routing protocols and concepts

© 2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 1

Routing Protocols and Concepts

VLSM and CIDR

Chapter 6Modified by Pete Brierley

ITE PC v4.0Chapter 1 2© 2007 Cisco Systems, Inc. All rights reserved. Cisco Public

What will we Learn from chapter 6?

Compare and contrast classful and classless IP addressing.

Review VLSM and explain the benefits of classless IP addressing.

Describe the role of the Classless Inter-Domain Routing (CIDR) standard in making efficient use of scarce IPv4 addresses


Introduction

Prior to 1981, IP addresses used only the first 8 bits to specify the network portion of the address

In 1981, RFC 791 modified the IPv4 32-bit address to allow for three different classes

IP address space was depleting rapidly

the Internet Engineering Task Force (IETF) introduced Classless Inter-Domain Routing (CIDR)

–CIDR uses Variable Length Subnet Masking (VLSM) to help conserve address space.

-VLSM is simply subnetting a subnet


Classful and Classless IP Addressing Classful IP addressing As of January 2007, there are over 433 million hosts on

internet Initiatives to conserve IPv4 address space include:

-VLSM & CIDR notation (1993, RFC 1519)

-Network Address Translation (1994, RFC 1631)

-Private Addressing (1996, RFC 1918)


Classful and Classless IP Addressing

The High Order Bits

These are the leftmost bits in a 32 bit address



Classes of IP addresses are identified by the decimal number of the 1st octet

Class A address begin with a 0 bit

Range of class A addresses = 0.0.0.0 to 127.255.255.255

Class B address begin with a 1 bit and a 0 bit

Range of class B addresses = 128.0.0.0 to 191.255.255.255

Class C addresses begin with two 1 bits & a 0 bit

Range of class C addresses = 192.0.0.0 to 223.255.255.255.



The IPv4 Classful Addressing Structure (RFC 790)

An IP address has 2 parts:

-The network portion

Found on the left side of an IP address

-The host portion

Found on the right side of an IP address



Purpose of a subnet mask

It is used to determine the network portion of an IP address



Classful Routing Updates

-Recall that classful routing protocols (i.e. RIPv1) do not send subnet masks in their routing

updates The reason is that the Subnet mask is directly related to the network address



Classless Inter-domain Routing (CIDR – RFC 1517)Advantage of CIDR :

-More efficient use of IPv4 address space

-Route summarizationRequires subnet mask to be included in routing update

because address class is meaningless

Recall purpose of a subnet mask:

-To determine the network and host portion of an IP address



Classless IP Addressing

CIDR & Route Summarization

-Variable Length Subnet Masking (VLSM)

-Allows a subnet to be further sub-netted according to individual needs

-Prefix Aggregation a.k.a. Route Summarization

-CIDR allows for routes to be summarized as a single route


Classful and Classless IP Addressing Classless Routing Protocol

Characteristics of classless routing protocols:

-Routing updates include the subnet mask

-Supports VLSM

Supports Route Summarization



RoutingProtocol

Routing updatesInclude subnet Mask

Supports VLSM

Ability to sendSupernet routes

Classful No No No

Classless Yes Yes Yes

Classless Routing Protocol


VLSM

Classful routing

-only allows for one subnet mask for all networks

VLSM & classless routing

-This is the process of subnetting a subnet

-More than one subnet mask can be used

-More efficient use of IP addresses as compared to classful IP

addressing


VLSM VLSM – the process of sub-

netting a subnet to fit your needs

-Example:

Subnet 10.1.0.0/16, 8 more bits are borrowed again, to create 256 subnets with a /24 mask.

-Mask allows for 254 host addresses per subnet

-Subnets range from: 10.1.0.0 / 24 to 10.1.255.0 / 24


Classless Inter-Domain Routing (CIDR)

Route summarization done by CIDR

-Routes are summarized with masks that are less than that of the default classful mask

-Example:

172.16.0.0 / 13 is the summarized route for the 172.16.0.0 / 16 to 172.23.0.0 / 16 classful networks


Classless Inter-Domain Routing (CIDR) Steps to calculate a route

summary

-List networks in binary format

-Count number of left most matching bits to determine summary route’s mask

-Copy the matching bits and add zero bits to determine the

summarized network address

© 2007 Cisco Systems, Inc. All rights reserved. Cisco PublicITE PC v4.0Chapter 1 19

