ccent notes part 3

CCENT Notes

Part 3 – IP Routing

Ref : CCENT/CCNA ICND1 Official Exam Certification Guide, Second Editionby Wendell Odom

It is highly recommended that you read at least once the above study guide to make full use of this notes, it is expected that there may be minor errors in this notes, please always refer the study guide for accurate information.

(Jojo Jacob - CCENT)

TABLE OF CONTENTS

Chapter 12 – IP Addressing and Subnetting ........................................................................ 3 Chapter 13 – Operating Cisco Routers .............................................................................. 38 Chapter 14 – Routing Protocol Concepts and Configuration ............................................ 47 Chapter 15 – Troubleshooting IP Routing ......................................................................... 58

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Chapter 12 – IP Addressing and Subnetting

List of all possible valid network numbers….reference table for the number of network, size of the network part, size of the host part, for Class A,B and C ip networks.

Class A Class B Class CFirst Octect range 1 to 126 128 - 191 192 – 223Valid Network Numbers 1.0.0.0 to

126.0.0.0128.0.0.0191.255.0.0

192.0.0.0 to223.255.255.0

Number of networks in this Class 2 ^ 7 – 2= 128

2 ^ 14 = 16,384

2 ^ 21=2,097,152

Number of hosts per network 2 ^ 24 – 2 = 16,777,214

2 ^ 16 – 2 = 65,534

2 ^ 8 – 2 = 254

Size of network part of the address (bytes)

1 2 3

Size of hosts part of the address (bytes)

3 2 1

^ raised toCLASS A (7 Network Bits)0 N N N N N N N H H H H H H H H H H H H H H H H H H H H H H H H1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 CLASS B (6 + 8 = 14 Network Bits)1 0 N N N N N N H H H H H H H H H H H H H H H H H H H H H H H H1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 CLASS C (5 + 16 = 21 Network Bits)1 1 0 N N N N N H H H H H H H H H H H H H H H H H H H H H H H H1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8 1 2 3 4 5 6 7 8

Class A , B and C network, network and host parts and default maskClass of Address

Size of network part of address in bits

Size of host part of address in bits

Default mask for each class of network

A 8 24 255.0.0.0.B 16 16 255.255.0.0C 24 8 255.255.255.0

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RFC 1918 Private IP Address SpacePrivate IP Networks Class of Networks Number of Networks10.0.0.0 through 10.0.0.0 A 1172.16.0.0 through 172.31.0.0 B 16192.168.0.0 through 192.168.255.0 C 255

IPv6 – 128 bit IP address : can provide 10 ^ 38 IP Addresses.Currently in the earth there are 10 ^ 10 people living.

IPv4 vs IPv6Feature IPv4 IPv6Size of address 32 bits , 4 octects 128 bits , 16 octectsExample address 10.1.1.1 0000:0000:0000:0000:FFFF:FFFF:0A01:0101Same address abrreviated

---- ::FFFF:FFFF:0A01:0101

Number of possible addresses, ignoring reseved addresses

2 ^ 32 Approx 4 billion

2 ^ 128 , or roughly 3.4 x 10 ^ 38

Bitwise Boolean AND exampleDecimal Binary

Address 150.150.2.1 10010110 10010110 00000010 00000001Mask 255.255.255.0 11111111 11111111 11111111 00000000Result of AND 150.150.2.0 10010110 10010110 00000010 00000000

255.255.255.0 – 11111111 11111111 11111111 00000000 Is written as a /24 subnet mask.

Binary process to convert between Dotted decimal and prefix notation

Step 1. Covert the dotted decimal mask to binaryStep 2. Count the number of binary 1s in the 32 bit binary mask, this is the value of the prefix notation mask

Eg. 255.255.240.0 converts to….11111111 11111111 11110000 00000000 The mask has 20 binary 1s, so the prefix notation of the same mask is /20.

Convert prefix notation to a dotted decimal format…

Step 1. write down x binary 1s, where x is the value listed in the prefix notation of the mask

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Step 2. Write down binary 0s after binary 1s until you have written down all the 32 bits

Step 3. Convert this binary number, 8 bit at a time to dotted decimal format.

Eg. /20 converts to 11111111 11111111 11110000 00000000 255.255.240.0

Nine possible Decimal Numbers in a subnet maskSubnet mask’s Decimal Octect

Binary Equivalent Number of Binary 1s

Number of Binary 0s

0 00000000 0 8128 10000000 1 7192 11000000 2 6224 11100000 3 5240 11110000 4 4248 11111000 5 3252 11111100 6 2254 11111110 7 1255 11111111 8 0

Convert a dotted decimal subnet mask format to a prefix format….

Step 1. Start with a prefix value of 0Step 2. For each dotted decimal octect, add the number of binary 1s, listed for that decimal value in the table aboveStep 3. Prefix length is /x , where x is the sum calculated in Step 2.

Eg. 255.255.240.0

Start from 0First octect 255 add 8Second octect 255 add 8Third octect 240 add 4Fourth octect 0 add 0

Prefix is 20

Converting a /20 subnet into dotted decimal format….

Step 1. Divide x by 8 ( x / 8) , noting the number of times 8 goes fully into x , the dividend represented as d, and the number left over, the reminder represented as r.

Step 2. write down d octets of 255,

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Step 3. for the next octet, find the decimal number that begins with r, binary 1s, Followed by all binary 0s.

Step 4. for any remaining octets write down value 0

20 / 8 = 2 and a reminder of 4

2 octects with all binary 1s , third octed with 1 birary 1 and 4 binary 0 , and last one with all binary 0s.

255.255.240.0

Appendix D. Problem Set 1.

255.240.0.0 - 8+4+0+0+ = /12255.255.192.0 – 8+8+2+0 = /18255.255.255.224 – 8+8+8+3 = /27 255.254.0.0 – 8+7+0+0 = /15255.255.248.0 - 8+8+5+0 = /21

/30 - 255.255.255.252/25 - 255.255.255.128/11 - 255.224.0.0/22 - 255.255.252.0/24 - 255.255.255.0

255.240.0.0 - 11111111 11110000 00000000 00000000 - /12255.255.192.0 – 11111111 11111111 11000000 00000000 /18255.255.255.224 – 11111111 11111111 11111111 11100000 /27 255.254.0.0 – 11111111 11111110 00000000 00000000 /15255.255.248.0 - 11111111 11111111 11111000 00000000 /21

/30 - 11111111 11111111 11111111 11111100 255.255.255.252/25 - 11111111 11111111 11111111 10000000 255.255.255.128/11 - 11111111 11100000 00000000 00000000 255.224.0.0/22 - 11111111 11111111 11111100 00000000 255.255.252.0/24 - 11111111 11111111 11111111 00000000 255.255.255.0

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ANALYSING AND CHOOSING SUBNET MASKS

SLSM – Static Length Subnet MaskVLSM – Variable Length Subnet Mask

How to find the size of the Network , Subnet and Host part of an IP Address…

• The network part of the address is always defined by the class rule• The host part of the address is defined by the subnet mask, the number of binary zeros

in the subnet mask defines the number of host bits.• Subnet part of the address is what is left over in the 32 bit address

IP Address : 8.1.4.5 Mask : 255.255.0.0Class : ANetwork Bits : 8Host bits : 16Subnet bits : 32 – 24 = 8

IP Address : 130.4.102.1Mask : 255.255.255.0Class : BNetwork Bits : 16Host bits : 8Subnet bits : 32 – 24 = 8

IP Address : 199.1.1.100Mask : 255.255.255.0Class : CNetwork Bits : 24Host bits : 8Subnet bits : 32 – 32 = 0

Facts about how the subnet mask identifies part of the structure of an IP address

• A subnet masks binary 1s define the combined network and subnet part of an IP address

• The masks binary 0s define the hosts part of the IP address• Class rules define the size of the network part

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Finding the Network, Subnet and Host part using binary…..

Step 1. Compare the first octet of the address to the table of Class A,B or C addresses, write down the number of network bits depending on the address class.

Step 2. Find the number of hosts bits by

a. Converting subnet masks to binaryb. Counting the number of binary 0s in the mask

Step 3. Calculate the number of subnet bits by subtracting the combined network and hosts bits from 32.

Decimal Process : Finding the Network, Subnet and Hosts bits in a subnet mask

Step 1. Compare the first octet of the address to the table of Class A,B or C addresses; write down the number of network bits based on the address class.

