- •Warning and Disclaimer
- •Feedback Information
- •Trademark Acknowledgments
- •About the Author
- •About the Technical Reviewers
- •Dedication
- •Acknowledgments
- •Contents at a Glance
- •Contents
- •Icons Used in This Book
- •Command Syntax Conventions
- •Cisco’s Motivation: Certifying Partners
- •Format of the CCNA Exams
- •What’s on the CCNA Exams
- •ICND Exam Topics
- •Cross-Reference Between Exam Topics and Book Parts
- •CCNA Exam Topics
- •INTRO and ICND Course Outlines
- •Objectives and Methods
- •Book Features
- •How This Book Is Organized
- •Part I: LAN Switching
- •Part II: TCP/IP
- •Part III: Wide-Area Networks
- •Part IV: Network Security
- •Part V: Final Preparation
- •Part VI: Appendixes
- •How to Use These Books to Prepare for the CCNA Exam
- •For More Information
- •Part I: LAN Switching
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Brief Review of LAN Switching
- •The Forward-Versus-Filter Decision
- •How Switches Learn MAC Addresses
- •Forwarding Unknown Unicasts and Broadcasts
- •LAN Switch Logic Summary
- •Basic Switch Operation
- •Foundation Summary
- •Spanning Tree Protocol
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Spanning Tree Protocol
- •What IEEE 802.1d Spanning Tree Does
- •How Spanning Tree Works
- •Electing the Root and Discovering Root Ports and Designated Ports
- •Reacting to Changes in the Network
- •Spanning Tree Protocol Summary
- •Optional STP Features
- •EtherChannel
- •PortFast
- •Rapid Spanning Tree (IEEE 802.1w)
- •RSTP Link and Edge Types
- •RSTP Port States
- •RSTP Port Roles
- •RSTP Convergence
- •Edge-Type Behavior and PortFast
- •Link-Type Shared
- •Link-Type Point-to-Point
- •An Example of Speedy RSTP Convergence
- •Basic STP show Commands
- •Changing STP Port Costs and Bridge Priority
- •Foundation Summary
- •Foundation Summary
- •Virtual LANs and Trunking
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Review of Virtual LAN Concepts
- •Trunking with ISL and 802.1Q
- •ISL and 802.1Q Compared
- •VLAN Trunking Protocol (VTP)
- •How VTP Works
- •VTP Pruning
- •Foundation Summary
- •Part II: TCP/IP
- •IP Addressing and Subnetting
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •IP Addressing Review
- •IP Subnetting
- •Analyzing and Interpreting IP Addresses and Subnets
- •Math Operations Used to Answer Subnetting Questions
- •Converting IP Addresses from Decimal to Binary and Back Again
- •The Boolean AND Operation
- •How Many Hosts and How Many Subnets?
- •What Is the Subnet Number, and What Are the IP Addresses in the Subnet?
- •Finding the Subnet Number
- •Finding the Subnet Broadcast Address
- •Finding the Range of Valid IP Addresses in a Subnet
- •Finding the Answers Without Using Binary
- •Easier Math with Easy Masks
- •Which Subnet Masks Meet the Stated Design Requirements?
- •What Are the Other Subnet Numbers?
- •Foundation Summary
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Extended ping Command
- •Distance Vector Concepts
- •Distance Vector Loop-Avoidance Features
- •Route Poisoning
- •Split Horizon
- •Split Horizon with Poison Reverse
- •Hold-Down Timer
- •Triggered (Flash) Updates
- •RIP and IGRP
- •IGRP Metrics
- •Examination of RIP and IGRP debug and show Commands
- •Issues When Multiple Routes to the Same Subnet Exist
- •Administrative Distance
- •Foundation Summary
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Link-State Routing Protocol and OSPF Concepts
- •Steady-State Operation
- •Loop Avoidance
- •Scaling OSPF Through Hierarchical Design
- •OSPF Areas
- •Stub Areas
- •Summary: Comparing Link-State and OSPF to Distance Vector Protocols
- •Balanced Hybrid Routing Protocol and EIGRP Concepts
- •EIGRP Loop Avoidance
- •EIGRP Summary
- •Foundation Summary
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Route Summarization and Variable-Length Subnet Masks
- •Route Summarization Concepts
- •VLSM
- •Route Summarization Strategies
- •Sample “Best” Summary on Seville
- •Sample “Best” Summary on Yosemite
- •Classless Routing Protocols and Classless Routing
- •Classless and Classful Routing Protocols
- •Autosummarization
- •Classful and Classless Routing
- •Default Routes
- •Classless Routing
- •Foundation Summary
- •Advanced TCP/IP Topics
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Scaling the IP Address Space for the Internet
- •CIDR
- •Private Addressing
- •Network Address Translation
- •Static NAT
- •Dynamic NAT
- •Overloading NAT with Port Address Translation (PAT)
- •Translating Overlapping Addresses
- •Miscellaneous TCP/IP Topics
- •Internet Control Message Protocol (ICMP)
- •ICMP Echo Request and Echo Reply
- •Destination Unreachable ICMP Message
- •Time Exceeded ICMP Message
- •Redirect ICMP Message
- •Secondary IP Addressing
- •FTP and TFTP
