2011-02-10 vlsm an intro to classless routing - 71

7/27/2019 2011-02-10 VLSM an Intro to Classless Routing - 71

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CCNA v3.0 Semester 3

CCNA v4.0

Introduction to Classless

Routing



CCNA v3.0 Semester 3 2

What is VLSM?

A Variable Length Subnet Mask (VLSM) is a means of allocating IP addressing resources to subnets

according to their individual need rather than some

general network-wide rule.

VLSM allows an organization to use more than one

subnet mask within the same network address space.

It is often referred to as „subnetting a subnet‟, and

can be used to maximize addressing efficiency.

Large subnets are created for addressing LANs and

small subnets are created for WAN links (a 30 bit

mask is used to create subnets with only two host).




Subnetting vs. VLSM

• Subnetting allows you to divide big networksinto smaller, equal-sized slices.

• VLSM allows you to divide big networks into

smaller, different-sized slices. This enables

you to make maximum use of your valuable

IP address space.

• So basically, you are now utilizing subnetmasks in the same IP address space.




Routing Protocols Supporting

VLSM• RIP v2

• EIGRP

• OSPF




Addressing a Network with

Standard Subnetting

• Site A has two Ethernet networks• Site B had one Ethernet network

• Site C had one Ethernet network

207.21.24.0 /24

How many network addresses are needed?

How many hosts are needed for the largest LAN?

How many bits need to be borrowed to address thisnetwork?

Site A Site B Site C

25 users 25 users 10 users 8 users




Addressing a Network with

Standard Subnetting

• Site A has two Ethernet networks• Site B had one Ethernet network

• Site C had one Ethernet network



Class C addressBorrow 3 bits [NNNHHHHH] giving eight networks (2^3),only use six. Each network has 32 addresses (2^5) only30 usable.

Giving us __, __, __ Ethernet network addresses at eachsite and ____extra networks.

Consider a PTP WAN connection between the routers.These two connections will take up the remaining two

networks.


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Addressing a Network with Standard

Subnetting

Borrowing 3 bits meets the current needs, but it leavesno room for growth.

Each network has 30 usable addresses, including the

WAN links (each requiring only two addresses).



Subnet # Subnet AddressBits

Masked

0 207.21.24.0 /27

1 207.21.24.32 /27

2 207.21.24.64 /27

3 207.21.24.96 /27

4 207.21.24.128 /27

5 207.21.24.160 /27

6 207.21.24.192 /27

7 207.21.24.224 /27

207.21.24.0



We can use subnet 0To enable subnet 0 on a Cisco router (if not already

enabled), use the global configuration command ip

subnet-zero.

Router# configure terminal (config t)

Router(config)# ip subnet-zero

To disable subnet 0, use the no form of this command.

Router(config)# no ip subnet-zero



Subnetting in a Box

0

255

In a class C

network there are

256 addresses.

One network of

256 addresses.

Subnet mask:

255.255.255.0

We can sub divide

this address into

smaller units

called subnets.

256 addresses



Subnetting in a Box

0

255

128

127

Borrowing 1 bit

divides the 256

addresses in totwo parts

Resulting in 2

networks each

with 128 addresses.

Subnet mask:

255.255.255.128.

128 addresses 128 addresses



Subnetting in a Box

0

255

128

127

64

63

Borrowing 2 bits

would divide each

of the 2 networksin half again.

Resulting in 4

networks, each

with 64 addresses.Subnet mask:255.255.255.192.

64 addresses

64 addresses

64 addresses

64 addresses

192

191



Subnetting in a Box

0

255

128

127

64 192

63 191

Borrowing 3 bits

would break each

of these 4networks in half

again.

Providing 8

networks, eachwith 32 addresses.

Subnet mask:255.255.255.224.

32

addresses

32

addresses

31

32

32

addresses

32

addresses

95

96

32

addresses

32

addresses

159

160

32

addresses

32

addresses

223

224



Subnetting in a Box

0

255

128

127

64 192

63 191

Borrowing 4 bits

would break each

of these 8networks in half

again.

