 26, 27l
show running-config, 9, 9l
Split-horizon technique, 303-305, 304f
SPT (shortest-path tree) see Link-state routing protocols
SSH (secure shell), interface with router, 3-5, 3f, 4f
Static routes
address ranges, 133-135, 134f, 134l-135l
backup, 244-247, 245f, 266-267
dial, 267-268
configuration parameters, 72-73, 73l, 243, 268
usage illustrations, 73l-74l
cooperation with dynamic routes, 143-144
and discard routes, 253-255, 253f
host routes, 266
implementation scenarios, 255
central and branch offices (partial mesh), 259-265, 260f
ISP and one customer, 255-259, 256f
loops, 246, 250, 251f, 252-253
in NBMA and dial-up environments, 247-248, 248f
over remote networks, 75, 75l, 76l, 266
processing, 126
adding/deleting, 126-127
delete unresolvable routes, 128, 128l-129l
deletion examples, 132-133
installation examples, 130-132
objectives, 133
of pending static routes, 129-130
scheduling, 128
resolution of intermediate network address, 79-80
and classful routing table lookup, 139-143
routing protocol, 41, 42-44, 43f
sanity check, 135-139
types, 241
both interfaces and next-hop addresses, 242
interfaces only reference, 242
intermediate network addresses only, 243-244, 244f
automatic route summarization, 53
and discontiguous networks, 54
mask, 82, 86l-87l
procedure, 18-20
Supernet routes, 58, 62
Switching tables, 34
Switching technologies (Cisco), 207t
Autonomous and SSE switching, 206, 214-215
Cisco Express Forwarding (CEF), 206
adjacency table, 223-224
characteristics, 220-221
configuration, 225
cooperation with switching-methods code, 221-222, 221f
enabling, 239
FIB (Forwarding Information Base), 220, 222-223
load sharing, 234-237
Fast switching, 205, 208-209, 208f, 214
administrative display, 212-214
dynamic cache, 211-212
half-fast switched features, 211
invalidation timers, 212
per destination load sharing, 232-233
and route cache entries, 209-211
NetFlow switching, 206
configuration, 227
and data export, 228
display, 227l-228l
flow cache entries, 226-227
objectives, 225
and packet processing, 226
Optimum switching, 206, 215-216
datagram networks, 33, 34-35
IP networks, 39
virtual-circuits (VC), 33-34
process switching, 205, 208f
and load sharing, 230-232
performance considerations, 206-208
Tag switching, 206
TADInfo data structure, 280
TELNET, interface with router, 3-5, 3f, 4f, 4l
terminal monitor command, 5, 5l
and debug output, 11
Test lab, 14
EIGRP configuration examples, 586-587, 586f
OSPF configuration examples, 524, 524f
and Frame Relay Cloud, 529, 529f
hub and spoke, 532
multiple areas, 535f
RIP configuration examples, 356-357, 356f
topology, 14f
TFTP server, and copy of router's active configuration, 9
Time-to-Live (TTL) field, 40, 48
TLVs, 567, 567f
authentication, 567
IP external routes, 568, 570f
IP internal routes, 568, 569f
next multicast sequence, 568
parameter, 567, 568f
peer info, 568
route attributes, 568-569
external route, 569-571
sequence, 567
SW version, 567
TNetworkInfo type, 84, 86l
Topology broadcast see Link-state routing protocols
TPathDescr, 84l-85l
TPrefixDescr type, 84
pseudocode example of routing entry, 83l-84l
Traceroute tool, 13
Traffic flow, 225-226
Triggered updates, 306-307, 335
TRoute types, 84
TRPInterfaceInfo, 278, 278l-279l
see also Interface data structures
TTY (terminal teletype) lines, interface with routers, 3-4, 4f
2500-series routers, 2, 162
ValidateRT() function, 117-118, 117l-118l, 128
VLSM (variable-length subnet masks), 22, 59
decision issues, 30
and forwarding engine, 159
VTY (virtual teletype) lines, interface with routers, 3-4, 4f
WAN interfaces, 183
X.25 PAD (packet assembler/disassembler)
interface with router, 3-5, 3f, 4f
static routes, 247-248
and packet-delivery process, 51-52
point-to-multipoint, 183


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