Router Basics

MM Clements

05/03/23 ITCN2

Introduction

WAN introduction and devices MODEM technologies Router Functions Router Hardware Connecting to a router for the first time

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Router Function

Linking WANs and LANs Interconnecting communication lines Path determination and packet switching Application of security rules (ACLs) Protocol conversion (encapsulation)

– E.g. HDLC, PPP etc.

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Introduction to WANs

A wide area network (WAN) is a data communications network spanning a large geographic area such as a region, country or the entire planet

A WAN may interconnect LANs May use microwave, satellite, fibre-optic,

phone lines etc. to cover distances

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WAN Devices

•Switch connects PCs, servers etc. to a router

•Ethernet cannot travel large distances so MODEM is used

•MODEM terminates a data circuit – data circuit-terminating equipment (DCE)

•Communication server allows many users access to a MODEM link

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Use of MODEM in WANs

•MODEM = Modulator and Demodulator

•Translates between Ethernet and WAN technology and back again

•Permits long distance communications

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Routers Connected by WAN Technologies

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Some WAN Terminologies

T1=1.544 Mbps, T3=44.736 Mbps , X.25, SMDS (SWITCHED MULTIMEGABIT DATA SERVICE), ATM, xDSL, Modem, Cable Modem, ISDN, OC-x

X.25 and ISDN are used less today than in the past but can still be found in operation

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Cisco Routers

Operating system is known as Internetwork Operating System (IOS)

Held in Flash memory (non-volatile) CLI not GUI Based on UNIX heritage

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Router Operation

Layer 3 device Accepts PDUs on incoming network Examines PDU data Makes decision(s) for next stage of PDU

journey May modify PDU contents (not payload) Passes PDU on to outgoing network

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Router Sorts Incoming Traffic

•Acts like Internet roundabout

•Allows data to get closer to its destination

•Best path decision made by router

•Incoming packets are switched to the most appropriate outgoing network

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Data Link Layer and Routing

Router does not pass on layer 2 data– e.g. broadcasts do not pass router

Layer 2 header is modified by router Source and destination MAC addresses are

changed at each router

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Network Path Determination

Router accepts packet and views inside Network Layer header

IP address of destination carried in Network Layer header and other information

Destination IP address looked up in routing table

Packet passed to appropriate exit interface

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Transport Layer Determination

Transport Layer header contents examined Source and destination port checked May trigger security of an Access Control List May drop packets under heavy load

– UDP often first casualty

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Access Control Lists

Used to identify incoming packets Can be used for security purposes E.g. do not allow TELNET traffic

– Identified by destination port number 23– Found in Transport Layer header

More on ACLs later in course

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Inside a Router

Router is a dedicated computer Contains hardware found in most PCs Does not have hard disk – Flash memory is

used instead to hold IOS NVRAM used to hold configuration files DRAM used to hold routing tables, buffering,

ARP cache etc CPU, ROM and interfaces too

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Internal Components of a 2600 Router

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External Connections

Configuration connections– Console and AUX

LAN connections– FastEthernet (usually)

WAN connections – often via WAN Interface Cards (WICs)

Newer hardware is modular Makes upgrading cheaper

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External Connections on a Router

Router Connections

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Connecting to a Router

First-time connection must be via console cable attached to a PC

PC runs terminal emulator e.g. Hyperterminal

Correct parameters must be set

9600 baud 8 data bits 1 stop bit No parity No flow control

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Physical Connection to Router

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Conclusion

Routers choose paths and switch data packets IOS runs on Cisco hardware Apply security etc. No hard disk – all solid state New routers have modular chassis for flexibility Terminal emulator and rollover cable to connect