07 Understanding Routing

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1-1CSE-Network ManagementIntroduction 1999, Cisco Systems, Inc.www.cisco.com

Module 7:

Understanding

Routing


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7-2CSE: Networking FundamentalsRouting 1999, Cisco Systems, Inc.www.cisco.com

Agenda

What Is Routing?

Network Addressing

Routing Protocols


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1-3CSE-Network ManagementIntroduction 1999, Cisco Systems, Inc.www.cisco.com

www.cisco.com 1999, Cisco Systems, Inc.

What is Routing?What is Routing?


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What Is Routing?What Is Routing?

Routing is:

Finding a path between a source anddestination (path determination)

Moving information across an internetworkfrom a source to a destination (switching*)

Very complex in large networks becauseof the many potential intermediate nodes

A router is:

A network layer device that forwards packetsfrom one network to another and determinesthe optimal path for forwarding network traffic

* The term switching, when used to describe a routersfunction, is different from a switch (the network device).


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Routing TableRouting Table

192.168.3.0192.168.3.0 Frame RelayFrame Relay

192.168.1.0192.168.1.0 EthernetEthernet

192.168.2.0192.168.2.0 FDDIFDDI

Network 192.168.2.0

FDDI

Network 192.168.3.0Frame Relay

Remote

Location

Network 192.168.1.0

Ethernet

Main Site

RoutersLayer 3RoutersLayer 3


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LAN-to-LAN Connectivity

Routers encapsulate and de-encapsulate data packets

as they are transferred from system X to system Y

X Y

AA

BB

CC

Presentation

Data Link

Physical

Data Link

Physical

A B C

Data Link

Physical

Data Link

NetworkTransport

Session

Presentation

Application

PhysicalData Link

Network

Transport

Session

Application

Physical

Network Network Network


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Path Determination

1

2

3

4

5

6

7

8 9

10 11

Routers find the best path through the network

Routing tables contain route information

Network addresses represent the pathof media connections to a destination

Which Path?Which Path?


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Multiprotocol Routing

IP 15.17.132.6

IP 15.16.42.8

IP 15.16.50.3

Routing Tables

IP

TokenRing

TokenRing

AppleTalk 200.167

AppleTalk 100.110

Apple

IPX 4b.0800.0121.ab13

IPX 3a.0800.5678.12ab

Novell

DECnet 5.8

DECnet 10.1

DEC

VAX

VAX


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Routing Tables

Routing algorithms

Initialize and maintainrouting tables to helpwith path determination

Route information types

Destination/next-hop associations

Path desirability

Vary depending on routing algorithm

Message = Routing tablemaintenance communications

Routing update messages

Link-state advertisement

To Reach Send

Network: To:

27 Node A

57 Node B

17 Node C

24 Node B

11 Node B

72 Node A


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Routing Algorithm GoalsRouting Algorithm Goals

Optimality

Selecting the best route based on metrics andmetric weightings used in the calculation

Simplicity and low overhead

Efficient routing algorithm functionality with aminimum of software and utilization overhead

Robustness and stability

Correct performance in the face of unusualor unforeseen circumstances (e.g., high load)

Rapid convergence

Fast agreement, by all routers, on optimal routes

Flexibility

Quick and accurate adaptation to changes in

router availability, bandwidth, queue size, etc.


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Routing MetricsRouting Metrics

Path length

Total hop count or sum of cost per network link

Reliability Dependability (bit error rate) of each network link

Delay

Useful because it depends on bandwidth, queues, networkcongestion, and physical distance

Communication cost

Operating expenses of links (private versus public)

Bandwidth and load


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1-13CSE-Network ManagementIntroduction 1999, Cisco Systems, Inc.www.cisco.comwww.cisco.com 1999, Cisco Systems, Inc.

Network AddressingNetwork Addressing


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Addressing Examples

Network Node/Host

Protocol Address Address

General 1. 4

TCP/IP 10. 8.2.48

Novell IPX 1aceb0b 0000.0c00.6e25

AppleTalk 10. 1.

X.25 DNIC NTN

NTN: National Terminal Number


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Routing Algorithm TypesRouting Algorithm Types

Single-path versus multi-path

Flat versus hierarchical

Host-intelligent versus router-intelligent

Intradomain versus interdomain

Static versus dynamic routing

Link state versus distance vector


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Dynamic Routing

A network change blocks

the established path...

A B

CD

XA B

CD

X

and an alternate route is

found dynamically.

Most internetworks use dynamic routing


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Distance Vector versus Link StateDistance Vector versus Link State

Distance vector

Sends routing table info only to neighbors, sochange communication may need one min/router

Also called routing by rumor Easy to configure, but slow

Link state

Floods routing information about itself to all nodes,

so changes are known immediately

Efficient, but complex to configure

Ciscos EIGRP hybrid

Efficient and easy to configure


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Routed versus Routing Protocols

Routed protocols

used between

routers to direct user

traffic; also called

network protocols Examples: IP, IPX,

DECnet, AppleTalk,

NetWare, OSI, VINES

1.02.03.0

1.12.13.1

DestinationNetwork

NetworkProtocol

Protocol name

Exit Portto Use

Routing protocols

used betweenrouters to maintain

routing tables

Examples: RIP, IGRP,

OSPF, BGP, EIGRP


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Routing Protocol EvolutionsRouting Protocol Evolutions

Distance Vector

Link State

RIPDistance vector

Most common IGP

Uses hop count

IGRP Distance vector

Developed by Cisco

Addresses problems in

large, heterogeneous

networks

OSPF Link state, hierarchical

Successor to RIP Uses least-cost routing,

multipath routing, and

load balancing

Derived from IS-IS

EIGRP

Hybrid protocol

Developed by Cisco

Superior convergence

and operating efficiency

Merges benefits of linkstate & distance vector

Hybrid


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RIP and IGRP

RIPIndustry standard that

selects the path with thefewest hops

19.2 k

64k64k 64k

IGRPCisco protocol that selects

the fastest path (usingload, distance, etc.)

19.2 k

64k64k 64k


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OSPF and EIGRP

Aspect OSPF EIGRP

Topology Hierarchical Not restricted

Memory & CPUrequirements

High Moderate

Routing table size Large Moderate

Controlling body Industry standard Cisco proprietary

Convergence Fast Fast

Supportedprotocols

IPIP

IPXAppleTalk

Configuration Difficult Easy


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SummarySummary

Routers move data across networksfrom a source to a destination

Routers determine the optimal pathfor forwarding network traffic

Routing protocols communicatereachability information betweenrouters


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27Presentation_ID 1999, Cisco Systems, Inc. www.cisco.com

07 Understanding Routing

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