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Chapter 1 1

Chapter 1 Introduction to Routing and Packet Forwarding

Routing Protocols and Concepts – Dr. C. BouSaba

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Router Functions

 Connect networks together

 Direct packets to their proper destination efficiently

 Find best routes

 Switch packets from one network to another

 Provide

– Security (Permitting or denying specified types of packet)

– Quality of service (Prioritizing packets)

ITE PC v4.0

Router as a computer

 CPU: control unit handles instructions, ALU for calculations

 RAM: volatile working storage

 ROM: permanent storage for POST and start-up instructions

 Operating System: software that runs the computer

 System bus, Power supply  Long term storage is Flash and

NVRAM, not hard disk

 Range of different interfaces all on different networks

 No input/output peripherals. Connect via a console PC and use PC’s keyboard and screen

Router VS computer

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Hardware Components of a Router

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Integrated Services Routers

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RAM  Dynamic random access memory

 Temporary memory while router is on

 Loses content when the router loses power or is restarted

 Holds running configuration

 Holds routing tables

 Holds ARP cache

 Holds fast-switching cache etc.

NVRAM  Non-volatile RAM: keeps its contents

when the router is off

 Stores the startup configuration file

 When you have configured a router, you must save your configuration to NVRAM if you want to keep it

Flash  Electronically erasable, programmable

ROM (EEPROM)

 Keeps its contents when the router is off

 Holds the operating system image (IOS)

 Allows the IOS to be updated

 Can store multiple versions of IOS software if it has enough capacity

 Can be upgraded by adding SIMMs

ROM  Permanent memory: cannot be upgraded

without replacing the chip

 Holds power-on self test (POST) inst.

 Stores bootstrap program

 Stores ROM monitor SW (for emergency download of IOS, for password recovery)

 May store basic IOS for emergency use (less common than it was)

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Router storage

 ROM

 Permanent

 Holds POST, boot instructions, basic IOS

 Flash

 Keeps contents

 Holds IOS image

 NVRAM

 Keeps contents

 Holds startup configuration file

 RAM

 Volatile

 Holds running config, tables, queues etc

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Interfaces

 Can be attached directly to the motherboard (like our Fast Ethernet or Ethernet interfaces)

 Can be on removable and interchangeable modules (like our serial interfaces)

 Modules for different serial connections

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Operating system

 As specialized computers, routers and switches need operating systems.

 Cisco devices use the Cisco Internetwork Operating System (IOS)

 There are versions for different models of router and switch, and different feature sets

 The IOS can be upgraded periodically

ITE PC v4.0

Naming IOS image files

 Platform-features-format-version

 c2600-i-mz.122-8.T5

 c2600 is the platform: Cisco 2600 series router

 i is a code for the set of features in this IOS, another is ipbase

 mz is a code to say that the IOS runs in RAM and the file is zip compressed

 122-8.T5 is the upgrade version

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IOS storage

 The IOS is stored in the router’s flash memory, often in compressed form

 Most routers copy the IOS to RAM when they start up

 You need enough space in flash and in RAM if you upgrade the IOS

 Some of our routers have more features than others – it depends on the IOS.

ITE PC v4.0

IOS modes

 ROM monitor:

Used to recover from system failure or loss of password

Needs direct access from console port

 Boot ROM (optional, may not have this):

Used when upgrading IOS

 Cisco IOS

Normal operation, stored in Flash, runs in RAM

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Router startup

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‘Normal’ start up

1. Run POST and bootup instructions from ROM

2. Load IOS file from flash

3. Load configuration from NVRAM

4. Fully operational

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Configuration register

 Has 4 hex digits – that’s 16 binary digits

 Configuration register is saved in NVRAM

 show version to see its value

 Value of last hex digit tells how to load IOS

 Usual is 0x2102 (2 means load from flash)

 Third hex digit controls whether configuration file is loaded. (0 means load, 4 means do not)

Check Configuration Register value (NVRAM)

0 = ROM Monitor mode

1 = ROM IOS

2 - 15 = Boot system from Flash

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How a Cisco device locates and loads IOS

 Demo

config-register

The config-register can be Downloaded from:

http://www.lilligren.com/cisco/downloads.htm

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Configuration register: 0, 1, and 2 and above

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Configuration register: 2102 and 2142

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Configuration register Router(config)#config-register value

1 2

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Loading IOS  You see ############# as IOS loads from flash memory.

