03 01 classless rtg - ppt

8/12/2019 03 01 Classless Rtg - Ppt

1/34

1

Introduction to

Classless Routing

CCNA 3/Module 1


2/34

2

Overview: Classful/Classless Routing

Classful routing - a network must use the same subnet maskfor the entire network

Network IP 192.168.187.0

Network Subnet Mask 255.255.255.0

Classless routingusing more than one subnet mask for anetwork address

subnetting a subnet

Network IP 192.168.187.0

Network Subnet Masks 255.255.255.252

255.255.255.0


3/34

3

Overview: (Classful) IPv4 Addressing Limits

IPv420 years old

IPv4even with subnetting, couldnt handle the global demand

for Internet connectivity

Class B space was on the verge of depletion.

Rapid and substantial increase in the size of the Internet's

routing tables.

As more Class C's came online, the flood of new network

information threatened Internet routers' capability to

cope.


4/34

4

Overview: (Classful) IPv4 Addressing Limits

Provides IP scheme with limitations:

Class A126 networks: 16,777,214 hosts each

Class B65,000 networks: 65,534 hosts each

Class C2 million networks: 254 hosts each

While available addresses were running out, only 3%of assigned addresses were

actually being used!

Subnet zero, broadcast addresses,

pool of unused addresses atClass A and B sites, etc.


5/34

5

Overview: Scalability & Routing Tables

Maximum theoretical routing table size is 60,000 entries.

Classful addressing would have hit this capacity by

mid-1994.

Internet growth would have ended.


6/34

6

1.1.1 What is VLSM and why is it used?

The purpose of VLSM is to alleviate the shortage of IP addresses

VLSM allows:

More than one subnet mask within the same NW

Or . . . Multiple SNMasks with ONE IP Address

Use of long maskon networks with few hosts

Use of short maskon networks with many hosts

In order to use VLSM, the routing protocol must support it.

Cisco routers with the following routing protocols support VLSM:

OSPF (Open Shortest Path First) IS-IS (Integrated Intermediate System to Intermediate System)

EIGRP (Enhanced Interior Gateway Routing Protocol)

RIP v2

Static Routing


7/347

1.1.1 What is VLSM and why is it used?

Classfulrouting protocols use one subnet mask for a single network

Ex: 192.168.187.0, must use subnet mask255.255.255.0

VLSMallows a single autonomous system to have networks withdifferent subnet masks, for example:

Use a 30-bitsubnet mask on network connections

(255.255.255.252)

Use a 24-bitsubnet mask for user networks up to 250 users

(255.255.255.0)

Use a 22-bitsubnet mask for user networks up to 1000 users

(255.255.252.0)


8/34


9/349

1.1.2 A waste of space

Network Address 192.168.187.0

Borrow 3 bits = SNM 255.255.255.224

Subnets = 0, 32, 64, 96, 128, 160, 192, 224


10/34


11/34


12/3412

1.1.3 When to use VLSM

Design addressing scheme that

allows:

Growth

Doesnt waste addresses on

point-to-point links

VLSM addressing applied instead results in:

Variable sized subnets

Take1 of the3 subnets andsubnet it againExample 192.168.187.224(last subnet)

Apply a 30 bit mask (225.225.225.252)

Creates a possible8 rangesof addresses with30 bits

Best solution forpoint-to-point linksuse 2 host addresses

instead of 30


13/34


14/34

14

1.1.4 Calculating subnets with VLSM

osters abulous ilms 2 routers 1 in Hollywood (100 hosts)

1 in Ravenna (50 hosts)

1 WAN link (2 needed)

IP/NW Address: 192.16.10.0 Class C

Use the BIGGEST first:

100

50

2


15/34

15


osters abulous ilms 2 routers 1 in Hollywood (100 hosts)

1 in Ravenna (50 hosts)

1 WAN link (2 needed)

IP/NW Address: 192.16.10.0 Class C

Use the BIGGEST first:

100 /25

50 /26

2 /30


16/34

16


If VLSM were used instead of classful routing:

A 24-bit maskcould be used for LAN segments for 250hosts

A 30-bit maskcould be used for WAN segments for 2hosts

172.16.32.0/20(would accommodate 4094 hosts)

Binary = 10101100.00010000.00100000.00000000

SNM = 11111111.11111111.11110000.00000000

VLSM address172.16.32.0/26(needed for 62hosts)

Binary = 10101100.00010000.00100000.00000000

SNM = 11111111.11111111.11111111.11000000

If 172.16.32.0/20used, but only 10 hostson segment, wouldprovide 4094 hostsand waste 4084addresses

By further subnetting /20 to /26, gain 64 subnets (26) eachsupporting 62 hosts


17/34

17

1.1.4 Calculating Subnets w/VLSM

Procedure to subnet a subnet /20 to /26 using VLSM:

1. Write 172.16.32.0 in binary form

Binary = 10101100.00010000.00100000.00000000

2. Draw a vertical line between the 20thand 21stbits (the originalsubnet boundary)

3. Draw a vertical line between the 26thand 27thbits extending the bits

to segment/host needs

4. Calculate the number of subnet addresses between the two vertical

lines (lowest to highest) in value


18/34

18

1.1.4 Calculating Subnets w/VLSM

Keep in mind that only unused subnets can be further

subnetted

If any address for a subnet is used cannot be further

subnetted


19/34


20/34

20

1.1.5 Route Aggregation w/VLSM

Using CIDR and VLSM prevents address wasteand promotes route

aggregationor summarization

Without summarization, Internet would collapse

Summarization reduces burdenon upstream routers

This process of summarization continues until entire networkis

advertised as a single aggregateroute

Summarization is also called supernetting

Possible if the routers of a network run a classless routing

protocolsuch as OSPF or EIGRP Consists of IP address and bit mask in routing updates

