Guide to TCP/IP, Third Edition

Chapter 10: Routing in the IP Environment

Routing in the IP Environment 2

Objectives

• Understand how basic routing works

• Describe various routing characteristics

• Understand interior gateway protocols, such as RIP, OSPF, and EIGRP

• Understand BGP, an exterior gateway protocol

• Describe how to manage routing on an in-house internetwork

Routing in the IP Environment 3

Objectives (continued)

• Be aware of the challenges associated with redistributing a network from a classless protocol to a class-oriented protocol

• Describe factors involved in choosing a routing protocol for your wide area network

• Describe router connections on internal and external networks, including the Internet

Routing in the IP Environment 4

Objectives (continued)

• Understand the importance of securing routers and routing protocols

• Explain basic router diagnostic troubleshooting concepts, tools, and techniques

Routing in the IP Environment 5

Understanding Routing

• Routing table– Database that lives in the memory of the router– Compilation of information about all the networks

that the router can reach

Routing in the IP Environment 6

Routing in the IP Environment 7

How Entries Are Placed in the Routing Table

• Route entry can be placed in a routing table in three basic ways– Through direct connection– It can be manually configured– An entry can be placed in a routing table is

dynamically, by using a routing protocol

Routing in the IP Environment 8

Routing Protocols and Routed Protocols

• Routing protocols – Used to exchange routing information– Routing Information Protocol (RIP) and OSPF are

routing protocols

• Routed protocols – Layer 3 protocols that are used to get packets

through an internetwork

Routing in the IP Environment 9

Grouping Routing Protocols

• Interior gateway protocols (IGPs)– Routing protocols used inside a routing domain are

called interior

• Exterior gateway protocols (EGPs)– Routing protocols used to connect these routing

domains

• Distance vector and link-state– Used to communicate

Routing in the IP Environment 10

Distance Vector Routing Protocols

• RIP

• Interior Gateway Routing Protocol (IGRP)

• Border Gateway Protocol (BGP) also is a distance vector routing protocol

Routing in the IP Environment 11

Routing in the IP Environment 12

Link-State Routing Protocols

• Differ from distance vector routing protocols in two ways– They do not route by rumor– They do not periodically broadcast their entire tables

Routing in the IP Environment 13

Routing in the IP Environment 14

Routing Characteristics

• Network– Converged when all the routers know a loop-free

path to get to all other networks– Ideally should be in a converged state

• Split horizon– Used to speed up the process of convergence and

resolve the counting-to-infinity problem

• Poison reverse– Technique for assigning costs to routes designed to

prevent routing loops

Routing in the IP Environment 15

Routing in the IP Environment 16

Routing Characteristics (continued)

• Time to Live– Ensures that packets cannot loop endlessly through

a network– Defined in the Network layer header

• Multicast Versus Broadcast Update Behavior– Routers can be configured to forward multicasts– RIPv1 sends broadcast updates– RIPv2 can send multicast updates

Routing in the IP Environment 17

ICMP Router Advertisements

• Some routers – Can be configured to send periodic ICMP Router

Advertisement packets

• Periodic ICMP Router Advertisements – Allow hosts to learn passively about available routes

• Unsolicited ICMP Router Advertisements – Sent periodically to the all-hosts multicast address

224.0.0.1

Routing in the IP Environment 18

Black Holes

• Occurs on a network when– ICMP is turned off and – Router discards packets without sending any

notification about its actions

Routing in the IP Environment 19

Routing in the IP Environment 20

Areas, Autonomous Systems, and Border Routers

• OSPF utilizes areas– To reduce the number of entries in the link-state

database

• OSPF specification – Defines the need for a backbone area, Area 0

• Autonomous systems (ASs)– Groups of routers under a single administrative

authority

Routing in the IP Environment 21

Routing in the IP Environment 22

Routing in the IP Environment 23

Interior Gateway Protocols

• Used to exchange routing information within an AS

• Also are referred to as intra-domain routing protocols

• RIP– Basic distance vector routing protocol– Two versions: RIPv1 and RIPv2– Communications are UDP based– RIP-based routers send and receive datagrams on

UDP port number 520

Routing in the IP Environment 24

Routing in the IP Environment 25

Routing in the IP Environment 26

Routing in the IP Environment 27

Open Shortest Path First

• Defined in RFC 2328

• The premier link-state routing protocol used on TCP/IP networks

• Based on – Configurable values (metrics) that may be based on

network bandwidth, delay, or monetary cost

Routing in the IP Environment 28

Routing in the IP Environment 29

Open Shortest Path First (continued)