Step 2. If the mask is in dotted decimal format, convert the mask to prefix format

Step 3. To find the number of host bits, subtract the prefix length from 32

Step 4. Calculate the number of subnet bits by subtracting the combined network and hosts bits from 32

Given a class full network number and a single subnet mask is used throughout the classfull network.

Number of subnets :- 2 ^ s , where s is the number of subnet bitsNumber of hosts :- 2 ^ h , where h is the number of host bits

When to use which formula for number of subnetsUse the 2 ^ s – 2 formula, and avoid using the zero subnet and broadcast subnet if…

Use the 2 ^ s formula, and use the zero and broadcast subnet if…

Classful routing protocol Classless routing protocolRIP version 1 or IGRP as the routing protocol

RIP Version 2, EIGRP or OSPF as the routing protocol

The no ip subnet zero command is configured

The ip subnet zero command is configured or ommited (default)VLSM is usedNo other clues provided

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Analysing the subnet mask

8.1.4.5 /16

Hosts bits : 32 – 16 = 16Class A network bits : 8Subnet bits : 32 – 24 = 8Number of subnets = 2 ^ 8 = 256Number of host = 2 ^ 16 – 2 = 65,534

130.4.102.1/24

Hosts bits : 32 – 24 = 8Class B network bits : 16Subnet bits : 32 – 24 = 8Number of subnets = 2 ^ 8 = 256Number of host = 2 ^ 8 – 2 = 254

199.1.1.100/24

Hosts bits : 32 – 24 = 8Class C network bits : 24Subnet bits : 32 – 32 = 0Number of subnets = 2 ^ 0 = 1Number of host = 2 ^ 8 – 2 = 254

130.4.102.1/22

Hosts bits : 32 – 22 = 10Class B network bits : 16Subnet bits : 32 – 26 = 6Number of subnets = 2 ^ 6 = 64Number of host = 2 ^ 8 – 2 = 1022

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199.1.1.100/27

Hosts bits : 32 – 27 = 5Class C network bits : 24Subnet bits : 32 – 29 = 3Number of subnets = 2 ^ 3 = 8Number of host = 2 ^ 5 – 2 = 30

Number of bits in the host or subnet field

Maximum number of hosts (2 ^ h – 2)

Maximum number of subnets (2 ^ s )

1 0 22 2 43 6 84 14 165 30 326 62 647 126 1288 254 2569 510 51210 1022 102411 2046 204812 4094 409613 8190 819214 16,382 16,384

Finding the only possible MASK

With a Class B network 130.1.0.0 , what is the only subnet mask you can use to have upto 200 subnets and 200 hosts per subnets???

To have 200 subnets you need to have atleast 8 subnet bits ( 2 ^ 8 = 256) , (2 ^ 7 is only 128) Only possible subnet so is…(16 bits for network part as it is a Class B network).

NNNNNNNN NNNNNNNN SSSSSSSS HHHHHHHH

11111111 11111111 11111111 00000000

255.255.255.0 or a /24 mask.

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Finding the multiple possible MASKs

With a Class B network, what are the subnet masks you can use to have upto 50 subnets and 200 hosts in the largest subnet???

To have 50 subnets you need to have atleast 6 subnet bits ( 2 ^ 6 = 64) , (2 ^ 5 is only 32) To have 200 hosts you need to have atleast 8 host bits ( 2 ^ 8 - 2 = 254) , (2 ^ 7 is only 128)

Possible subnet format is …(16 bits for network part as it is a Class B network).

NNNNNNNN NNNNNNNN SSSSSSXX HHHHHHHH

XX – can be either subnet or host bits

That mean you may get 2 ^ 2 = 4 possible combination of subnet masks, however

All masks must start with one unbroken consecutive string of binary 1s, followed by one unbroken consecutive string of binary 0s.

So the possible mask are listed below but only three are valid.

11111111 11111111 11111111 00000000 (8 subnets , 8 hosts)11111111 11111111 11111110 00000000 (7 subnets, 9 hosts)11111111 11111111 11111100 00000000 (6 subnets, 10 hosts)11111111 11111111 11111101 00000000 (INVALID MASK)

255.255.255.0 prefix format /24 255.255.254.0 prefix format /23255.255.252.0 prefix format /22

Choosing the subnet that maximizes the number of subnets or hosts

The mask with the most subnet bits : the mask with the wildcard bits set to binary 1s, there by increasing the subnet bits, maximizes the number of subnets and minimizes the number of hosts per subnet.

The mask with most host bits : the mask with the wildcard bits set to binrary 0s, thereby making the host part of the subnet address larger, maximizes the number of hosts per subnet and minimizes the number of subnets.

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Choosing a subnet mask steps …..

Step 1. Find the number of network bits (N) based on the Class A,B or C rulesStep 2 : Find the number of subnet bits (S) based on the formula 2 ^ s , such that 2 ^ s > or = the number of required subnets.Step 3. Find the number of host bits (H) based on the formula 2 ^ h – 2, such that 2 ^ h – 2 is >= the number of required hosts.Step 4. Write down starting from the left N+S binary 1sStep 5. Write down starting from right H binary 0sStep 6. If the binary 1s and 0s together adds up to less than 32 a. fill in the remaining ‘wildcard’ bit positions with X between binary 1s and 0s. b. find all combination of bits for the wildcard bit positions, meeting the requirement of having one unbroken consecutive string of binary 1s on left.Step 7. convert the mask to decimal or prefix format Step 8. To find the mask that maximizes the number of subnets pick the mask with most binary 1s init, and to find the mask that maximizes the number of hosts pick the mask with most binary 0s.

Problem set 2. Analysing unsubnetted IP addresses

10.55.44.3

Class of the address : ANumber of octets in the network part : 1Number of octets in the host part : 3Network number : 10.0.0.0Network broadcast number : 10.255.255.255

128.77.6.7

Class of the address : BNumber of octets in the network part : 2Number of octets in the host part : 2Network number : 128.77.0.0Network broadcast number : 128.77.255.255

192.168.76.54

Class of the address : CNumber of octets in the network part : 3Number of octets in the host part : 1Network number : 192.168.76.0Network broadcast number : 192.168.76.255

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190.190.190.190

Class of the address : BNumber of octets in the network part : 2Number of octets in the host part : 2Network number : 190.190.0.0Network broadcast number : 190.190.255.255

9.1.1.1

Class of the address : ANumber of octets in the network part : 1Number of octets in the host part : 3Network number : 9.0.0.0Network broadcast number : 9.255.255.255

200.1.1.1

Class of the address : CNumber of octets in the network part : 3Number of octets in the host part : 1Network number : 200.1.1.0Network broadcast number : 200.1.1.255

Problem Set 3. Interpreting existing subnet masks

10.66.5.99 255.255.254.0

StepsMask in prefix format /23 Class : A

ResultsNetwork bits 8Subnet bits 32 – (8+9) = 15Host bits 9Number of subnets in the network 2 ^ 15 = 32,768Number of hosts per subnet 2 ^ 9 – 2 = 510

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172.16.203.42 255.255.252.0

StepsMask in prefix format /22Class : B

ResultsNetwork bits 16Subnet bits 32 – (16+10) = 6Host bits 10Number of subnets in the network 2 ^ 6 = 64Number of hosts per subnet 2 ^ 10 – 2 = 1022

192.168.55.55 255.255.255.224

StepsMask in prefix format /27Class : C


10.22.55.87 /30

StepsClass : A

ResultsNetwork bits 8Subnet bits 32 – (8+2) = 22Host bits 2Number of subnets in the network 2 ^ 22 = 4,194,304Number of hosts per subnet 2 ^ 2 – 2 = 2

172.30.40.166 /26

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StepsClass : B


192.168.203.18 /29

StepsClass : C


Problem set 4 : Choosing Subnet Masks

Network 10.0.0.0 needs 50 subnets and 200 hosts/subnets

Class : ANetwork bits : 8Min. Subnet bits 6Min. Host bits 8

NNNNNNNN SSSSSSXX XXXXXXXX HHHHHHHH

10.0.0.0 /14 (Maximum number of hosts)10.0.0.0 /1510.0.0.0 /1610.0.0.0 /1710.0.0.0 /18

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10.0.0.0 /1910.0.0.0 /2010.0.0.0 /2110.0.0.0 /2210.0.0.0 /2310.0.0.0 /24 (Maximum number of subnets)

