ccna03 01 classless rtg - ppt

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Introduction toIntroduction to

Classless Routing Classless Routing

CCNA 3/Module 1

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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

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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.

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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.

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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.

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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 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

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1.1.1 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)

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1.1.2 A waste of space

• In classless routing, it was recommended that first and last subnet not be used• First (SN 0) had same address for the network and subnet• Last subnet (all-1’s) was the broadcast

• Always could have been used, was not recommended practice

• Address depletion has lead to use of these subnets• Now acceptable practice to use the first and last subnets in

conjunction with VLSM

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Network Address 192.168.187.0

Borrow 3 bits = SNM 255.255.255.224

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

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If subnet zero is used, there are 8 useable subnets• Each 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 = 0, 32, 64, 96, 128, 160, 192, 224

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FOSTER(config)#no ip subnet-zero

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

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1.1.3 When to use VLSM

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

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1.1.4 Calculating subnets with VLSM

VLSM helps to manage IP addresses• VLSM can use one SNM for a point-to-point link and

one SNM for a LAN

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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

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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 /25

50 /26

2 /30

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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

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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 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

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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

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1.1.5 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 carry only one route 172.16.14.0/23

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• 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 if the routers of a network run a classless routing

protocol such as OSPF or EIGRP• Consists of IP address and bit mask in routing updates• The summary route uses prefix common to all addresses of

organization

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

• A router must know subnets attached in detail• A router does not need to tell other routers about subnets• A router 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

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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 = 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)

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1.1.6 Configuring VLSM

• Point-to-point connections are:

• 192.168.10.128/30 (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 138/30 – host 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

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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 within AS is Interior Gateway ProtocolRouting protocol used between Autonomous 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

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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

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

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1.2.3 Comparing RIP v1 & v2

RIP v1 RIP v2Easy to configure Easy to configure

Supports classful routing Supports classless routing

No subnet info 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)

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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 (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

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1.2.5 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

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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

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1.2.6 Troubleshooting RIP v2

Foster#debug ip rip Displays RIP routing updates as they are sent and received

Foster#no debug all

Foster#undebug all

Turns off all debugging

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1.2.7 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

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Default Route Command:

FOSTER(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1

Default NW Tells that 8 bits of subnetting in effectNext hop router

Default Route Command:

FOSTER(config)# ip route 172.16.1.0 255.255.255.0 172.16.2.1

Default NW Tells that 8 bits of subnetting in effectNext hop router

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Used to:

1. Give packets that are not ID’d 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

ccna03 01 classless rtg - ppt

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