• Six basic types of LSAs:– Type 1 (Router Links Advertisement)– Type 2 (Network Links Advertisement)– Type 3 (Network Summary Link Advertisement)– Type 4 (AS Boundary Router Summary Link

Advertisement)– Type 5 (AS External Link Advertisement)– Type 7 (Not So Stubby Area Networks

Advertisement)

Routing in the IP Environment 30

Routing in the IP Environment 31

Routing in the IP Environment 32

Enhanced Interior Gateway Routing Protocol

• Interior Gateway Routing Protocol (IGRP)– Developed in the 1980s by Cisco Systems– Updated in the early 1990s (Enhanced Interior

Gateway Routing Protocol)

• Enhanced Interior Gateway Routing Protocol– Integrates the capabilities of link-state routing into a

distance vector routing protocol

Routing in the IP Environment 33

Border Gateway Protocol

• Exterior gateway protocols (EGP) – Used to exchange routing information between

separate autonomous systems– Defined in RFC 904– Border Gateway Protocol (BGP) replaces EGP

routing

Routing in the IP Environment 34

Border Gateway Protocol (continued)

• Offers three types of routing operations– Inter-autonomous system routing– Intra-autonomous system routing– Pass-through autonomous system routing

• When configured for intra-autonomous system routing– BGP routers are located within the same AS

• Pass-through autonomous system routing – Enables BGP peer routers to exchange routing

information across an AS that does not support BGP

Routing in the IP Environment 35

Routing in the IP Environment 36

Managing Routing on an In-House Internetwork

• IP routing protocols have a shortcoming – They do not discriminate between users, types of

traffic, and so on– They only know how to get to a network– Policy-based routing addresses this shortcoming

Routing in the IP Environment 37

Hybrid Networks

• What happens when you redistribute a network from a classless protocol to a class-oriented protocol that only understands the major network address but not the VLSM you’re using?

• How can you convert the bandwidth, delay, load, reliability, and MTU size of EIGRP and IGRP into the simple hop counts that RIP uses?

Routing in the IP Environment 38

Routing On and Off a Wide Area Network

• If the network in question is relatively small – Consider using no routing protocol

• Frame relay and ATM – Use virtual circuits

• So one physical interface can actually be logically partitioned into several logical interfaces.

• If your company also uses Novell’s IPX protocol– You may have to run one routing protocol for IP and

another routing protocol for IPX

Routing in the IP Environment 39

Mobile Users

• Mobile IP– Defined by the IETF in RFCs 2003 through 2006 and

RFC 3220 (which obsoletes RFC 2002)

• Local Area Mobility– Cisco proprietary feature that is similar to Mobile IP – Operates by using the routing table

Routing in the IP Environment 40

Routing To and From the Internet

• BGPv4 – The exterior routing protocol in use on the Internet– Should only be used by networks that connect to

multiple Internet providers – Tracks hops between pairs of autonomous systems

instead of tracking hop counts for actual routers

Routing in the IP Environment 41

Securing Routers and Routing Behavior

• Securing routers– Turn off unnecessary services– Shut down unnecessary listening ports– Configure strong access security to prevent

tampering– Secure physical access to the boxes

• Securing routing protocols – Requires cooperation from the protocols themselves

Routing in the IP Environment 42

Troubleshooting IP Routing

• ROUTE– View the host’s local routing table, and add and

remove route entries• PING

– Sends ICMP Echo messages and test connectivity• TRACERT

– Sends ICMP echoes with incrementally increasing TTL values to identify the path to a destination

• PATHPING– Utility used to discover path from host to destination

Routing in the IP Environment 43

Summary

• Routing protocols and routers– Provide a mechanism that can forward traffic from a

sender’s subnet to an intended receiver’s subnet

• Routers – Depend on various routing protocols to manage the

packet forwarding process

• Distance vector routing protocols such as RIP– Provides a crude metric of routing cost

Routing in the IP Environment 44

Summary (continued)

• The OSPF protocol– Supports much more sophisticated routing structures

that break up a network into routing areas

• Routing characteristics– Help to determine what kinds of routing protocols to

use in specific applications

• Managing routing on a complex network means – Understanding how and when to use exterior and

interior routing protocols

Routing in the IP Environment 45

Summary (continued)

• Router tables define the topology and behavior of IP networks– Essential to manage router security and updates as

safely as possible

• Troubleshooting tools for inspecting and diagnosing routing problems through – Windows 2000, Windows Server 2003, and Windows

XP IP host include route, tracert, ping, and pathping