Network 172.32.0.0 need 125 subnets and need 125 hosts/subnet

Class : BNetwork bits : 16Min. Subnet bits : 7Min. Host bits : 7

NNNNNNNN NNNNNNNN SSSSSSSX XHHHHHHH172.32.0.0 /23 255.255.254.0 (Maximum number of hosts)172.32.0.0 /24 255.255.255.0172.32.0.0 /25 255.255.255.128 (Maximum number of subnets)

Network 192.168.44.0 need 15 subnets and 6 host/subnet

Class : CNetwork bits 24Min. subnet bits : 4Min. host bits : 3

NNNNNNNN NNNNNNNN NNNNNNNN SSSSXHHH

192.168.44.0 /28 255.255.255.240 (Maximum number of hosts/subnet)192.168.44.0 /29 255.255.255.248 (Maximum number of subnets)

10.0.0.0 300 subnets and 500 hosts/subnet

Class : ANetwork bits : 8Min. subnet bits : 9Min. host bits : 9

NNNNNNNN SSSSSSSS SXXXXXXH HHHHHHHH

10.0.0.0 /17 255.255.128.0 Maximum hosts/subnets

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10.0.0.0 /18 255.255.192.010.0.0.0 /19 255.255.224.010.0.0.0 /20 255.255.240.010.0.0.0 /21 255.255.248.010.0.0.0 /22 255.255.252.010.0.0.0 /23 255.255.254.0 Maximum subnets

172.32.0.0 500 subnets and 15 hosts/subnet

Class : BNetwork bits : 16Min. subnets bits : 9Min. host bits : 5

NNNNNNNN NNNNNNNN SSSSSSSS SXXHHHHH172.32.0.0 / 25 255.255.255.128 Maximum hosts/subnet 172.32.0.0 / 26 255.255.255.192172.32.0.0 / 27 255.255.255.224 Maximum subnets

Network 172.16.0.0 2000 subnets 2 hosts/subnet

Class : BNetwork hosts : 16Min. subnet bits : 11Min. host bits : 2

NNNNNNNN NNNNNNNN SSSSSSSS SSSXXXHH

172.16.0.0 /27 255.255.255.224 Maximum number of hots/subnet172.16.0.0 /28 255.255.255.240172.16.0.0 /29 255.255.255.248172.16.0.0 /30 255.255.255.252 Maximum number of subnets

ANALYSING EXISTING SUBNETS : BINARY

Learning resident subnet number using Boolean AND operation….

Step 1. Convert the IP address from decimal to binaryStep 2. Convert the subnet mask to binary and write it down under the binary ip address Step 3. Perform a bit wise Boolean AND operation of the two numbers…Step 4. Convert the resulting binary number back to decimal, to get the subnet number

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Address 8.1.4.5 00001000 00000001 00000100 00000101 Mask 255.255.0.0 11111111 11111111 00000000 00000000Subnet Number 8.1.0.0 00001000 00000001 00000000 00000000

Address 130.4.102.1 10000010 00000100 01100110 00000001 Mask 255.255.255.0 11111111 11111111 11111111 00000000Subnet Number 130.4.102.0 10000010 00000100 01100110 00000000

Address 199.1.1.100 11000111 00000001 00000001 01100100Mask 255.255.255.0 11111111 11111111 11111111 00000000Sub. Number 199.1.1.0 11000111 00000001 00000001 00000000

Address 130.4.102.1 10000010 00000100 01100110 00000001Mask 255.255.252.0 11111111 11111111 11111100 00000000SubnetNumber 130.4.100.0 10000010 00000100 01100100 00000000

Address 199.1.1.100 11000111 00000001 00000001 01100100 Mask 255.255.255.224 11111111 11111111 11111111 11100000SubnetNumber 199.1.1.96 11000111 00000001 00000001 01100000

Binary Short cut….

• Record decimal mask in the first row of the table, and decimal IP address below it• For any mask octect of value 255, copy the IP address’s octet value for the same octet

of the decimal subnet number• Similarly for any mask octet value of 0, write down decimal 0 for the same octet of

the subnet number• If the subnet number has still has one remaining octet to be filled in, then

o Convert the remaining octet of the ip address to binaryo Convert the remaining octet of the mask to binaryo AND the two 8-bit numbers togethero Covert the 8 bit number to decimal, and place the value in the remaining octet

of the subnet number

Address 199.1.1.100Mask 255.255.255.0Sub. Number 199.1.1.0

Address 130.4.102.1 01100110 Mask 255.255.252.0 11111100 SubnetNumber 130.4.100.0 01100100

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Calculating the Broadcast address – Binary

Address 8.1.4.5Mask 255.255.0.0 nnnnnnnn nnnnnnnn hhhhhhhhh hhhhhhhSubnetAddress 8.1.0.0 00001000 00000001 00000000 00000000BroadcastAddress 8.1.255.255 00001000 00000001 11111111 11111111

Address 130.4.102.1Mask 255.255.255.0SubnetAddress 130.4.102.0BroadcastAddress 130.4.102.255Address 199.1.1.100Mask 255.255.255.0SubnetAddress 199.1.1.0BroadcastAddress 199.1.1.255

Address 130.4.102.1 01100110Mask 255.255.252.0 11111100SubnetAddress 130.4.100.0 01100100BroadcastAddress 130.4.103.255 01100111

Address 199.1.1.100 0110 0100Mask 255.255.255.224 1110 0000SubnetAddress 199.1.1.96 0110 0000BroadcastAddress 199.1.1.127 0111 1111

Steps to determine Subnet Broadcast Address…..

• Step 1. Write down the subnet number (IP address), and subnet mask in binary form, • Step 2. Separate the network/subnet and host bits by a vertical line• Step 3. To find the subnet broadcast address in binary

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o Copy the bits of the subnet number (IP address) that are to the left of the vertical line

o Write down binary 1s for the (HOST) bits to the right of the vertical line

• Step 4. Convert the 32 bit binary subnet broadcast address to decimal, 8 bits at a time, ignoring the vertical line

Steps to find the first and last IP address in a subnet

Step 1. To find the first IP address, Copy the subnet number but add 1 to the fourth octetStep 2. To find the last IP address, Copy the subnet broadcast address but substract1 from the fourth octet

Address 8.1.4.5Mask 255.255.0.0 nnnnnnnn nnnnnnnn hhhhhhhhh hhhhhhhSubnetAddress 8.1.0.0 00001000 00000001 00000000 00000000BroadcastAddress 8.1.255.255 00001000 00000001 11111111 11111111First Address 8.1.0.1LastAddress 8.1.255.254

Address 130.4.102.1Mask 255.255.255.0SubnetAddress 130.4.102.0BroadcastAddress 130.4.102.255First Address 130.4.102.1LastAddress 130.4.102.254

Address 199.1.1.100Mask 255.255.255.0SubnetAddress 199.1.1.0Broadcast

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Address 199.1.1.255First Address 199.1.1.1LastAddress 199.1.1.254

Address 130.4.102.1 01100110Mask 255.255.252.0 11111100SubnetAddress 130.4.100.0 01100100BroadcastAddress 130.4.103.255 01100111FirstAddress 130.4.100.1Last Address 130.4.103.254

Address 199.1.1.100 0110 0100Mask 255.255.255.224 1110 0000SubnetAddress 199.1.1.96 0110 0000BroadcastAddress 199.1.1.127 0111 1111First Address 199.1.1.97Last Address 199.1.1.126

Finding subnet address, broadcast address and range of address using Decimal – Difficult MaskSubnet Chart 130.4.102.1 / 255.255.252.0Octet 1 2 3 4 CommentsMask 255 255 252 0Address 130. 4 102 1Subnet Address 130 4 100 0 Magic number = 256 – 252 = 4

100 is the multiple of 4 closes to but not higher than 102

First Address 130 4 100 1 Add 1 to the subnets last octetLast Address 130 4 103 25

4Subtract 1 from broadcast address’s fourth octet

Broadcast Address 130 4 103 255

Subnet’s interesting octet + magic number – 1 (100+4 – 1)

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Summary of decimal process to find the subnet, broadcast and range

Step 1. Write down the subnet mask in the first empty row of the subnet chart, and the IP address in the second empty row.

Step 2. Find the octet for which subnet mask’s value is not 255 or 0. This octet is called the interesting octet. Draw a dark rectangle around the interesting octet’s column of the table, top to bottom.

Step 3. Record the subnet numbers value for the uninteresting octets as follows

a. for each octet to the left of the rectangle, drawn in step 2, copy the IP address value in the same octet.

b. for each octet to the right of the rectangle: write down decimal 0

Step 4. To find the subnet numbers value for this interesting octet

a. calculate the magic number by subtracting the subnet mask’s interesting octet value from 256

b. calculate the multiple of magic number starting from 0 through to 256

c. write down the interesting octet value, calculated as follows, Find the multiple of magic number that is closest to, but not higher than the IP Address’s interesting octet value.