An Introduction toAn Introduction to

Classless Routing Classless Routing

CCNA 3/Module 1


Overview: Classful/Classless Routing

Classful routing - a network must use the same subnet mask for the entire network

Network IP 192.168.187.0

Network Subnet Mask 255.255.255.0

Classless routing – using more than one subnet mask for a network address

• “subnetting a subnet”

Network IP 192.168.187.0

Network Subnet Masks 255.255.255.252

255.255.255.0


Overview: (Classful) IPv4 Addressing Limits

IPv4 – 20 years old

IPv4 – even with subnetting, couldn’t 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.


Overview: (Classful) IPv4 Addressing Limits

Provides IP scheme with limitations:Class A – 126 networks: 16,777,214 hosts each

Class B – 65,000 networks: 65,534 hosts each

Class C – 2 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 at

Class A and B sites, etc.


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.


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 mask on networks with few hosts

Use of short mask on 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


What is VLSM and why is it used?

Classful routing protocols use one subnet mask for a single network

Ex: 192.168.187.0, must use subnet mask 255.255.255.0

VLSM allows a single autonomous system to have networks with different subnet masks, for example:

Use a 30-bit subnet mask on network connections

(255.255.255.252)

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

(255.255.255.0)

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

(255.255.252.0)


A waste of space

In classless routing, recommended that first and last subnets have special use; not be used for host addresses

First (SN 0) had same address for the network and subnet

Last subnet (all-1’s) was the broadcast

Address depletion has lead to use of these subnets

Now acceptable practice to use the first and last subnets in conjunction with VLSM


A waste of space

Network Address 192.168.187.0

Borrow 3 bits = SNM 255.255.255.224

Subnets = (2^H) 0, 32, 64, 96, 128, 160, 192, 224


A waste of space

If subnet zero is used, there are 8 useable subnetsEach subnet can support 30 hosts

Cisco routers use subnet zero by default IOS v. 12.0+

If no ip subnet-zero command is used on the router, there are 7 useable subnets with 30 hosts per subnet

If supporting 4 routers (1 subnet each) that need 3 WAN links to each other, all subnets are used

No room for growth

Waste of 28 host addresses for each WAN (point-to-point) links or 1/3 of potential address space

Network Address 192.168.187.0

Borrow 3 bits = SNM 255.255.255.224

Subnets = (2^H) 0, 32, 64, 96, 128, 160, 192, 224


A waste of space

FOSTER(config)#no ip subnet-zero

Disables the capability to use subnets that include the network address of the unsubnetted network


When to use VLSM

Networking design addressing scheme that allows:

Growth

Doesn’t waste addresses on point-to-point links


VLSM addressing applied instead results in:

•Variable sized subnets•Take 1 of the 3 subnets and subnet it again •Example 192.168.187.224 (last subnet)

•Apply a 30 bit mask (225.225.225.252)•Creates a possible 8 ranges of addresses

with 30 bits•Best solution for point-to-point links – use 2 host addresses instead of 30

When to use VLSM


Calculating subnets with VLSMVLSM helps to manage IP addresses

VLSM can use one SNM for a point-to-point link and

one SNM for a LAN


Calculating subnets with VLSM

Foster’s Fabulous Films 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


If VLSM were used instead of classful routing:A 24-bit mask could be used for LAN segments for 250

hosts

A 30-bit mask could be used for WAN segments for 2 hosts

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 62 hosts)

Binary = 10101100.00010000.00100000.00000000

SNM = 11111111.11111111.11111111.11000000

If 172.16.32.0/20 used, but only 10 hosts on segment, would provide 4094 hosts and waste 4084 addresses

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

Calculating subnets with VLSM


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 20th and 21st bits (the original

subnet boundary)

3. Draw a vertical line between the 26th and 27th bits extending the bits to segment/host needs

4. Calculate the number of subnet addresses between the two vertical lines (lowest to highest) in value


Keep in mind that only unused subnets can be further subnetted

If any address for a subnet is used cannot be further subnetted

Calculating Subnets w/VLSM


Route Aggregation w/VLSM

Every network needs a separate entry in routing table

Each subnet needs a separate entry

Aggregation will reduce routing table size

When using VLSM keep subnetwork numbers grouped together in the network to allow for aggregation by using Classless InterDomain Routing (CIDR)