 If you see a prompt instead:

 rommon1>

 Then the IOS was not loaded and you are in ROM monitor mode.

 Try reload or boot

 If this fails, the IOS file is probably missing…

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Configuration

 If there is a startup configuration file in NVRAM then it will normally load into RAM as the running configuration.

 If not, the router may look for a configuration on a TFTP server. Wait until it gives up.

 It then prompts you to enter Setup mode: Would you like to enter the initial configuration dialog? [yes/no]: no (If it asks if you want to exit Autoinstall: yes)

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Show version

 IOS version

 Bootstrap version

 Router model and CPU

 Amount of RAM

 Number and type of interfaces

 Amount of NVRAM

 Amount of Flash

 Configuration register

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Basic Configuration (revision)

 Name

 Passwords

 Interfaces

 Routing

 Banner (Message of the day)

 Save configuration

 Check configuration

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Global configuration

 Router>enable

 Router#configure terminal (config t)

 Router(config)#

 Start in user exec mode

 Go to privileged exec mode (no configuration so no password)

 Go to global configuration mode

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Hostname

 Give the router a name to show at its prompt

 Do this in global configuration mode

 Router(config)# hostname ITT

 ITT(config)#

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Enable secret

 Protect privileged exec mode with an encrypted password.

 ITT(config)# enable secret class

 You could set an enable password but this is not encrypted

 There is no need to set both, but if you do then the enable secret will be used

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Passwords for login

 Set login password on console port for security

 ITT(config)# line con 0

 ITT(config-line) password cisco

 ITT(config-line) login

 ITT(config-line) exit

 You can also put a password on the AUX port in a similar way

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Passwords for Telnet login

 Set login password on virtual lines to allow you to Telnet to the router

 ITT(config)# line vty 0 4

 ITT(config-line) password cisco

 ITT(config-line) login

 ITT(config-line) exit

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Interface configuration

 ITT(config)# interface serial 0

 ITT(config-if)# ip address 192.168.3.1 255.255.255.0

 ITT(config-if)# no shutdown

 ITT(config-if)# exit

 This is for a DTE serial interface

 Ethernet interfaces are configured the same way

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Interface DCE configuration

 A DCE serial interface needs an extra line:

 ITT(config)# interface serial 0

 ITT(config-if)# ip address 192.168.3.1 255.255.255.0

 ITT(config-if)# clock rate 64000

 ITT(config-if)# no shutdown

 ITT(config-if)# exit

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Interface description

 You can give an interface a description

 This does not affect the operation of the router but it is useful documentation

 Do it in interface configuration mode for the required interface

 ITT(config-if) description Serial line to Rocco 01993 876543

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Message of the day

 You can configure a message to be shown before the user logs on

 Cisco recommend that you show a warning to unauthorized users (NOT “welcome”)

 ITT(config)# banner motd # authorized users only #

 # is a delimiter. Any character can be used.

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Routing

 The router knows its directly attached networks because you have put IP addresses on its interfaces

 It can put these networks in its routing table

 It needs to find routes to networks that are not directly attached

 You can give it static routes

 You can enable a routing protocol

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Routing protocol: RIP

 You choose the routing protocol

 Then you tell the router which directly attached networks it should advertise

 ITT(config) router rip

 ITT(config-router) network 192.168.1.0

 ITT(config-router) network 192.168.3.0

 ITT(config-router) exit

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Save configuration

 Your configuration is held in RAM as the running configuration

 If you want to keep this configuration then you must save it to NVRAM into the startup configuration file

 ITT# copy running-config startup-config

ITE PC v4.0

Shortened commands

 The Cisco IOS accepts shortened forms of commands

 You need to type enough to distinguish the command from other commands

 copy run start can be used instead of copy running- config startup-config

 int s 0 can be used instead of interface serial 0

ITE PC v4.0

Show commands

 Show running-config

 Show startup-config

 Show ip route

 Show ip interfaces

 Show ip interface brief

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OSI layers 1, 2 and 3

Receive signals

from cable, convert

to binary.