Step 5 : Find the subnet broadcast address as follows

a. for each subnet mask octet to the left of the rectangle, copy the IP address octet value

b. for each subnet mask octet to the right of the rectangle, write down 255

c. find the value for the interesting octet by adding the subnet number’s value in the interesting octet to the magic number and subtract 1.

Step 6. To find the first IP address, copy the decimal subnet number, but add 1 to the fourth octet.

Step 7. To find the last IP address, copy the decimal subnet broadcast address, but subtract 1 from the fourth octet.

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Problem 110.180.10.18 255.192.0.0

Size of Network Part 8Size of Subnet Part 2Size of Host part 22Number of hosts per subnet 2 ^ 22 – 2 = 4,194,302Number of subnets 2 ^ 2 = 4

255.192.0.010.180.10.18(256 – 192 ) = 6464 x 3 = 19264 x 2 = 128

Subnet Number 10.128.0.0Broadcast Address 10.191.255.255

Range of valid IP addressesFirst Address 10.128.0.1Last Address 10.191.255.254

Problem 210.200.10.18 255.224.0.0


255.224.0.010.200.10.18(256 – 224 ) = 3232 x 6 = 192



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Problem 310.100.18.18 255.240.0.0


255.240.0.010.100.18.18(256 – 240 ) = 1616 x 6 = 96



Problem 410.100.18.18 255.248.0.0

Size of Network Part 8Size of Subnet Part 5Size of Host part 19Number of hosts per subnet 2 ^ 19 – 2 = 524,286Number of subnets 2 ^ 5 = 32

255.248.0.010.100.18.18(256 – 248 ) = 88 * 12 = 96



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Problem 510.150.200.200 255.252.0.0


255.252.0.010.150.200.200(256 – 252 ) = 437 * 4 = 148



Problem 610.150.200.200 255.254.0.0


255.254.0.010.150.200.200(256 – 254 ) = 275 * 2 = 150



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Problem 710.220.100.18 255.255.0.0


255.255.0.010.220.100.18(256 – 255 ) = 1220 * 1 = 220



Problem 810.220.100.18 255.255.128.0


255.255.128.010.220.100.18(256 – 128 ) = 1280 * 128 = 0



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Problem 9172.31.100.100 255.255.192.0


255.255.192.0172.31.100.100(256 – 192 ) = 641 * 64 = 64



Problem 10172.31.100.100 255.255.224.0


255.255.224.0172.31.100.100(256 – 224 ) = 323 * 32 = 96



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Problem 11172.31.200.10 255.255.240.0


255.255.240.0172.31.200.10(256 – 240 ) = 1612 * 16 = 192



Problem 12172.31.200.10 255.255.248.0


255.255.248.0172.31.200.10(256 – 248 ) = 825 * 8 = 200



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Problem 13172.31.50.50 255.255.252.0

Size of Network Part 16Size of Subnet Part 6Size of Host part 10Number of hosts per subnet 2 ^ 10 – 2 = 1022Number of subnets 2 ^ 6 = 64

255.255.252.0172.31.50.50(256 – 252 ) = 412 * 4 = 48



Problem 14172.31.50.50 255.255.254.0


255.255.254.0172.31.50.50(256 – 254 ) = 225 * 2 = 50



29

Problem 15172.31.140.14 255.255.255.0


255.255.255.0172.31.140.14



Problem 16172.31.140.14 255.255.255.128


255.255.255.128172.31.140.14256 – 128 = 1280 * 128 = 0



30

Problem 17192.168.15.150 255.255.255.192


255.255.255.192192.168.15.150256 – 192 = 642 * 64 = 128



Problem 18192.168.15.150 255.255.255.224


255.255.255.224192.168.15.150256 – 224 = 324 * 32 = 128



31

Problem 19192.168.100.100 255.255.255.240


255.255.255.240192.168.100.100256 – 240 = 166 * 16 = 96



Problem 20192.168.100.100 255.255.255.248


255.255.255.248192.168.100.100256 – 248 = 812 * 8 = 96



A NOTE : In the above examples, Number of subnets means, maximum number of subnets possible for the subnets mask (eg. 255.255.255.248),

32

Subnet address and Broadcast address are the subnet and broadcast address for the subnet the given IP addresss 192.168.100.100 belongs to. (see the binary version in appendix d for more details)

Problem 21192.168.15.230 255.255.255.252


255.255.255.248192.168.100.100256 – 248 = 812 * 8 = 96



Problem 2210.1.1.1 255.248.0.0


255.248.0.010.1.1.1256 – 248 = 80 * 8 = 0



33

Problem 23172.16.1.200 255.255.240.0


255.255.240.0172.16.1.200256 – 240 = 160 * 16 = 0



Problem 24172.16.0.200 255.255.255.192


255.255.255.192172.16.0.200256 – 192 = 643 * 64 = 192



34

Problem 2510.1.1.1 255.0.0.0




Finding all subnets with fewer than 8 subnet bits

Generic list All Subnet ChartOctect 1 2 3 4Mask 255 255 252 0Magic Number 4Network number/Zero subnet 130 4 0 0Next subnet 130 4 4 0Next subnet 130 4 8 0Last subnet 130 4 248 0Broadcast subnet 130 4 252 0Out of range (used by process) 130 4 256 0

Step 1. Write down the subnet mask in decimal, in the first empty row of the tableStep 2. Identify the interesting octet, which is the octet with value other than 255 or 0, and draw a rectangle around the column of the interesting octet.Step 3. Calculate the magic number by subtracting the mask’s interesting octet from 256Step 4. Write down the classful network number (zero subnet number)Step 5. To find each successive subnet number

a. for the three un-interesting octets copy the previous subnet numbers valueb. for the interesting octet add the magic number to the previous subnet numbers interesting octet value.

Step 6. Once the sum calculated in stp 5.b becomes 256, stop the process, the number with 256 in it is out of range, and previous subnet number is the broadcast subnet number.

35

Finding all subnets with exact 8 subnet bits

The subnet octet is the interesting octet, to find all the subnets add 1 to the interesting octet till it reaches 256.

Finding all subnets with more than 8 subnet bits

The process follows the same five steps as with fewer than 8 subnet bits.

Step 6. When any steps addition results is in sum of 256 a. for the octet whose sum would have been 256 write down 0b. for the octet to the left add 1 to the previous subnet’s value in that octetc. for any other octet copy the value of the same octet in the previous subnet numberd. start again with step 5

Step 7. each time the process results in a sum of 256, repeat step 6 of this process

Step 8. Repeat the steps until the addition in step 6b, would actually change the value of the network portion of the subnet number

Octect 1 2 3 4Mask 255 255 255 192Magic Number 64Network number/Zero subnet 130 4 0 0First non-zero subnet number 130 4 0 64Next subnet 130 4 0 128Next subnet 130 4 0 192Next subnet (add 1 to the third octet, and write 0 in the fourth octet)

130 4 1 0

Next subnet 130 4 1 64Next subnet 130 4 1 128Next subnet 130 4 1 192

Broadcast subnet 130 4 255 192

Definitions….

Bitwise Boolean AND : A Boolean AND between two numbers of the same length where the first bit in each number is ANDed and the second bit and so on

Boolean AND : A math operation performed on a pair of one digit binary numbers, the result is another one digit binary number, binary 1 and 1 yields a result of binary 1, all other combinations yielding binary 0.

36

Broadcast subnet : When subnetting a Class A, B or C network, the one network in each classful network, for which all subnet bits have a value of binary 1s. The subnet broadcast address in this subnet has the same numeric value as the classful network’s network wide broadcast address.

Classful network : An IPv4 Class A,B or C network, called classful network, because these networks are defined by the class rules for IPv4 addressing.

Default Mask : The mask used in Class A,B or C network, that does not create any subnets, specifically mask 255.0.0.0 for Class A, 255.255.0.0 for Class B, and 255.255.255.0 for a Class C network.

Prefix notation , CIDR notation : A shorter way to write subnet mask, in which number of binary 1s in the mask is simply written in decimal. For instance /24 denotes the subnet mask with 24 binary 1 bits in the subnet mask.

Private IP address : IP addresses within Class A,B and C, networks that are set aside for use within a private organization. These addresses are defined by RFC 1918, and are not routable through internet.