Providing 16

networks, eachwith 16 addresses.

Subnet mask:255.255.255.240.

31

32

95

96

159

160

223

224

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

addresses

16

15

48

47

144

143

176

175

80

79

112

111

208

207

240

239



Subnetting in a Box

Borrowing 5 bits

would break each

of these ___ networks in half

again.

Providing ___

networks, eachwith ___

addresses.

Subnet mask:255.255.255. ___ .

0

255



Addressing a Network Using VLSM

• When using VLSM to subnet an address, not all of thesubnets have to be the same size.

• A different subnet mask may be applied to some of the

subnets to further subnet the address.

• In order to take advantage of VLSM, the proper routingprotocol must be selected.

• Not all routing protocols share subnetting information in

their routing table updates.

Classful Routing Protocols(do not share subnet info)

Classless Routing Protocols(do share subnet info)

RIP v1 RIP v2

IGRP EIGRP

OSPF

IS-IS




To subnet using VLSM, identify the LAN with the largestnumber of hosts. Subnet the address 207.21.24.0/24 based on this information.

• Site A has two Ethernet networks (25 hosts each)

• Site B had one Ethernet network (10 hosts)• Site C had one Ethernet network (8 hosts)



Subnet # Subnet AddressBits

Masked

0 207.21.24.0 /27

1 207.21.24.32 /27

2 207.21.24.64 /27

3 207.21.24.96 /27

4 207.21.24.128 /27

5 207.21.24.160 /27

6 207.21.24.192 /27

7 207.21.24.224 /27




Subnet 1 & 2 assigned to router A Ethernet networks.Subnet 4 & 5 assigned to routers B & C Ethernet networks.

Subnet 6 can be subnetted for WAN links.Site A Site B Site C


Free

Addresses

Site A

Subnet # Subnet Address

0 207.21.24.0 /27

1 207.21.24.32 /27

2 207.21.24.64 /27

3 207.21.24.96 /27

4 207.21.24.128 /275 207.21.24.160 /27

6 207.21.24.192 /27

7 207.21.24.224 /27

Site B & C

Sub-subnet 0 207.21.24.160 /28

Sub-subnet 1 207.21.24.176 /28

Site B

Site C


0 207.21.24.0 /27

1 207.21.24.32 /27

2 207.21.24.64 /27

3 207.21.24.96 /27

4 207.21.24.128 /275 207.21.24.160 /27

6 207.21.24.192 /27

7 207.21.24.224 /27

WAN links

Sub-subnet 0 207.21.24.192 /30

Sub-subnet 1 207.21.24.196 /30

Sub-subnet 2 207.21.24.200 /30

Sub-subnet 3 207.21.24.204 /30

Sub-subnet 4 207.21.24.208 /30

Sub-subnet 5 207.21.24.212 /30

Sub-subnet 6 207.21.24.216 /30

Sub-subnet 7 207.21.24.220 /30

Free

Addresses

WAN

1 & 2


0 207.21.24.0 /27

1 207.21.24.32 /27

2 207.21.24.64 /27

3 207.21.24.96 /27

4 207.21.24.128 /275 207.21.24.160 /27

6 207.21.24.192 /27

7 207.21.24.224 /27




Through applying VLSM, the topology will have two completesubnets available for future growth.



207.21.24.32 /27 207.21.24.64 /27 207.21.24.160 /28 207.21.24.176 /28

207.21.24.192 /30 207.21.24.196 /30





Exercise 1

Your company IP network is 195.39.71.0 /24.Headquarters is connected to five branch offices by aWAN link, and to an ISP.

Determine an appropriate IP addressing scheme. (the ISP owns the addresses on its link)

Headquarters

Branch 1

60 users

12 users 12 users 12 users 12 users 12 usersBranch 2 Branch 3 Branch 4 Branch 5

ISP




195.39.71.0 /24

Subnet according

to the largest

subnet needed.