172.16.14.0

172.16.15.0

Router needs to hold only one route 172.16.14.0/23



•Using CIDR and VLSM prevents address waste and promotes route aggregation or summarization

•Without summarization, Internet would collapse

•Summarization reduces burden on upstream routers

•This process of summarization continues until entire network is advertised as a single aggregate route

•Summarization is also called supernetting

•Possible only if the routers of a network run a classless routing protocol such as OSPF or EIGRP

•IP address and bit mask included in routing updates

•The summary route uses a prefix common to all addresses of an organizational group



•Carefully assign addresses in a hierarchical fashion to share same high-order bits for summarization

•A router:

• Must know subnets attached in detail

• Does not need to tell other routers about subnets

• Using aggregate routes has fewer entries in routing table

•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


Configuring VLSMIf 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 = 64 – 2 = 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 = 32 – 2 = 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 = 16 – 2 = 14) 192.168.10.96/28 (leaves 4 bits for hosts)

192.168.10.112/28 (leaves 4 bits for hosts)


Configuring VLSMPoint-to-point connections are:

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



Choices = .136 .137 .138 .139

Configuration of the 192.168.10.136/30 subnet

(.136/30 - network address; .137/30 and 138/30 – host addresses .139/30 - broadcast address; :

(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


RIP HistoryInternet 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 within AS is Interior Gateway Protocol

Routing protocol used between Autonomous Systems is an Exterior Gateway Protocol

RIP v1:

• is an IGP that is classful

• 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


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 or /8

Class B default classful mask is 255.255.0.0 or /16

Class C default classful mask is 255.255.255.0 or /24


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 encryption – typically used to encrypt enable secret passwords (Message-Digest 5)


Comparing RIP v1 & v2

RIP v1 RIP v2Easy to configure Easy to configure

Supports classful routing Supports classless routing

No subnet mask sent with routing updates (considered a limitation of v1)

Sends subnet mask with 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)


Configuring RIP v2

To enable a dynamic routing protocol:

1. Select routing protocolFOSTER(config)#router rip

FOSTER(config-router)#version 2

2. Configure routing protocol with the network IP address (identify physically 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


Verifying RIP v2

FOSTER#show ip protocols •Shows protocol name

•Tells when updates are sent and when the next is due

FOSTER#show ip route •Tells if routers have learned about a newly added network

•Displays IP routing table

FOSTER#show ip interface brief •Summary of information

•status of interface

FOSTER#show running-config Checks for a misconfigured routing protocol


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


1.2.6 Troubleshooting RIP v2Foster# debug ip rip Displays RIP routing

updates as they are sent and received

Foster# no debug all

Foster# u all (undebug all)

Turns off all debugging


Default Routes

Three ways a router learns about paths:

1. Static routes – manual configuration of routes (next hop)

Uses ip route command

2. Default routes – manually defined path to take when there is no known route to a destination

3. Dynamic routes – routers lean paths by receiving updates from other routers


1.2.7 Default Routes

FOSTER(config)# ip route 172.16.1.0 255.255.255.0

Default NW24 bit prefix

Default Route Command:

Next hop router


1.2.7 Default Routes

Used to:

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


What did I Learn from chapter 6?

Classful IP addressingIPv4 addresses have 2 parts:

-Network portion found on left portion of an IP address

-Host portion found on right portion of an IP addressClass A, B, & C addresses were designed to provide IP addresses for different sized organizationsThe class of an IP address is determined by the decimal value found in the 1st octetIP addresses are running out so the use of Classless Inter Domain Routing (CIDR) and Variable Length Subnet Mask (VLSM) are used to conserve address space


What did I Learn from chapter 6? (con’t)

Classful Routing Updates

–Subnet masks are not sent in routing updates

Classless IP addressing

–Benefit of classless IP addressing

Can create additional network addresses using a subnet mask

that fits your needs

–Uses Classless Interdomain Routing (CIDR)



CIDR

Uses IP addresses more efficiently through use of VLSM

-VLSM is the process of subnetting a subnet

Allows for route summarization

-Route summarization is representing multiple contiguous

routes with a single route



Classless Routing Updates

Subnet masks are included in updates


VLSM & CIDR

NextRIP Ver2

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