Public IP Address : An IP address that is part of a registered network number, as assigned by an Internet Assigned Numbers Authority (IANA) member agency. Routers in the internet forward (route) publicly assigned network numbers.

Subnet : Sub division of Class A, B or C network as configured by the network administrator. Subnets allow single Class A,B or C network to be used, instead of multiple networks but still allow multiple groups of ip addresses. Subnet Mask : A 32 bit number that numerically represents the format of an IP address, by representing the network and subnet part with a mask bit value of 1, and host part with a mask bit value of binary 0s.

Subnet number/ Subnet address : In IPv4 a dotted decimal number that represents all addresses in a single subnet. Numerically smallest value in the range of numbers in a subnet, reserved so that it cannot be used as a uni cast IP address by a host.

Zero Subnet : For every class ful IPv4 network that is subnetted, the one subnet whose subnet number has all binary 0s in the subnet part. In decimal zero subnet can be easily identified, because it is the same number as the classful network number.

37

Chapter 13 – Operating Cisco Routers

Physical installationSteps required to install a router…..Step 1. Connect any LAN cables to LAN portsStep 2. If using an external CSU/DSU connect the router’s serial interface to CSU/DSU and the CSU/DSU to the line from telco.Step 3. If using internal CSU/DSU connect the routers’ serial interface to the line from telcoStep 4. connect the router’s console port to a pc using a rollover cable as needed to configure the routerStep 5. Connect the power cable to power port on the routerStep 6. Turn on the router

Comparison between Switch CLI and Router CLI

The configuration commands used for the following features are the same on both routers and switches…

Similarities between switch CLI and router CLI…

• Use and Enable (privileged) mode• Entering and exiting configuration mode, using configure terminal, end and exit

commands, and ctrl z key sequence• Configuration of console, telnet and enable secret password• Configuration of SSH encryption keys, and username/password login credentials.• Configuration of host names and interface descriptions• Speed and Duplex commands• Shutdown and no shutdown commands to administratively disable and enable an

interface respectively• Navigation through different configuration context modes, using commands like line

console 0 , and interface • CLI help, command editing and command recall• The meaning and use of start-up config in NVRAM, running config in RAM, external

servers like TFTP, along with copy commands.• The process of reaching a setup mode either by re-loading the router with an empty

start-up config, or by using setup command

Differences….

• The configuration of IP address differ in someway between switches and routers• Questions asked in setup mode differ

38

• Routers have an auxiliary port, intended to be connected to an external modem and phone line, to allow remote users to dial into the router and access the CLI, by making a phone call.

There is no show mac address-table dynamic command in a router, but a router has show ip route command.

Router Interfaces

Routers generally have two types of physical interfaces : Ethernet Interfaces and Serial Interfaces.

Ethernet Interfaces…Interface Ethernet numberInterface fastethernet numberInterface gibabitethernet number

Serial Interfaces…For PPP links and Frame relay links and uses HDLC or PPP protocols HDLC being the default.

Valid configuration command formats….

Interface Ethernet 0Interface fastethernet 0/1Interface serial 1/0/1

Commands to view information about interfaces….

Show ip interface briefShow protocols fa0/0Show interfaces s1/0/1

Please refer to page 413 for a detailed listing example

39

Router Interface status codes and their meaningName Location General MeaningLine Status First Status

codeRefer to the layer – 1 status, eg. If the cable is installed, is it the right/wrong cable, is the device on the other end powered on.

Protocol Status Second Status code

Refer generally to the layer – 2 status, It is always down if the line status is down. If the line status is up and protocol status is down, usually is caused by mismatch in the data link layer protocol configurations.

Typical combinations of two interface status codes and likely reasons….Line and protocol status Typical ReasonsAdministratively down,down

The interface has shutdown command configured on it

Down, down The interface has no shutdown command configured but the physical layer has a problem. Eg. No cable has been connected to the interface, or with Ethernet, the switch interface on the other end of the cable is shutdown, or the switch is powered off.

Up, down Almost always refer to data link layer problems, most often configuration problems. Eg. Serial link have this combination when one router was configured to use PPP and the other default to use HDLC.

Up, Up All is well, interface is functioning

Router Interface IP Address

Routers need an IP address on each interface.

Router1# configure terminalRouter1 <config> # interface fa0/1Router1 <config-if> # ip address 10.1.1.1 255.255.255.0Router1 <config-if> # ^zRouter1# show ip interface brief

Please refer to page 415 for the command output listing

Bandwidth and clock rate on serial interfaces

40

The clock rate speed sub command sets the rate in bits per second on the router that has the DCE cable plugged into it. Show controllers serial 0/1/0 command will show if a DCE/DTE cable is connected to a router interface. IOS accepts the clock rate command on an interface only if it has a DCE cable attached to it, or if no cable is installed. If a DTE cable is installed IOS silently rejects the clock rate command.

Bandwidth speed command tells the IOS the speed of the link, in kilobits, which are mainly used by routing protocols (EIGRP, OSPF) etc in their default routing matrices.

For serial link the default bandwidth is 1544 , 1544 kbps, 1.544 Mbps, (a T1 line).Router ethernett interface default to a bandwidth setting that reflects the current speed of the interface. If router’s fasterethernet interface is running at 100 Mbps then the bandwidth is 100,000 Kbps.

Clock rate uses a unit of kbps, whereas bandwidth command uses a unit of Kbps

Auxiliary Port can be configured using the Line aux 0 command to reach the aux line configuration mode.

Summary of facts about Initial configuration Setup Mode dialogue.

• Setup mode is intended to allow basic configurations by prompting the CLI user via a series of questions.

• You can reach the setup mode, either by booting the router after erasing the startup-config file, or by using the setup enable mode EXEC command.

• At the end of the process you get three options (0,1,2), to either ignore the answers and go back to the CLI (0), ignore the answers but begin again in setup mode (1), or to use the resulting config (2).

• Ctrl C key combination to eject the user out of the setup mode• If the user selects to use the resulting config, the router writes the configuration file to

the startup-config file as well as the running-config file.

The questions asked differ between switches and routers especially on IP configurations.

Cisco IOS Software boot sequence…When a router first powers on, it follows these four steps…

1. The router performs Power On Self Test (POST) , to discover hardware components, and to verify that all components work properly.

41

2. The router copies the bootstrap program from ROM into RAM, and runs the bootstrap program

3. Bootstrap program decides which IOS (or other OS) to load into the RAM, and loads the OS. After loading the IOS the bootstrap program hand over the control of the router hardware to the newly loaded OS.

4. If the bootstrap program loads IOS, IOS find the configuration file typically the startup-config from the NVRAM and loads it into the RAM as running-config.

Loading the Cisco IOS

ROM

RAM

Bootstrap

Cisco IOS

Running Config File

Flash

TFTP

ROM

Step 2

Step 3

NVRAM

TFTP

Console

Step 4

Router can get the Cisco IOS image from three different locations, and running config file from other three different locations.

42

The Three Router Operating SystemsComparison of ROMMON and RxBoot Operating Systems…Operating Environment

Common Name Stored in Used in

ROM Monitor ROMMON ROM Older and new routersBoot ROM Rx Boot, Boot helper ROM Only in older routers

Cisco routers use different OSs to perform some troubleshooting, to recover router passwords, and to copy new IOS into the flash when the flash has been erased or corrupted.

The configuration register The configuration register is a special 16 bit number, that can be set on any cisco router. Eg. The console speed, what IOS iamge to load etc are set using the configuration register bits.

Config-register global configuration command sets the configuration register values.Eg. Config-register 0x2100 sets the value to hex 2100, which causes the router to load ROMMON OS, instead of IOS - a common practice when recovering lost passwords.

Config-register values are automatically saved, to both running config and startup-config, but these new values will not be used until the router is re-loaded.

The show version command lists the configuration register’s current value, if different the value that will be used once the router is re-loaded.

In most cisco routers the default configuration register setting is Hex 2102.

How routers choose which OS to load

A router chooses the OS to load based on the low order 4 bits in the configuration register, and the details configured in any boot system global configuration command found in start-up configuration command. The low order 4 bits, (the 4th hex digit) is the configuration register are called boot-field.

Steps a routers uses to choose which IOS to load…

Step 1. If boot field = 0 , use the ROMMON OSStep 2. If boot field = 1, load the first IOS file found in Flash memoryStep 3. If boot field = 2-Fa. try each boot system command in the startup-config file, in order until one worksb. if none of the boot system commands works, load the first IOS file found in the flash memory

43

Bootstrap and ROMMON

1st IOS File2ND IOS File3rd IOS File

…….