(Headquarters 60

hosts)

0

255

128

127

64 192

63 191

Borrow 2 bits or

/26. This would

give you 4

networks with 64host addresses on

each subnet.




Playing it safe, we

will not use the

first subnet

(subnet 0).

0

64

128

192We will start

addressing with

195.39.71.64 /26.

Headquartersneeds 60 hosts, so

we will assign

them .64 - .127.

Headquarters

60 hosts

26 bit mask or /26

(255.255.255.192)




The 5 Branch

offices need 12

hosts each.

0

64

128

192

The next address

block available is

the .128 - .191

block. Use VLSM.

Headquarters

60 hosts

26 bit mask or /26

(255.255.255.192)

Using a /28 mask

will give us 16hosts at each

location. This will

take care of 4 of

the Branch offices.

160

144 176

Branch 1

12 hosts

/28

(255.255.255.240)

Branch 2

12 hosts

/28

(255.255.255.240)

Branch 3

12 hosts

/28

(255.255.255.240)

Branch 4

12 hosts

/28

(255.255.255.240)




To obtain a block

for Branch 5, we

will need to subnet

the .192 - .255block using a /28

mask.

0

64

128

192

Headquarters

60 hosts

26 bit mask or /26

(255.255.255.192)

160

144 176

Branch 1

12 hosts

/28

(255.255.255.240)

Branch 2

12 hosts

/28

(255.255.255.240)

Branch 3

12 hosts

/28

(255.255.255.240)

Branch 4

12 hosts

/28

(255.255.255.240)

224

208

Branch 5

12 hosts

/28

(255.255.255.240)




Now connect the 5

WAN links to the

Branch offices.

These are point-

to-point

connections and

only require 2

addresses.

0

64

128

192

Here we will use a

/30 mask to further

subnet thesubnets.

Headquarters

60 hosts

26 bit mask or /26

(255.255.255.192)

160

144 176

Branch 1

12 hosts

/28

(255.255.255.240)

Branch 2

12 hosts

/28

(255.255.255.240)

Branch 3

12 hosts

/28

(255.255.255.240)

Branch 4

12 hosts

/28

(255.255.255.240)

224

208

Branch 5

12 hosts

/28

(255.255.255.240)

232

228 236

WAN

1

WAN

2

WAN

3

WAN

4

248

244

WAN

5

240




Any remaining

networks could be

used for future

growth of either LANs or WANs.

Subnet 0 could

also be further

subnettedaccording to the

needs of the

network.

0

64

128

192

Headquarters

60 hosts

26 bit mask or /26

(255.255.255.192)

160

144 176

Branch 1

12 hosts

/28

(255.255.255.240)

Branch 2

12 hosts

/28

(255.255.255.240)

Branch 3

12 hosts

/28

(255.255.255.240)

Branch 4

12 hosts

/28

(255.255.255.240)

224

208

Branch 5

12 hosts

/28

(255.255.255.240)

232

228 236

WAN

1

WAN

2

WAN

3

WAN

4

248

244

WAN

5

240




Address

provided by ISP195.39.71.64 /26

195.39.71.128 /28 195.39.71.144 /28 195.39.71.160 /28 195.39.71.176 /28 195.39.71.192 /28

Applying the Addresses to the Topology




Classful Addressing

The IPv4 address architecture uses (a/n)8 bit network number for Class A addresses

16 bit network number for Class B addresses

24 bit network number for Class C addresses

1 - 126

128 - 191

192 - 223

Class B

Network Host

1 0

Class C

Network Host

1 1 0

Class A

Network Host

0




Classful Addressing

Classful addressing (A, B, C…) is obsolete.

Address

ClassApplication

Number of

Network Bits

Number of

Host Bits

Decimal

Address

Range

Number of

Addresses

Number of

Possible Host

Class ALarge

Networks8 bits 24 bits 1 - 126 126 16,777,214

Class BMedium-sized

Networks16 bits 16 bits 128 - 191 65,534 65,534

Class C Small Networks 24 bits 8 bits 192 - 223 2,097,152 254

The Class System




Classless Interdomain RoutingCIDR (pronounced “cider ”) ignores class.