Check layer 2

address,

decapsulate

Find destination network, check routing table for route,

direct packet to correct outgoing interface

Encapsulate with

frame for next link

Encode binary,

place signals on

cable

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What the router does 1

 Ethernet frame received from PC1 through port Fa0/0

 Destination MAC address is router’s address

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What the router does 2

 Strip off frame header and trailer (decapsulate)

 Read destination IP address 192.168.4.9

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What the router does 3

 Logical AND with IP address 192.168.4.9 and subnet mask 255.255.255.0 (/24) gives destination network address 192.168.4.0

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What the router does 4

 Look in routing table for network address 192.168.4.0

 Route found via 192.168.3.2 through S0/0

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What the router does 5

 S0/0 connects to a WAN link using PPP

 Encapsulate packet in PPP frame

 Send frame out through S0/0

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No route found

 If the destination network is not in the routing table:

 Use a default route if one exists

 Otherwise drop the packet and send an ICMP destination unreachable message to the source host.

ITE PC v4.0

Routing tables

 A router uses the routing table to select the best path to a network

 Directly connected networks are taken from the interface configuration

 Static routes can be added by administrator

 Routes can be learned dynamically from other routers by using a routing protocol

ITE PC v4.0

Show ip route

List of codes

List of routes

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Routing table

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0

S 192.168.3.0/24 [1/0] via 192.168.2.2

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0

Directly connected Network and mask

Exit port

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Routing table

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0

S 192.168.3.0/24 [1/0] via 192.168.2.2

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0

Static route Network and mask

Administrative

distance and metric Address of next

hop router

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Routing table

C 192.168.1.0/24 is directly connected, FastEthernet0/0

C 192.168.2.0/24 is directly connected, Serial0/0

S 192.168.3.0/24 [1/0] via 192.168.2.2

R 192.168.4.0/24 [120/1] via 192.168.2.2, 00:00:20, Serial0/0

Dynamic route, RIP Network and mask

Administrative

distance and metric Address of next

hop router Time since

last update

Exit port

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Static routes Dynamic routes

 Entered by administrator

 Time consuming, different for each router

 Must be updated if routes change

 Little processing

 No bandwidth used

 Gives nothing away

 Learned from other routers

 Start the protocol then it runs by itself

 Automatically updates when routes change

 More processing

 Uses bandwidth

 Gives away information

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Routing protocols

Interior, used within

an organization’s

networks

Exterior, used

between different

organizations’

networks

BGP OSPF

IS-IS

Distance vector Link state

RIP

(IGRP)

EIGRP

Routing Information Protocol Interior Gateway Routing Protocol Enhanced Interior Gateway RP

Opens Shortest Path First Intermediate System to…

Border Gateway Protocol

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Routing Table Principles

1. Every router makes its decision alone, based on the information it has in its own routing table.

2. The fact that one router has certain information in its routing table does not mean that other routers have the same information.

3. Routing information about a path from one network to another does not provide routing information about the reverse, or return, path.

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Metrics

 A routing protocol may learn of several possible routes to a destination.

 It uses metrics to pick the best route.

 RIP uses hop count as its only metric.

 OSPF uses “cost” based on bandwidth.

 EIGRP uses bandwidth and delay and can use load and reliability as well.

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Metrics

RIP uses hop count. It

picks this route as the

best.

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Metrics

OSPF uses cost based

on bandwidth. It picks

this route as the best.

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Administrative distance

 There may be more than one routing protocol running. There may also be static routes.

 Static routes have administrative distance 1 or 0 by default.

 RIP routes have administrative distance 120

 OSPF routes have administrative distance 110

 The route with the lowest administrative distance goes in the routing table

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