Last IOS File

RAMBOOT = 0

ROM

Boot System command 1Boot System command 2

….Boot system command last

FLASH

BOOT = 1

BOOT = 2...FNVRAM (Startup Config)

TFTP

IP Network

If all three steps fails, as it may be possible that flash memory is erased, the router send broadcasts looking for tftp server and guessing IOS file name to load, if that fails, router loads the ROMMON which provides the tools to recover.

From the factory Cisco routers have not boot system command configured, and comes with a configuration register value of 0x2102, meaning boot field hex 2, the process tires step 3, finds no boot system commands and loads the first IOS image from the flash memory.Boot System commandsBoot system command ResultBoot system flash The first file from flash memory is loadedBoot system flash filename IOS with name filename is loaded from flash memoryBoot system tftp filename 10.1.1.1

IOS with name filename loaded from TFTP server

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The SHOW VERSION command

Show version command supplies a wide variety of information about the router, including the current and future configuration register.

1. IOS Version2 The uptime – the length of time passed since the last reload3. The reason for the last reload of the IOS (reload command, power off/on, software failure)4. The time of the last loading of IOS (if the router’s clock has been set)5. The source from which the router has loaded the current IOS6. The amount of RAM memory7. The number and types of interfaces8. The amount of NVRAM memory9. The amount Flash memory10. The configuration register’s current and future settings (if different)

Please refer to page 430 for a detailed listing of the show version command

Bandwidth : A reference to the speed of a network link. Its origins come from earlier communication technologies where the range or width of frequency bands dictated how fast communication could occur.

Boot field : Low order 4 bits of the configuration register in a cisco router. The value in the boot field in part tells the router where to look for the cisco IOS image to load.

Clock Rate : The speed at which a serial link encodes bits on the transmission medium

Configuration register : In cisco route a 16 bit use configurable value that determines how router the router functions during initialization. In software, the bit position is set by specifying a hex value using configuration commands.

IOS Image : A file that contains IOS – Cisco operating system that provides majority of the router’s or switch’s features with the hardware providing the remaining features.

Power On Self Test (POST) : The process on any computer, including routers and switches , in which computer hardware first runs hardware diagnostics on required hardware before even trying to load a bootstrap program.

ROMMON : A shorter name of ROM Monitor , which is a low level operating system, that can be into cisco routers for several seldom needed maintenance tasks, including password recovery, and loading new IOS when flash memory has been corrupted.

45

RxBoot : A limited function version of IOS stored in the ROM, in some older models of Cisco routers, for the purpose of performing seldom needed low level functions including loading new IOS into flash memory, when flash memory has been corrupted or erased.

Configuration Command refernce…Bandwidth kbps Interface command that sets the router’s perception

of bandwidth in kpbsClock rate rate Interface command that sets the speed at which the

router supplies a clocking signal, applicable when the router has a DCE cable installed. The unit is bit/second

Config-register value Global command that sets the hexadecimal value of the configuration register

Boot system {file-url | filename} Global command that identifies an externally located IOS image using a URL.

Boot system flash [flash:fs:] [filename]

Global command that identifies the location of an IOS image in flash memory

Boot system rom Global command that tells the router to load RxBoot OS found in ROM, if one exitsts.

Boot system {rcp|tftp|ftp} filename [ip address]

Global command that identifies an external server, protocol and file name to use to load IOS from an external server

EXEC Command reference….Command PurposeShow interfaces type number Lists a large set of information about each interface, or

about the one, if one is specifiedShow ip interface brief List a single line of information about each interface,

including the IP address, line and protocol status, and the method with which address was configured. (manual or DHCP).

Show protocols type numebr Lists a single line of information about the specified interface including ip address, line,protocol status.

Show controllers type number List many line of information per interface, or for the specified interface, for the hardware contoller of the interface. On serial interfaces, this command identifies the cable as either a DCE or DTE cable.

Show version Lists IOS version as well as a lots of other usefil information

setup Starts the setup dialogue Copy url-from url-to Copies the file from source url to destination urlShow flash List the contents of the flash memory, including used

and available memoryreload Enable mode command to reboot the router

46

Chapter 14 – Routing Protocol Concepts and Configuration

Albequerque

`

`

Fa0/0

S0/1/0S0/0/010.1.128.251 10.1.130.251

10.1.1.251

10.1.128.0/24 10.1.130.0/24

10.1.1.0/24

10.1.128.252S0/0/1

10.1.130.253S0/0/1

10.1.2.0/24

`` `

10.1.3.0/24

10.1.2.252 10.1.3.253

A router adds routes to its routing table for the subnets connected to each of the router’s interfaces. For this to occur the router must have an ip address and mask configured on the interface, and the interface must be in an up/up status.

Show running config – will show the ip address and mask on each interfaceShow ip interface brief – will show the interface status (line/protocol)

Show I p route – will show the routing table entries

10.0.0.0/24 is subnetted , 3 subnets

c 10.1.1.0 directly connected, Fastethernet 0/0c 10.1.128.0 directly connected, serial 0/0/0c 10.1.130.0 directly connected, serial0/1/0

terminal ip netmask-format decimal – will change the mask format to decimal

47

Static Route

Adding a static route to router alberquerque for the subnet 10.1.2.0/24 and 10.1.3.0/24

Albuquerque# configure terminalAlbuquerque(config)# ip route 10.1.2.0 255.255.255.0 10.1.128.252Albuquerque(config)# ip route 10.1.3.0 255.255.255.0 10.1.130.253

Albuquerque# show ip route static

10.0.0.0 /24 is subnetted , 5 subnets

S 10.1.3.0 [1/0] via 10.1.130.253S 10.1.2.0 [1/0] via 10.1.128.252

Ip route global configuration command supplies the subnet number, mask and the next hop id address.

If the outgoing interface of a router (Albuquerque’s s0/0/0 or s0/1/0) interface is not in up/up state the static route will not be listed in the routing table.

Extended PING - a router enable mode command allows the CLI user to change many option on Ping command, including the source ip address, which enables to issue a ping command from a router which resembles more closely to a ping command issues by an end user.

Cisco ping command by default uses the output interface’s ip address as the packet’s source address, unless otherwise specified in an extended ping.

Default Routes

R1 R2172.16.3.0

Fa0/0

S0/1

172.16.3.2

Rest of the enterprises network

Subnet 1

Subnet 2

Subnet 3

As part of the routing process a router compares a packet’s destination ip address to its routing table, if a no matching route is found the router discards the packet. A default

48

route is a route that matches all destination ip addresses, and the router forwards using the default route, when no other route is matched with packets destination ip address.

In the above example the router R1 can be configured in three ways…..

• Configure hundreds of static routes on R1, but all with an outgoing interface of s0/1 and next hop id address 172.16.3.2 (R2).

• Enable routing protocols on routers to learn routes• Add a default route on R1 with outgoing interface as S0/1

R1(config)# ip route 0.0.0.0 0.0.0.0 172.16.3.2

R1# show ip route

172.16.0.0/24 is subnetted, three subnets

C 172.16.3.1 directly connected fa0/0C 172.16.3.2 directly connected s0/1

S* 0.0.0.0/0 [1/0] via 172.16.3.2 S* - statically configured default route

49

Routing Protocol Overview

RIP2 – Basic Concepts.

Example of how RIP-2 advertises routes

R1

R3

R2

`

`

Fa0/0

S0/1S0/0

172.16.5.253

172.16.1.251Fa0/0

S0/1

`` `

172.16.3.252

S0/0

Fa0/1

172.16.2.252

172.16.6.252

S0/0

S0/1

12

I have a route to172 .16.3.0/24 , Metric 1

2I have a route to

172.16.3.0/24 , Metric 1

3

R3 IP Routing TableSubnet Out.Int Next hop Metric

------------------------------------------------------------ 172.16.3.0 s0/1 172.16.6.252 1

4

R1 IP Routing TableSubnet Out.Int Next hop Metric

------------------------------------------------------------ 172.16.3.0 s0/0 172.16.2.252 1

5I have a route to

172.16.3.0/24 , Metric 2

5I have a route to

172.16.3.0/24 , Metric 2

1. Router r2 learns a connected route for subnet 172.6.3.02. R2 sends routing updates to its neighbors’ listing subnet, mask and a distance , metric 3. R3 hears the routing updates and add a route to its routing table for the subnet 172.16.3.0/24 with R2 as the next hop router.4. At the same time R1 also hears the routing update sent directly from R2 to R1 and adds the route to its routing table 5. R1 and R3 then send routing updates to each other for the subnet 172.16.3.0/24 with metric value 2.