Using CIDR, a router views a bit mask to determine the

network and host portions of an address.

This allows CIDR to craft network address spaces

according to the size of a network instead of force-

fitting networks into pre-sized network address

spaces.




Classless Interdomain Routing

CIDR sounds a lot like VLSM

CIDR is usually discussed in general Internet

context (ISPs)

– Uses custom length prefixes to reduce workload in

key Internet routers

VLSM is usually discussed in enterprise context

– Uses custom length prefixes to have better usageof enterprise address space




Classless Interdomain RoutingRouters use the network-prefix, rather than the first 3

bits of the IP address, to determine the dividing point

between the network number and the host number.

In the CIDR model, each piece of routing information is

advertised with a bit mask or prefix-length ( /x ). The

prefix-length is a way of specifying the number bits in

the network-portion of each routing table entry.




Classless Interdomain RoutingFor example, a network with 20 bits of network-number

and 12 bits of host-number would be advertised with

a 20 bit prefix (/20).

The clever thing is that the IP address advertised with

the /20 prefix could be a former Class A, Class B, or

Class C.

All addresses with a /20 prefix represent the sameamount of address space (212 or 4,096 host

addresses).

20 bits network + 12 bits host




Classless Interdomain Routing Address space can now be assigned in “chunks” that fit

the need.

If an organization needs 254 host addresses, what

difference does it make whether they are given: – a Class C (200.23.76.0 /24)

– 1/256th of a Class B (145.38.20.0 /24)

– 1/65,536th of a Class A (91.187.7.0 /24)

Using a /24 prefix, each of these specifies eight hostbits which will support 254 hosts.

P fi E i l t




Network Prefix Equivalent Number of Class Addresses Number of Hosts

/27 1/8th of a Class C 32

/26 1/4th of a Class C 64

/25 1/2 of a Class C 128

/24 1 Class C or 1 /24 256

/23 2 Class C or 2 /24s 512

/22 4 Class C or 4 /24s 1,024

/21 8 Class C or 8 /24s 2,048

/20 16 Class C or 16 /24s 4,096

/19 32 Class C or 32 /24s 8,192

/18 64 Class C or 64 /24s 16,384

/17 128 Class C or 128 /24s 32,768

/16 256 Class C or 1 Class B 65,536

/15 512 Class C or 2 Class B 131,072

/14 1,024 Class C or 4 Class B 262,144

/13 2048 Class C or 8 Class B 524,288

/12 4096 Class C or 16 Class B 1,048,576

/11 8192 Class C or 32 Class B 2,097,152

/10 16384 Class C or 64 Class B 4,194,304

/9 32768 Class C or 128 Class B 8,388,608

/8 65,536 Class C or 256 Class B or 1 Class A 16,777,216

Prefix Equivalents




Route Aggregation w/ CIDR or

(Summarization)You need 500 addresses.

Given two consecutive /24 addresses:

(200.201.202.0 /24 and 200.201.203.0 /24)

This address space could be advertised to the rest of

the Internet as 200.201.202.0 /23.

Why? (the two /24s have the first 23 bits in common).

11001000.11001001.11001010.00000000

11001000.11001001.11001011.00000000

23 bits network prefix




CIDR Scenario continued

If the ISP owns all of the 200.201.0.0 networks (256 /24s),

why should it advertise all of them separately?

Instead, it could simply advertise 200.201.0.0 /16 (which

would be 200.201.0.0 /24 through 200.201.255.0 /24).This would reduce the size of the routing tables on the

router to which the routes are advertised.

11001000.11001001.00000000.00000000

11001000.11001001.11111111.00000000

16 bits network prefix

.0.0

.255.0




CIDR Scenario continued

The summary of route 200.201.202.0 /23 is called a“CIDR block” or a supernet.