The summary routeuses prefixcommon to all addresses of

organization


21/34

21

1.1.5 Route Aggregation w/VLSM

Carefully assign addressesin a hierarchicalfashion to share same

high-order bits for summarization

A router must know subnetsattached in detail

A router does notneed to tell other routers about subnets

A router using aggregateroutes has fewerentries in routingtable

VLSM allows for summarization of routes

Works even if networks are not contiguous

VLSM increases flexibly by summarization on higher-order bits

Used to calculate the network number of the summary route

Uses only shared highest-order bits


22/34

22

1.1.6 Configuring VLSM

If VLSM is chosen, it must be configured correctly

Example: 192.168.10.0

One router has to support 60 hosts, needs 6 bits in host

portion of address to provide 62 possible address

(26

= 642 = 60)192.168.10.0/26(leaves 6 bits for hosts)

One router has to support 28 hosts, needs 5 bits in host

portion of address to provide 30 possible hosts

(25= 322 = 30) 192.168.10.64/27(leaves 5 bits for hosts)

Two routers have to support 12 hosts each, needs 4 bits in

host portion of address to provide 14 possible hosts

(24= 162 = 14) 192.168.10.96/28(leaves 4 bits for hosts)

192.168.10.112/28 (leaves 4 bits for hosts)


23/34

23

1.1.6 Configuring VLSM

Point-to-point connections are:

192.168.10.128/30(2 address required, 2 bits = 2 host addresses)

192.168.10.132/30 (2 address required, 2 bits = 2 host addresses)

192.168.10.136/30 (2 address required, 2 bits = 2 host addresses)

Choices = .136 .137 .138 .139

Configuration as follows for the 192.168.10.136/30 network (.136/30 -

network address;.139/30 - broadcast address; .137/30 and 138/30host

addresses:

(config)#interface serial 0

(config-if)#ip address 192.168.10.137 255.255.255.252

(config)#interface serial1

(config-if)#ip address 192.168.10.138 255.255.255.252


24/34

24

1.2.1 RIP History

Internet is a collection of autonomous systems (AS) Each AS is administered by a single entity

Each AS has its own routing technology

Routing protocol used withinAS is InteriorGateway Protocol

Routing protocol used betweenAutonomous Systems is an Exterior Gateway

Protocol

RIP v1:

is an IGP that is classful

was designed to work within moderate-sized AS

is a distance vector routing protocol by default, broadcasts entire routing table every 30 seconds

uses hop count as metric (16 max)

is capable of load balancing 6 equal-cost paths (4 default)

Does not send subnet mask information in its updates

Is not able to support VLSM or CIDR


25/34

25

1.2.1 RIP History

If the router receives information about a network, and the receiving

interface belongs to same network but is on a different subnet, the

router applies the one subnet mask configured on the receiving

interface

Class A default classful mask is 255.0.0.0

Class B default classful mask is 255.255.0.0

Class C default classful mask is 255.255.255.0


26/34

26

1.2.2 RIP v2 Features

RIP v2 is an Improved version of RIP v1 with following features:

Distance vector protocol

Uses hop count as metric

Uses hold-down timers (prevent routing loops), default 180 sec.

Uses split horizon to prevent routing loops

Uses 16 hops as infinite distance

Provides prefix routing (sends subnet mask with route update)

Supports use of classless routing (VLSM)

Multicasts updates using 224.0.0.9 address for better efficiency

Provides authentication in updates

Clear text - default

MD5 encryptiontypically used to encrypt enable secret

passwords (Message-Digest 5)


27/34

27

1.2.3 Comparing RIP v1 & v2

RIP v1 RIP v2

Easy to configure Easy to configure

Supports classful routing Supports classlessrouting

No subnet infosent with routing

updates (considered a limitation of v1)

Sends subnet maskwith routing

update

No authentication Provides for authentication

Uses hop count Uses hop count

16 hops as metric for infinite distance 16 hops as metric for infinite distance

Broadcasts routing table updates

255.255.255.255

Multicasts updates 224.0.0.9

Does not support prefix routing (all

devices in same network must use

same subnet mask)

Supports prefix routing (VLSM,

different subnet masks can be used

in same network)


28/34

28

1.2.4 Configuring RIP v2

To enable a dynamic routing protocol:

1. Select routing protocol

FOSTER(config)#router rip

FOSTER(config-router)#version 2

2. Configure routing protocol with the network IP address (identifyphysically connected network that will receive routing tables)

FOSTER(config-router)#network 10.0.0.0

FOSTER(config-router)#network 172.16.0.0

3. Assign IP/SNM to interfaces


29/34


30/34

30

1.2.5 Verifying RIP v2

RIP updates table every 30 seconds

If no update received in 180 seconds, route marked as down If no update after 240 seconds, removes from routing table entry


31/34


32/34

32

1.2.7 Default Routes

Three ways a router learns about paths:

1. Static routesmanual configuration of routes (next hop)

Uses ip route command

2. Default routesmanually defined path to take when there is no

known route to a destination

3. Dynamic routesrouters lean paths by receiving updates from

other routers


33/34

33


Default Route Command:

FOSTER(config)# ip route 172.16.1.0255.255.255.0

172.16.2.1

Default NW Tells that 8 bits of

subnetting in effectNext hop router

Default Route Command:

FOSTER(config)# ip route 172.16.1.0255.255.255.0

172.16.2.1

Default NW Tells that 8 bits of

subnetting in effectNext hop router


34/34


Used to:

1. Give packets that are not IDd in the routing table a place to go

Usually a router that connects to the Internet

2. Connect a router with a static default route

DYNAMIC PROTOCOL Default Route Command

FOSTER(config)# ip default-network 192.168.20.0

Default NW

03 01 classless rtg - ppt

Documents