Tài liệu học CCNA kỳ 3 Module1

Module 1 – Introduction to Classless Routing CCNA 3 version 3.1 Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 2 Overview • Define VLSMand briefly describe the reasons fo

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Module 1 – Introduction to Classless

Routing

CCNA 3 version 3.1

Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com 2

Overview

• Define VLSMand briefly describe the reasons for its use

• Divide a major network into subnets of different sizes using VLSM

• Define route aggregation and summarizationas they relate to VLSM

• Configure a router using VLSM

• Identify the key features of RIP v1 and RIP v2

• Identify the important differences between RIP v1 and RIP v2

• Configure RIP v2

• Verify and troubleshoot RIP v2 operation

• Configure default routes using the ip route and ip

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Advanced IP Management

IPv4 Address Classes

• No medium size host networks

• In the early days of the Internet, IP addresses were allocated to

organizations based on request rather than actual need

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IPv4 Address Classes

Class D Addresses

• A Class D address begins with binary 1110 in the first octet

• First octet range 224 to 239

• Class D address can be used to represent a group of hosts called a

host group, or multicast group

Class E Addresses

First octet of an IP address begins with 1111

• Class E addresses are reserved for experimentalpurposes and should

not be used for addressing hosts or multicast groups

IP addressing crisis

• Address Depletion

• Internet Routing Table Explosion

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IPv4 Addressing

Subnet Mask

• One solution to the IP address shortage was thought to be the

subnet mask

• Formalized in 1985 (RFC 950), the subnet mask breaks a single

class A, B or C network in to smaller pieces

But internal routers think all these addresses are on different networks, called subnetworks

Externet routers still “see” this net as 190.52.0.0

Given the Class B address 190.52.0.0

Subnet Example

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Using the 3rd octet, 190.52.0.0 was divided into:

All Zeros and All Ones Subnets

Using the All Ones Subnet

• There is no command to enable or disable the use of the all-ones subnet, it is

enabled by default.

Router(config)#ip subnet-zero

• The use of the all-ones subnet has always been explicitly allowed and the use

of subnet zero is explicitly allowed since Cisco IOS version 12.0.

RFC 1878 states, "This practice (of excluding all-zeros and all-ones subnets) is

obsolete! Modern software will be able to utilize all definable networks "

Today, the use of subnet zero and the all-ones subnet is generally accepted

and most vendors support their use, though, on certain networks,

particularly the ones using legacy software, the use of subnet zero and the

all-ones subnet can lead to problems.

CCO: Subnet Zero and the All-Ones Subnet

http://www.cisco.com/en/US/tech/tk648/tk361/technologies_tech_note09186a

0080093f18.shtml

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Long Term Solution: IPv6 (coming)

• IP v6, or IPng (IP – the Next Generation) uses a 128-bit address

space, yielding

340,282,366,920,938,463,463,374,607,431,768,211,456

possible addresses 3,4 e38

• IPv6 has been slow to arrive

• IPv4 revitalized by new features, making IPv6 a luxury, and not

a desperately needed fix

• IPv6 requires new software; IT staffs must be retrained

• IPv6 will most likely coexist with IPv4 for years to come

• Some experts believe IPv4 will remain for more than 10 years

Short Term Solutions: IPv4 Enhancements

• CIDR (Classless Inter-Domain Routing) – RFCs 1517,

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• By 1992, members of the IETF were having serious concerns about the

exponential growth of the Internet and the scalability of Internet routing

tables

• The IETF was also concerned with the eventual exhaustion of 32-bit

IPv4 address space

• IETF’s response was the concept of Supernetting or CIDR, “cider”

• To CIDR-compliant routers, address class is meaningless

– The network portion of the address is determined by the network

subnet mask, network-prefix or prefix-length (/8, /19, etc.)