Because we are dealing with binary, the block size is

always a power of two (2, 4, 8, 16, 32, etc.). The

starting point of the block must be a multiple of the power of two that is being used (21 … 2, 4, 6,

8, etc.).

– 200.201.202.0

– 200.201.204.0

– 200.201.206.0

– 200.201.208.0

– 200.201.210.0

Examples

of starting

addresses




Network Prefixes200.201.200.0 11001000.11001001.11001000.00000000

200.201.201.0 11001000.11001001.11001001.00000000

200.201.202.0 11001000.11001001.11001010.00000000

200.201.203.0 11001000.11001001.11001011.00000000

200.201.204.0 11001000.11001001.11001100.00000000

200.201.205.0 11001000.11001001.11001101.00000000

200.201.206.0 11001000.11001001.11001110.00000000

200.201.207.0 11001000.11001001.11001111.00000000

200.201.208.0 11001000.11001001.11010000.00000000

200.201.209.0 11001000.11001001.11010001.00000000

200.201.210.0 11001000.11001001.11010010.00000000

200.201.211.0 11001000.11001001.11010011.00000000

23 bits

200.201.200.0/23

200.201.202.0/23

200.201.204.0/23

200.201.206.0/23

200.201.208.0/23

200.201.210.0/23





200.201.201.0 11001000.11001001.11001001.00000000

200.201.202.0 11001000.11001001.11001010.00000000

200.201.203.0 11001000.11001001.11001011.00000000

200.201.204.0 11001000.11001001.11001100.00000000

200.201.205.0 11001000.11001001.11001101.00000000

200.201.206.0 11001000.11001001.11001110.00000000

200.201.207.0 11001000.11001001.11001111.00000000

200.201.208.0 11001000.11001001.11010000.00000000

200.201.209.0 11001000.11001001.11010001.00000000

200.201.210.0 11001000.11001001.11010010.00000000

200.201.211.0 11001000.11001001.11010011.00000000

22 bits

200.201.204.0/22

200.201.200.0/22

200.201.208.0/22





200.201.201.0 11001000.11001001.11001001.00000000

200.201.202.0 11001000.11001001.11001010.00000000

200.201.203.0 11001000.11001001.11001011.00000000

200.201.204.0 11001000.11001001.11001100.00000000

200.201.205.0 11001000.11001001.11001101.00000000

200.201.206.0 11001000.11001001.11001110.00000000

200.201.207.0 11001000.11001001.11001111.00000000

200.201.208.0 11001000.11001001.11010000.00000000

200.201.209.0 11001000.11001001.11010001.00000000

200.201.210.0 11001000.11001001.11010010.00000000

200.201.211.0 11001000.11001001.11010011.00000000

21 bits

200.201.200.0/21




CIDR in a NutshellHand out pieces of classful networks (to avoid wasting

addresses)

Identify the network portion of an address with anetwork prefix ( /x)

Advertise blocks of networks (to reduce the size of

routing tables).




CIDR Example

ObjectiveCreate an addressing scheme using VLSM.

Scenario

You are assigned the CIDR address 200.32.108.0 /22 and

you must support the network shown in the diagram. Createan addressing scheme that will meet the diagram

requirements.

300 users 100 users 100 users

100 users




Dissect the problemGiven the CIDR address 200.32.108.0 /22

How many /24 networks do we have?How many host addresses do we have?

What is the largest LAN requirement?


100 users

Address given 200 32 108 0 /22




Address given - 200.32.108.0 /22

Host required - 300, 100, 100, 100, and 3 WAN links

200.32.108.0 200.32. 110.0

0 0

0 0255 255

255 255

200.32. 109.0 200.32. 111.0

Address given 200 32 108 0 /22




Address given - 200.32.108.0 /22


0 0

0 0255 255

255 255

3 0 0

h o s t s

2 0 0 .

3 2 .

1 0 8 . 0 /

2 3

200.32.108.0 200.32. 110.0

200.32. 109.0 200.32. 111.0

Address given - 200 32 108 0 /22




Address given - 200.32.108.0 /22


0 0

0 0255 255

255 255

3 0 0

h o s t s

2 0 0 .