- •TFTP
- •MTU and Fragmentation
- •Foundation Summary
- •Part III: Wide-Area Networks
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Review of WAN Basics
- •Physical Components of Point-to-Point Leased Lines
- •Data-Link Protocols for Point-to-Point Leased Lines
- •HDLC and PPP Compared
- •Looped Link Detection
- •Enhanced Error Detection
- •Authentication Over WAN Links
- •PAP and CHAP Authentication
- •Foundation Summary
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •ISDN Protocols and Design
- •Typical Uses of ISDN
- •ISDN Channels
- •ISDN Protocols
- •ISDN BRI Function Groups and Reference Points
- •ISDN PRI Function Groups and Reference Points
- •BRI and PRI Encoding and Framing
- •PRI Encoding
- •PRI Framing
- •BRI Framing and Encoding
- •DDR Step 1: Routing Packets Out the Interface to Be Dialed
- •DDR Step 2: Determining the Subset of the Packets That Trigger the Dialing Process
- •DDR Step 3: Dialing (Signaling)
- •DDR Step 4: Determining When the Connection Is Terminated
- •ISDN and DDR show and debug Commands
- •Multilink PPP
- •Foundation Summary
- •Frame Relay
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Frame Relay Protocols
- •Frame Relay Standards
- •Virtual Circuits
- •LMI and Encapsulation Types
- •DLCI Addressing Details
- •Network Layer Concerns with Frame Relay
- •Layer 3 Addressing with Frame Relay
- •Frame Relay Layer 3 Addressing: One Subnet Containing All Frame Relay DTEs
- •Frame Relay Layer 3 Addressing: One Subnet Per VC
- •Frame Relay Layer 3 Addressing: Hybrid Approach
- •Broadcast Handling
- •Frame Relay Service Interworking
- •A Fully-Meshed Network with One IP Subnet
- •Frame Relay Address Mapping
- •A Partially-Meshed Network with One IP Subnet Per VC
- •A Partially-Meshed Network with Some Fully-Meshed Parts
- •Foundation Summary
- •Part IV: Network Security
- •IP Access Control List Security
- •“Do I Know This Already?” Quiz
- •Foundation Topics
- •Standard IP Access Control Lists
- •IP Standard ACL Concepts
- •Wildcard Masks
- •Standard IP ACL: Example 2
- •Extended IP Access Control Lists
- •Extended IP ACL Concepts
- •Extended IP Access Lists: Example 1
- •Extended IP Access Lists: Example 2
- •Miscellaneous ACL Topics
- •Named IP Access Lists
- •Controlling Telnet Access with ACLs
- •ACL Implementation Considerations
- •Foundation Summary
- •Part V: Final Preparation
- •Final Preparation
- •Suggestions for Final Preparation
- •Preparing for the Exam Experience
- •Final Lab Scenarios
- •Scenario 1
- •Scenario 1, Part A: Planning
- •Solutions to Scenario 1, Part A: Planning
- •Scenario 2
- •Scenario 2, Part A: Planning
- •Solutions to Scenario 2, Part A: Planning
- •Part VI: Appendixes
- •Glossary
- •Answers to the “Do I Know This Already?” Quizzes and Q&A Questions
- •Chapter 1
- •“Do I Know This Already?” Quiz
- •Chapter 2
- •“Do I Know This Already?” Quiz
- •Chapter 3
- •“Do I Know This Already?” Quiz
- •Chapter 4
- •“Do I Know This Already?” Quiz
- •Chapter 5
- •“Do I Know This Already?” Quiz
- •Chapter 6
- •“Do I Know This Already?” Quiz
- •Chapter 7
- •“Do I Know This Already?” Quiz
- •Chapter 8
- •“Do I Know This Already?” Quiz
- •Chapter 9
- •“Do I Know This Already?” Quiz
- •Chapter 10
- •“Do I Know This Already?” Quiz
- •Chapter 11
- •“Do I Know This Already?” Quiz
- •Chapter 12
- •“Do I Know This Already?” Quiz
- •Using the Simulation Software for the Hands-on Exercises
- •Accessing NetSim from the CD
- •Hands-on Exercises Available with NetSim
- •Scenarios
- •Labs
- •Listing of the Hands-on Exercises
- •How You Should Proceed with NetSim
- •Considerations When Using NetSim
- •Routing Protocol Overview
- •Comparing and Contrasting IP Routing Protocols
- •Routing Through the Internet with the Border Gateway Protocol
- •RIP Version 2
- •The Integrated IS-IS Link State Routing Protocol
- •Summary of Interior Routing Protocols
- •Numbering Ports (Interfaces)
228 Chapter 7: Advanced Routing Protocol Topics
The most interesting part of the routing tables is Yosemite’s route to 10.2.0.0/16, with the outgoing interface set to null0. Routes pointing out the null0 interface mean that packets matching this route are discarded. EIGRP added this route, with interface null0, as a result of the ip summary-address command. The logic works like this:
Yosemite needs this route because now it might receive packets destined for other 10.2 addresses besides the four existing 10.2 subnets. If a packet destined for one of the 4 existing 10.2.x subnets arrives, Yosemite has a correct, more specific route to match the packet. If a packet whose destination starts with 10.2 arrives, but it is not in one of those 4 subnets, the null route matches the packet, causing Yosemite to discard the packet—as it should.