RIP routers send periodic routing message about every 30 seconds.

50

Interior and Exterior Routing Protocols

Interior Gateway Protocol (IGP) : A routing protocol that was designed and intended for use inside a single autonomous system

Exterior Gateway Protocol (EGP) : A routing protocol that was designed and intended for use between different autonomous systems.

BGP – Border Gateway Protocol is the used to exchange routes between routers in different autonomous systems and is an EGP.

ICANN – assigns an ASN – Autonomous System Number

Routing protocols classes/algorithms and Protocols that use themClass/Algorithm IGPsDistance vector RIP-1, RIP-2, IGRPLink-state OSPF , Integrated IS-IS Balanced Hybrid (also called advanced distance verctor)

EIGRP

Metrics

Each routing protocol defines a metric that gives an objective numeric value to the goodness of each route. The lower the metric the better the route.

RIP uses a metric called hop count, which counts the number of routers (hops) between a router and a subnet.

EIGRP uses a metric which by default considers both the interface bandwidth and interface delay settings as input into the mathematical formula to calculate the metric.

ROUTERS generally perform routing more quickly with smaller routing table, Route Summarization (Auto summarization and Manual Summarization) helps shorten the routing table while retaining all the needed routes in the network.

51

Routing protocol that must consider Class (A,B,C) rules are called Classful Routing Protocols, and that do not need to consider Class rules are called Classless Routing Protocols.

Comparing Classful and Classless Routing Protocols

Feature Classless ClassfulSupport VLSM Yes NoSend subnet masks in routing updates Yes NoSupport manual route summarization Yes No

The process used by routing protocols to recognize changes in a network (a link comes up or fail, a router is added or removed), to figure out now-best route to each subnet, and to change each routers routing table is called Convergence. Summary of Interior Gateway (routing) ProtocolsFeature RIP-1 RIP-2 EIGRP OSPF IS-ISClassless No Yes Yes Yes YesSupports VLSM No Yes Yes Yes YesSends mask in updates No Yes Yes Yes YesDistance Vector Yes Yes No No NoLink-state No No No Yes YesSupport autosummarization

No Yes Yes No No

Support Manual Summarization

No Yes Yes Yes Yes

Proprietary No No Yes No NoRouting updates send to a multi cast ip address

No Yes Yes Yes n/a

Support Authentication No Yes Yes Yes YesConvergence Slow Slow V.Fast Fast Fast

Configuring and Verifying RIP-2

RIP-2 Configuration

Step 1. Router RIP configuration command to move into the RIP configuration modeStep 2. Version 2 RIP subcommand to tell the router to use RIP Version 2Step 3. Use one or more network net-number to enable RIP on the correct interfaceStep 4. passive-interface type number to disable RIP on an interface

The RIP network command uses a classful network number as its net-number parameters.

52

For any of the router’s interface ip address within that clasful network, the router does the following three things…

• The router multicast routing updates, to a reserved IP multicast address 224.0.0.9• The router listens for the incoming routing updates on the same interface• The router advertises about the subnet connected to the interface

Sample RIP configuration…..

R1199.1.2.1

Fa0/0

S0/1Fa0/1

S0/0199.1.1.1

10.1.4.2.

10.1.1.2.

R1# configure terminalR1(config)# router ripR1 (config-router)# version 2R1(config-router)# network 199.1.1.0 R1(config-router)# network 199.1.2.0R1(config-router)# network 10.0.0.0

S0/0 – 199.1.1.1. is in a class C network 199.1.1.0S0/1 – 199.1.2.1. is in a class C network 199.1.2.0Fa0/1 and fa0/0 10.1.4.2. and 10.1.1.2 are in class A network 10.0.0.0

To disable RIP on fa0/1 interface….

R1(config-router)# passive-interface fa0/1

53

RIP-2 Verification

RIP Operational commands…Command PurposeShow ip interface brief List one line per interface, including ip address and interface

status; an interface must have an ip address and up/up status for the RIP protocol to work

Show ip route [rip] List the routing table, including RIP learned routes, and optionally just RIP learned routes

Show ip protocols Lists information about the RIP configuration, plus IP addresses of neighbouring RIP routers, from which local router has learned the routes.

Show ip routeShow ip route rip Show ip route 10.1.2.1

Please refer to page 460 for a detailed listing of show ip route command.

Show ip route … listing contains…..

• Subnet number with the mask at the heading line• Next hop router’s ip address• Local router’s outgoing interface • Length of time since the router heard a routing update about this route• The RIP metric for this route, second number in the square brackets [120/1]• Administrative Distance of the route first number in the bracket [120/1]

Administrative Distance : is numeric number representing the routing protocol, which is used to determine the lower number (best route) in a network that uses multiple routing protocols.

IOS defaults for administrative distanceRoute Source Administrative DistanceConnected Route 0Static routes 1EIGRP 90IGRP 100OSPF 110IS-IS 115RIP (V1 and V2) 120

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Unknown or unbelievable 255

Normally a static route has lower administrative distance than a RIP learned route, however a backup static route can be configured with an administrative status higher than any default administrative distances say 150, so that this static route will be added to the routing table only if no route is learned by any routing protocols.

Show ip protocolPlease refer to page 464 for a detailed listing…..

Q. How it can be used to trouble shoot RIP problems???Ans. By checking the Version Information and Routing Information Sources of the show ip protocol command output. It is possible that one Router is configured as RIP Version 2 and other one default Version 1. Also you could verify the router is getting RIP messages from all the expected sources in the network.

Examining RIP messages using debug

Debug ip ripUndebug allShow processService timestamps

Please refer to page 465 for a detailed listing

RIP2 routing messages are sent to multicast IP address : 224.0.0.9RIP1 routing messages are sent to broadcast IP address : 255.255.255.255

Definitions…

Administrative Distance : In cisco routers a mean for a router to choose between multiple routes to reach the same subnet, when those routes learned by different routing protocols. The lower the administrative distance, the better the source of the routing information.

Autonomous Systems : An internetwork in the administrative control of a single organization., in side which that organization typically runs a Interior Gateway Protocol (IGP).

Backup Static Route : A static route configured with an administrative status higher than any of the default routing protocol administrative statuses.

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Balanced Hybrid : A general type of routing algorithm, other than distance vector and link state, EIGRP is the only routing protocol that uses Balance Hybird algorithm.

Classful Routing Protocol : Does not transmit mask information along with the subnet number, and therefore must consider Class A, B or C network boundaries, and perform autosummarization at those boundaries. Does not support VLSM.

Classless Routing Protocol : An inherent characteristic of a routing protocol, specifically that routing protocols send subnets masks in it routing updates, thereby removing any need to make assumptions about the addresses in a particular network or subnet. Making it able to support VLSM and manual route summarization.

Covergence : Time required for a routing protocol to react to a change in the network, removing bad routes, and adding new, better routes so that the current best routes are in all routers’ routing tables.

Default Route : On a router, the route that is considered to match all packets that are not otherwise matched some more specific routes.

Distance Vector : Logic behind some of the Interior Routing Protocols such as RIP. Distance Vector algorithm calls for each router to send its entire routing table in each update, but only to its neighbours. Distance vector algorithm can be prone to routing loops but are computationally simpler than the link state algoritm.

Exterior Gateway Protocol (EGP) :A routing protocol that was designed to exchange routing information between two different autonomous systems.

Interior Gateway Protocol (IGP) : A routing protocol designed to use within an organization (autonomous system).

Link State : A classification of underlying algorithm used in some routing protocols. Link state protocols build a detailed database that lists links (subnets) and their states (up/down) from which the best route is calculated.

Metric : A unit of measure used by routing protocol algorithms, to determine the best route, for the traffic to use to reach a particular destination, in RIP-2 it is the hop-counts.

Routing Updates : A generic reference to any routing protocol message, in which it sends the routing information to its neighbours.

Variable Length Subnet Masking (VLSM) : The capacity to specify different subnet masks for the same Class A,B, or C network number on different subnets. VLSM can help optimize available address space.

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Configuration command reference…Command DescriptionRouter rip Global configuration command that moves the user

to the RIP configuration mode.Network network-number RIP subcommand that lists a clasful network

number, enabling RIP on all of that router’s interfaces in that classful network.