3 2 .

1 0 8 . 0 /

2 3

127

128

1 0 0

h o s t s

2

0 0 .

3 2 .

1 1 0 .

0 / 2

5

1 0 0

h o s t s

2 0

0 .

3 2 .

1 1 0 .

1 2 8 /

2 5

200.32.108.0 200.32. 110.0

200.32. 109.0 200.32. 111.0

Address given - 200 32 108 0 /22




Address given - 200.32.108.0 /22


0 0

0 0255 255

255 255

3 0 0

h o s t s

2 0 0 .

3 2 .

1 0 8 . 0 /

2 3

127

128

1 0 0

h o s t s

2

0 0 .

3 2 .

1 1 0 .

0 / 2

5

1 0 0

h o s t s

2 0

0 .

3 2 .

1 1 0 .

1 2 8 /

2 5

127

128

1 0 0

h o s

t s

2 0 0 .

3 2 .

1 1 1 .

0 /

2 5

200.32.108.0 200.32. 110.0

200.32. 109.0 200.32. 111.0

Address given - 200 32 108 0 /22




Address given - 200.32.108.0 /22


0 0

0 0255 255

255 255

3 0 0

h o s t s

2 0 0 .

3 2 .

1 0 8 . 0 /

2 3

127

128

1 0 0

h o s t s

2

0 0 .

3 2 .

1 1 0 .

0 / 2

5

1 0 0

h o s t s

2 0

0 .

3 2 .

1 1 0 .

1 2 8 /

2 5

127

128

1 0 0

h o s

t s

2 0 0 .

3 2 .

1 1 1 .

0 /

2 5

191192

223

224

248

247

243252

251244

WAN links

/30

240239

200.32.108.0 200.32. 110.0

200.32. 109.0 200.32. 111.0




CIDR ResultGiven the CIDR address 200.32.108.0 /22


100 users

200.32.108.0 /23 200.32.110.0 /25 200.32.110.128 /25

200.32.111.0 /25

Two /24s




Classless Interdomain RoutingFor the router to operate in a classless manner and

match destination IP addresses to a CIDR network

address,

The global command: ip classless must beconfigured.

Router(config)# ip classless

Routing Information Protocol




Routing Information Protocol

(RIP)

•RIP is a relatively old, but still commonly used interior gateway protocol (IGP).

•It was created for use in small homogeneous networks.

•It is a distance-vector protocol that is used with classful

IP addressing only.•RIP v1 sends routing update messages at regular

intervals (30 seconds) and when the network

topology changes.

•RIP uses hop count as its only metric and maintains

only the best route to a destination.

RIP Version 2




RIP Version 2

•Known as RIP V2

In RIP v2 all of the operation procedures, timers, andstability functions of RIP v1 remain the same in version

2, with the exception of the broadcast updates.

RIP v2 has become the standard version of RIP used

in networks today.

RIP V2 is RIP V1 with extensions




RIP V2 is RIP V1 with extensions

• Subnet masks carried with each route entry

• Authentication of routing updates

• Next-hop addresses carried with each route entry

• External route tags

• Multicast route updates

RIP v2




RIP v2

The most important of these extensions is the additionof a Subnet Mask field

This enables the use of variable-length subnet masks

(VLSMs) and qualifies RIP v2 as a classless routingprotocol.

RIP v2 Packet Format





RIP v2

• RIP v2 allocated a 4-octet field to associate a subnetmask to a destination IP address.

• When used in tandem, the IP address and its subnet

mask enable RIP v2 to specifically identify the type of

destination that the route leads to.

• This allows RIP v2 to route specific subnets,

regardless of whether the subnet mask is fixed or of

variable length.


RIP v2




RIP v2

RIP v2 differs from RIP v1 in the way update are

sent out.

• RIP v1 sends updates as a broadcast (all stations

receive the broadcast message)

• RIP v1 does not send subnet mask information inits updates.

