3 - routers and routing

8/2/2019 3 - Routers and Routing

1/32

Understanding

Routing


2/32

7-2

Agenda

Routing and a Router? Network Addressing

Routing Protocols


3/32

Router and Routing


4/32

7-4

What Is Routing, Router?

Routing is:

Finding a path between a source anddestination (path determination)

Moving information across an internetwork

from a source to a destination (switching*) Very complex in large networks because

of 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 routers

function, is different from a switch (the network device).


5/32

7-5

Routers

A router is a more sophisticated device than ahub or a switch. It determines the appropriatenetwork path to send the packet along bykeeping an up-to-date network topology inmemory, its routing table.

Routers keep track of each others routes byalternately listening, and periodicallysending,

route information

Bridging is faster than Routing, but unlikeBridging, Routing provides LAN Segmentation,Broadcast Control, Security and Scalability.


6/32

7-6

Routing Table

192.168.3.0 Frame Relay192.168.1.0 Ethernet192.168.2.0 FDDI

Network 192.168.2.0FDDI

Network 192.168.3.0Frame Relay

RemoteLocation

Network 192.168.1.0

Ethernet

Main Site

RoutersLayer 3


7/327-7

Generalized Router Hardware

System Bus

Interface

Network

Controller

Interface:

Network

Controller

Interface:

Network

Controller

Flash

NVRAM

ROM

CPU

RAM

Bus

Interface


8/327-8

Software Components

# ROM Monitor: # Device Micro code:

Startup Diagnostic code Part of IOS that deals with network

Contains exception handling controllers. Also deals with

modular interface processor.

# RxBoot: # Configuration Register:

Host mode Operating System 16 Bits, Specifies router startup

Used for downloading full parameters.

CISCO IOS

# CISCO IOS: # Configuration Files:

Contains process scheduler, memory Startup-config: contains

manager, parser configuration information

Also contains protocol specific code Running-config: contains

for packet handling active configurations


9/327-9

Startup Procedure

ROM MonitorDiagnostic, Console Setup, Memory Sizing

Config Register Check.

Loads RxBoot, or stays in ROMMON

RxBoot

Builds Basic Data Structures, Interface Setup,

Host Mode Functionality, Startup-config Check.

Loads CISCO IOS or Stays in RxBoot

Prompt [router(boot)]

IOS

Interface Setup, Router Functionality,Allocate Buffers, Loads

Startup-config.

Boot Process Completes. [router>] Prompts Appears


10/32


11/327-11

LAN-to-LAN connectivity

WAN-to-WAN connectivity

Remote access

Where are Routers Used?

Popular WAN service options include ISDN(Integrated Services Digital Network),Leased Lines, Frame Relay and X.25


12/327-12

Packet Flow in a Routed Network

Routers encapsulate and de-encapsulate data packetsas they are transferred from system X to system Y

X Y

AA

BB

CC

Presentation

Data LinkPhysical

Data LinkPhysical

A B C

Data LinkPhysical

Data Link

NetworkTransport

Session

Presentation

Application

PhysicalData Link

Network

Transport

Session

Application

Physical

Network Network Network


13/327-13

From the OSI model reference point of view -

The router de-encapsulates and examinesthe frame to determine what type of network

layer data is being carried. The network layerdata is sent to the appropriate network layerprocess, and the frame itself is discarded.

The network layer process examines the

header to determine the destination networkand then references the routing table thatassociates networks to outgoing interfaces.

The packet is again encapsulated in the linkframe for the selected interface and sent on.

Packet Flow in a Routed Network


14/327-14

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?


15/327-15

Multiprotocol Routing

IP 15.17.132.6

IP 15.16.42.8

IP 15.16.50.3

Routing Tables

IP

Token

Ring

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

Routers can support multiple independent routing algorithms & maintain

associated routing tables for several routed protocols


16/327-16

Routing Tables

Routing algorithms

Initialize and maintainrouting tablesto 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 SendNetwork: To:

27 Node A

57 Node B

17 Node C

24 Node B

11 Node B

72 Node A


17/327-17

Routing Algorithm Goals

Optimality Selecting the best route based on metrics and

metric 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.


18/327-18

Routing 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


19/32

Network Addressing


20/32

7-20

Network and Node Addresses

Network Node

1

2

123

1

3 1

1.1

2.1

3.1

1.2

1.3

Network addressPath part used by the router

Node addressSpecific port or device on the network

1

2

3


21/32

7-21

Addressing Examples

Network Node/HostProtocol 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


22/32

7-22

131.108.0.0INTERNET131.108.1.0

131.108.2.0131.108.3.0

131.108.5.0

131.108.4.0

131.108.8.0131.108.6.0

131.108.7.0

131.108.10.0

131.108.9.0

Subnetwork Addressing

Manufacturing

R&D

Subdividing addressspace into smaller blocks

Helps organize network

Security (keeps HRseparately addressable)

ScalabilityKeeps traffic toappropriate segments

Allows single, summarizedrouting entry (131.108.0.0) tobe advertised to externalnetworks

Specific route entries(131.108.8.0) required onlyfor routers in the subnetted

block

HR


23/32

7-23

Routing 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


24/32

7-24

Static Routing

Stub Network

A

B

Manual table updates bya network administrator

Benefits

Reflects administrators special

topology knowledge PrivateNot conveyed to other

routers in updates

Avoids the overhead of dynamicrouting

Stub network When a node is accessible by

only one path, a static route issufficient

Point-to-point or circuit-switched

connection


25/32

7-25

Dynamic Routing

A network change blocksthe established path...

A B

CD

XA B

CD

X

and an alternate route is

found dynamically.

Most internetworks use dynamic routing


26/32

Routing Protocols


27/32

7-27

Routed versus Routing Protocols

Routed protocolsused betweenrouters to direct usertraffic; also callednetwork protocols

Examples: IP, IPX,DECnet, AppleTalk,NetWare, OSI, VINES

1.02.03.0

1.12.13.1

DestinationNetworkNetworkProtocol

Protocol name

Exit Portto Use

Routing protocols

used betweenrouters to maintainrouting tables

Examples: RIP, IGRP,

OSPF, BGP, EIGRP


28/32

7-28

Routing Protocols

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 (shortest path first)

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


29/32

7-29

Routing Protocol Evolutions

Distance Vector

Link State

RIPDistance vector

Most common IGP

Uses hop count

IGRPDistance vector

Developed by Cisco

Addresses problems in

large, heterogeneousnetworks

OSPF Link state, hierarchical

Successor to RIP

Uses least-cost routing,multipath routing, andload balancing

Derived from IS-IS

EIGRP Hybrid protocol Developed by Cisco

Superior convergenceand operating efficiency

Merges benefits of link

state & distance vector

Hybrid


30/32

7-30

RIP and IGRP

RIP

Industry standard thatselects the path with the

fewest hops

19.2 k

64k64k 64k

IGRP

Cisco protocol that selectsthe fastest path (using

load, distance, etc.)

19.2 k

64k64k 64k


31/32

7-31

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


32/32

Summary

Routers move data across networks from asource to a destination

Routers determine the optimal path forforwarding network traffic

Routing protocols communicatereachability information between routers

3 - routers and routing

Documents