– The network address is NOT determined by the first octet (first two

bits), 200.10.0.0/16 or 15.10.160.0/19

• CIDR helped reduced the Internet routing table explosion with

supernetting and reallocation of IPv4 address space

CIDR (Classless Inter-Domain Routing)

2 Add all zeros after the last matching bit:

172.24.0.0 = 10101100 00011 000 00000000 00000000 Steps:

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CIDR (Classless Inter-Domain Routing)

• By using a prefix address to summarizes routes,

administrators can keep routing table entries manageable,

which means the following

– More efficient routing

– A reduced number of CPU cycles when

recalculating a routing table, or when sorting through the

routing table entries to find a match

– Reduced router memory requirements

• Route summarization is also known as:

– Route aggregation

– Supernetting

• Supernetting is essentially the inverse of subnetting.

• Company XYZ needs to address 400 hosts

• Its ISP gives them two contiguous Class C addresses:

– 207.21.54.0/24

– 207.21.55.0/24

• Company XYZ can use a prefix of 207.21.54.0 /23 to supernet

these two contiguous networks (Yielding 510 hosts)

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• With the ISP acting as the addressing authority for a CIDR block of

addresses, the ISP’s customer networks, which include XYZ, can be

advertised among Internet routers as a single supernet

Supernetting Example

CIDR and the Provider

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CIDR and the provider

• Dynamic routing protocols must send network address and mask

(prefix-length) information in their routing updates

• In other words, CIDR requires classless routing protocolsfor dynamic

routing

CIDR Restrictions

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Example from online curriculum

Another example from online curriculum

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VLSM (Variable Length Subnet Mask)

• Limitation of using only a single subnet mask across a

given network-prefix (network address, the number of

bits in the mask) was that an organization is locked into a

fixed-number of of fixed-sized subnets.

• 1987, RFC 1009 specified how a subnetted network could

use more than one subnet mask.

• VLSM = Subnetting a Subnet

– “If you know how to subnet, you can do VLSM!”

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VLSM – Simple Example

65,536 hosts per subnet.

10.0.0.0/8

10.0.0.0/ 16

1st octet 2nd octet 3rd octet 4th octet

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256 /24 subnets with 254 hosts each.

the subnet mask with the network address in the routing updates.

10.2.8.0/24 10.8.0.0/16

10.2.6.0/24 10.2.1.0/24

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Another VLSM Example using /30 subnets

207.21.24.0/24 network subnetted into eight /27 (255.255.255.224)

subnets

• This network has seven /27 subnets with 30 hosts each

AND eight /30 subnets with 2 hosts each.

• /30 subnets are very useful for serial networks.

207.21.24.192/27 subnet, subnetted into eight /30

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/30 subnets with 2 hosts each (one left over).

serial networks

VLSM and the Routing Table

Routing Table without VLSM

RouterX#show ip route

207.21.24.0/27 is subnetted, 4 subnets

C 207.21.24.192 is directly connected, Serial0

C 207.21.24.204 is directly connected, FastEthernet0

Routing Table with VLSM

RouterX#show ip route

207.21.24.0/24 is variably subnetted, 4 subnets, 2 masks

C 207.21.24.192 /30 is directly connected, Serial0

C 207.21.24.96 /27 is directly connected, FastEthernet0

• Parent Route shows classful mask instead of subnet mask of the child

routes

• Each Child Routes includes its subnet mask

Displays one subnet mask for all child routes

Classful mask is assumed for the parent route.

Each child routes displays its own subnet mask

Classful mask is included for the parent route.

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Route flapping

• Route flapping occurs when a router interface alternates rapidly between the

up and down states

• Route flapping, and it can cripple a router with excessive updates and

recalculations.

• However, the summarization configuration prevents the RTC route flapping

from affecting any other routers.

• The loss of one network does not invalidate the route to the supernet

• While RTC may be kept busy dealing with its own route flap, RTZ, and all

upstream routers, are unaware of any downstream problem

• Summarization effectively insulates the other routers from the problem of route

flapping.

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Global addresses must be obtained from a provider or a registry at some expense.