The routing table on Seville is similar to Yosemite’s in terms of the table entries and why they are in the table.
VLSM
VLSM occurs when more than one mask is used in a single Class A, B, or C network. Although route summarization causes more than one mask to be used, requiring support for VLSM, you can also simply design a network to use multiple subnet masks. By using VLSM, you can reduce the number of wasted IP addresses in each subnet, allow for more subnets, and avoid having to obtain another registered IP network number from the NIC.
Figure 7-2 depicts the same familiar network, but this time without summarization and with two different subnet masks used in network 10.0.0.0.
Figure 7-2 VLSM in Network 10.0.0.0: Masks 255.255.255.0 and 255.255.255.252
10.2.1.0 |
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Albuquerque |
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10.3.4.0 |
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10.2.2.0 |
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10.1.4.0/30 |
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10.1.6.0/30 |
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10.3.5.0 |
10.2.3.0 |
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S0/0 |
S0/1 |
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10.3.6.0 |
10.2.4.0 |
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10.3.7.0 |
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Yosemite |
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Seville |
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10.1.1.0
Mask: 255.255.255.0 Except Where Shown
This figure shows a typical choice of using a /30 prefix (mask 255.255.255.252) on point-to- point serial links, with some other mask (255.255.255.0 in this example) on the LAN subnets. The only real requirements for VLSM are that the subnets do not overlap and that the routing protocol supports VLSM. Subnets overlap when the range of IP addresses in one subnet includes some addresses in the range of valid addresses in another subnet. When using a single mask in a single Class A, B, or C network, you can usually avoid overlapping subnets easily. However, with VLSM, you can more easily overlook cases in which you assign subnets that overlap.
Route Summarization and Variable-Length Subnet Masks 229
Example 7-5 lists the routing table on Albuquerque again, this time with the masks shown in Figure 7-2 and with summarization still configured on Yosemite and Seville.
Example 7-5 Albuquerque Routing Table with Two Separate Masks Configured on Interfaces, Plus Route Summarization
Albuquerque#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is not set
10.0.0.0/8 is variably subnetted, 5 subnets, 3 masks
D10.2.0.0/16 [90/2172416] via 10.1.4.2, 00:00:34, Serial0/0
D10.3.0.0/16 [90/2172416] via 10.1.6.2, 00:00:56, Serial0/1 C 10.1.1.0/24 is directly connected, Ethernet0/0
C 10.1.6.0/30 is directly connected, Serial0/1 C 10.1.4.0/30 is directly connected, Serial0/0
Albuquerque uses VLSM with three masks now. Albuquerque knows the /24 and /30 prefixes (masks 255.255.255.0 and 255.255.255.252, respectively) from the configuration of the connected interfaces. Albuquerque learns about the 255.255.0.0 mask from the summarized routes advertised by Yosemite and Seville.
Regardless of how Albuquerque learns about these routes, to share these routes with other routers, it must use a routing protocol that supports VLSM. For a routing protocol to support VLSM, it must advertise not only the subnet number, but also the subnet mask, when advertising routes. Also, because VLSM support is required to support route summarization, the same routing protocols that support VLSM also support summarization. Table 7-2 lists the routing protocols and their support (or nonsupport) of VLSM.
Table 7-2 Interior IP Routing Protocol VLSM Support
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Sends Mask/Prefix |
Route Summarization |
Routing Protocol |
VLSM Support |
in Routing Updates |
Support |
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RIP-1 |
No |
No |
No |
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IGRP |
No |
No |
No |
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RIP-2 |
Yes |
Yes |
Yes |
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EIGRP |
Yes |
Yes |
Yes |
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OSPF |
Yes |
Yes |
Yes |
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