• RIP v2 sends updates as a multi-cast. Multi-casting

is a technique for simultaneously advertising routing

information to multiple RIP devices via the class D

address 224.0.0.9




RIP v1 & RIP v2 comparisons

• Both use hop count as a metric

• Both have the same metric value for

infinite distance (16)

• Both use split horizon to prevent routing

loops.

• RIP v1 broadcasts routing table updates,while RIP v2 multicasts its updates

Configuring RIP v1




Configuring RIP v1

To configure RIP v1 on a router, enter the followingcommands:

Router# config t

Router(config)# router rip

Router(config-router)# network 192.168.12.0

NOTE - If no version is specified in the configuration, version

1 will be used. The router will listen for version 1 and 2

updates but send only version 1.

Configuring RIP v2




Configuring RIP v2

To take advantage of version 2s features, it is necessary toturn off version 1 support and enable version 2 updates withthe following commands:


Router(config-router)# version 2


NOTE - The default behavior can be restored by entering the

command no version in the config-router mode.


Router(config-router)# no version




Verifying & Troubleshooting

RIP• show ip route to make sure routers have

learned all networks dynamically

• show ip protocols to see information about

the routing protocols used.

• debug ip RIP to see live routing updates

Overriding Default Behavior of RIP




You can override the default behavior of RIP by configuring aparticular interface to behave differently.


Interface e0

sends and

receives

version 1

updatesonly.

RIP v2

configured

on the

router.


Router(config-router)# version 2


Router(config-router)# exit

Router(config)# int e0

Router(config-if)# ip address 192.168.12.33255.255.255.224

Router(config-if)# ip rip send version 1

Router(config-if)# ip rip receive version 1





You can override the default behavior of RIP by configuring aparticular interface to behave differently.

Interface e2

has no

special

configuration

and thereforesends and

receives

version 2 by

default.


Interface e1

sends and

receives both

version 1 and

2 updates.



Router(config-if)# ip rip send version 1 2

Router(config-if)# ip rip receive version 1 2






Review of Static &

Default Routing

Configuring static routes w/




Configuring static routes w/

outgoing interface

Administrative distance of 0 - default

outgoing

interface

Configuring static routes w/ next




Configuring static routes w/ next-

hop IP address

Administrative distance of 1 - default

Next hop

interface

Configuring Static Routes




Configuring Static RoutesRemember, an administrator actually enters these

routes into the routing table.

That makes them static route entries – because therouter is not “discovering” those routes.

If for some reason that outgoing interface goes down or

is not available for some reason, then at that time theroute will be removed from the routing table.

Show ip route shows the routing table.

The route would still be in the configuration (because itwas entered globally), but that route could now nolonger be used by the router because the interface itrefers to is down for some reason.




Administrative Distance

What is the default for a outgoing interface?

What is the default for the next-hop address?

Defaults can always be changed!!!

Just make it higher if you want it to be a“backup” route.

ip route 192.168.2.0 255.255.255.0 192.188.4.1120




Router A Router B Router C

S0 192.168.2.1/24

S1 192.168.2.2/24

S0 192.168.4.1/24

S1 192.168.4.2/24

192.168.1.0/24 192.168.3.0/24 192.168.5.0/24

What would you enter to configure a static route from Router C to the LAN on Router A using outgoing interface?

The LAN on Router B from Router A using next-hop?

The static default route




The static default route

A router should be configured with a special typeof static route – a default route.

This default route routes packets with destinationsthat do not match any of the other routes in the

routing tableIt is a “gateway of last resort” that allows the

router to forward “destination unknown”packets out a particular interface

ip route 0.0.0.0 0.0.0.0 [next-hop-

address | outgoing interface]

Default Route on non-directly




Default Route on non-directly

connected networks




Default Route on non-directly connected

networks



CCNA v4.0

Introduction to Classless

Routing

2011-02-10 vlsm an intro to classless routing - 71

Documents