Version {1|2} RIP subcommand that sets the RIP versionPassive-interface [default] interface type, number

RIP subcommand that tells the RIP to no longer advertise RIP updates on the listed interface

Ip address ip-address mask Interface subcommand that sets the routers inteface’s ip address and mask

Ip route prefix mask {ip-address | interface type, interface number}(eg. Ip route 10.102.0 255.255.255.0 10.1.128.252)

Global command that defines a static route

Service timestamp Global command that tells the router to put a timestamp on log messages, including the debug messages

EXEC command reference…Command DescriptionShow ip interface brief Lists one line per router interface, including ip

address and interface statusShow ip route [rip|static|connected]

Lists the routing table

Show ip route ip-address List the details of the routes the router would match for a packet sent to the listed ip-address

Show ip protocols Lists information about RIP configurations, plus the IP address of the neighbouring routers from which the local router has learned routes.

Show process Lists information on processes running in IOS, and also overall CPU utilization status

Terminal ip netmask-format decimal

For the length of the user session, causes the router to display mask information in dotted decimal instead of prefix format

Debug ip rip Tells the router to generate detailed message logs for each send and received RIP updates.

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Chapter 15 – Troubleshooting IP Routing

IP Troubleshooting Tips and Tools

Avoiding reserved IP addresses

• Addresses that are always reserved• Two addresses that are reserved in each subnet (subnet number and subnet broadcast

address)• Addresses in two special subnets of each classful network, namely zero subnet and

broadcast subnet

First octet values of addresses that are always reserved, and that cannot be assigned to hosts. Reserved IP addresses….as recognized based on the value of the first octect….• 0 (because network 0.0.0.0 is always reserved) • 127 (because network 127.0.0.0 is always reserved)• 224 – 239 (all Class D – Multicast IP Addresses)• 240 – 255 (all Class E – Experimental IP Addresses)

Summary of reasons why an exam question should or should not allow the use of the zero and broadcast subnets. Determine whether a question allows the use of the Zero and Broadcast subnetsClue in the question Subnet reserved?Says nothing about it (default for the exam) NoList the ip subnet-zero configuration command NoUse a classless routing protocol (RIP-2, EIGRP, OSPF) NoList the no ip subnet-zero configuration command YESUse a classful routing protocol (RIP-1) YES

Summary of 4 tips when approaching IP Addressing related questions on the exam…

• Check the mask used on each device in the same LAN; if different then the devices cannot have the same view of the range of addresses in the subnet.

• On a point-to-point WAN link, check the IP addresses and masks on both end of the link, and confirm that the two ip address are in the same subnet.

• When checking to confirm that hosts are in the same subnet, do not just examine the subnet number. Also check the subnet mask and the implied range of IP addresses.

• Be ready to quickly use the commands in the table below, to find the ip address, masks, and subnet numbers.

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Host Networking

Summary of how hosts think about routing, address assignment, name resolution and ARP…

Routing : If the packet’s destination is in the same subnet, send the packet directly, if not send the packet to the default gateway.

Address Assignment : Before sending any packets, the host may use DHCP client services to learn its IP address, mask, default gateway, and DNS ip address. The host could also be statically configured with the same details.

Name resolution : When the user directly or indirectly when a host references a host name, the host typically uses DNS name resolution request to ask DNS to identify the host’s ip address, unless the host already has the information in its name cache.

IP-to-MAC resolution: The host uses ARP requests to find the other host’s MAC address, or the default gateway’s ip address, unless the information is already in the host’s ARP cache.

Command FunctionIpconfig/all Displays detailed IP configuration information for all

interface, including IP address, mask, default gateway, and DNS IP address.

Ipconfig/release Releases any DHCP leased IP addressesIpconfig/renew Acquires an IP address and related information using DHCPNslookup name Sends a DNS request for the listed nameArp –a Lists the host’s ARP cacheIpconfig/displaydns List host’s name cacheIpconfig/flushdns Remove all dns-found name cache entriesArp -d Flushes (removes) the host’s ARP cacheNetstat -rn Displays host’s routing table

Troubleshooting Host Routing Problems.Two typical reasons why a hosts cannot ping other hosts in the same subnet. If a ping of a same subnet host fails, the root cause typically falls into two categories…

• The two hosts have incorrect ip address, and mask configuration, so that at least one of the two hosts thinks it is in a different subnet.

• The two hosts have correct ip address and mask configuration, but the underlying Ethernet has a problem

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When a host can ping hosts in the same subnet, but not the hosts in a different subnet…..the root cause could be…..

• There is a mismatch between the host’s default gateway configurations and the router acting as the default gateway. The problems include mismatched masks between hosts and the router, which impacts the perceived range of addresses in the subnet, or the host simply referring the wrong router IP address.

• If the default gateway settings are correct but the ping of the default gateway still fails, then probably there is a layer 1 or 2 problem in the LAN.

• If the default gateway setting are correct, and ping works, but if ping of one of the other router interface fails, then the router’s other interface must have failed.

Tips regarding how a router matches a packet’s destination IP address as part of the routing process. Finding the matching route on a router…key points to remember…..

• When a destination IP address matches more than one route in a router’s routing table, the router uses the most specific route, the route with the longest prefix length.

• Although the router uses binary maths to match the destination IP address to the routing table entries, you can simply compare the destination address to each subnet in the routing table. If a subnet’s implied address range include the packet’s destination address, the route matches packet’s destination.

• If the question include a simulator, you could easily find the matched route by issuing a command show ip route address command, which lists the route matched for the IP address listed in the command.

Please see a listing of show ip route rip , in page 484 , and determine the matching route for a list of destination ip address from page 487.

Troubleshooting Commands

Show ip arp – lists the contents of a router’s ARP cache.

R1# show ip arp

Protocol address age(min) hardware address typeInterface

Internet 172.161.1.1 8 0013.197b.2f58 ARPA Fa0/1

Please refer to command output in page 485

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It lists the ip-address, mac address and interface. The age heading if lists a number, the value represents the number of minutes since the router last received a packet from the host and the timer is reset each time a matching packet is received. If the age heading lists a – the arp entry represents the ip address assigned to the router.

Traceroute command list the ip addresses of the routers in the routePlease refer to the listing on page 486

telnet newYork

Ctrl + Shift + 6 and x (to exit/suspend a telnet session

show sessionswhere (lists all the suspended telnet sessions)

resume 1 (resumes telnet session 1)disconnect 1 (disconnects telnet session 1)

show users (lists the users logged in to the telnet session)

Routing Troubleshooting Scenarios….please refer to page 491 through to 505.

Reminders that are helpful when thinking about source and destination MAC, and IP addresses that are used at various points in an internetwork.

• IP packet flows from the sending host to the destination host• The data link header and trailer that encapsulates the ip packet, do not flow over the

complete end-to-end route-instead each data link helps move packet from a host to a router, between routers or from a router to a destination host.

• For the process to work, the data link destination lists the next device’s data link address

• The IP header lists the sending host’s ip address, and destination hosts IP address, and never changes throughout the journey through the route

• Routers discard the received data link header and trailer for received frame, and build a new header and trailer-appropriate for the outgoing interface-before forwarding the frame.

• On LANs, hosts and routers use ARP to discover Ethernet MAC address used by other devices on the same LAN

• On Point-to-point WAN links ARP is not required, and data link addressing is uninteresting and can be ignored.

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Two key requirements for a router to add a connected route…

• The interface’s two status codes are up and up• The interface has an IP address correctly configured

Command PurposeShow sessions Lists the suspended telent and ssh sessions, from which the telnet

and ssh session were createdWhere Does the same thing as show sessionstelnet { hostname| ip address}

Connect the CLI to another host using telnet

Ssh –l username{hostname | ip address}

Connects CLI to another host using ssh

Disconnect [connection number]

Disconnects currently suspended telnet or ssh session, based on the connection number as seen with the show sessions command

Resume[connection number]

Connects CLI to a currently suspended telnet or ssh session, based on the connection number as seen with the show sessions command

Traceroute {hostname|ip address}

Discovers if a path from a router to a destination ip address is working, listing each next hop router in the route.

Ctrl + Shift + 6 and x

The key sequence required to suspend a telnet or ssh session

Show ip arp Lists the contents of router’s arp casheShow arp Lists the contents of router’s arp casheShow ssh Lists the information about the users logged into the router using

sshShow users Lists the information about the users logged into the router,

including telnet, ssh , and console users.

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ccent notes part 3

Documents

binary number

x binary

binary mask

dotted decimal mask

c network

binary process

withall binary

network bits1