Discontiguous subnets

• “Mixing private addresses with globally unique addresses can create

discontiguous subnets.” – Not the main cause however…

• Discontiguous subnets, are subnets from the same major network that

are separated by a completely different major network or subnet

do the routing updates look like between Site A router and Site B router?

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• Classful routing protocols, notably RIPv1 and IGRP, can’t support

discontiguous subnets, because the subnet mask is not included in routing

updates

• RIPv1 and IGRP automatically summarize on classful boundaries.

• Site A and Site B are all sending each other the classful address of

• A classless routing protocol (RIPv2, EIGRP, OSPF) would be needed:

– to not summarize the classful network address and

– to include the subnet mask in the routing updates

• RIPv2 and EIGRP automatically summarize on classful boundaries

• When using RIPv2 and EIGRP, to disable automatic summarization (on both

routers):

Router(config-router)#no auto-summary

• SiteB now receives 207.21.24.0/27

• SiteA now receives 207.21.24.32/27

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1518, 1519, 1520

• VLSM (Variable Length Subnet Mask) – RFC 1009

• Private Addressing - RFC 1918

• NAT/PAT (Network Address Translation / Port Address

Translation) – RFC – Latter on Semester 4

Classless routing protocols

• The true defining characteristic of classless routing protocols is the

capability to carry subnet masks in their route advertisements

• “One benefit of having a mask associated with each route is that the

all-zeros and all-ones subnets are now available for use.”

– Cisco allows the all-zeros and all-ones subnets to be used with

classful routing protocols

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Học viện mạng Cisco Bách Khoa - Website: www.ciscobachkhoa.com

RIP version 2

0 1 2 3 3

0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1 2 3 4 5 6 7 8 9 0 1

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+

| command (1) | version (1) | must be zero (2) |

+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+-+ | Address Family Identifier (2) | Route Tag (2) |

+ -+ -+

| IP Address (4) |

+ -+

| Subnet Mask (4) |

+ -+

| Next Hop (4) |

+ -+

| Metric (4) |

+ -+

• ClasslessRouting Protocol, sent over UDP port 520

• Includes the subnet maskin the routing updates

• Automatic summarization at major network boundaries can be disabled.

• Updates sent as multicasts unless the neighbor command is uses which

sends them as unicasts

RIP v2 operation

• All of the operational procedures, timers, and stability functions of RIP v1 remain the same in RIP v2, with the exception of the broadcast updates

• RIP v2 updates use reserved Class D

address 224.0.0.9.

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Issues addressed by RIP v2

The following four features are the most significant new features added to

RIP v2:

• Authentication of the transmitting RIP v2 node to other RIP v2

nodes

• Subnet Masks– RIP v2 allocates a 4-octet field to associate a subnet

mask to a destination IP address

• Next Hop IP addresses– The inclusion of a Next Hop identification

field helps make RIP v2 more efficient than RIP v1 by preventing

unnecessary hops

• Multicasting RIP v2 messages– Multicasting is a technique for

simultaneously advertising routing information to multiple RIP or RIP v2

devices

Same limitations of RIPv2 as with RIPv1

• Slow convergence and the need of holddown timers to

reduce the possibility of routing loops.

Note: See CCNA 2 for review if needed.

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• RIP v2 continues to rely on counting to infinity as a means

of resolving certain error conditions within the network

• Dependent upon holddown timers.

• Triggered updates are also helpful.

Note: See CCNA 2 for review if needed.

• Perhaps the single greatest limitation that RIP v2 inherited from RIP is

that its interpretation of infinity remained at 16

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Basic RIPv2 configuration

Other:

For RIP and IGRP, the passive interface command stops the router from

sending updates to a particular neighbor, but the router continues to

listen and use routing updates from that neighbor (More later.)

Router(config-router)# passive-interface interface

Default behavior of version 1 restored:

Router(config-router)# no version

Compatibility with RIP v1

NewYork

interface fastethernet0/0

ip address 192.168.50.129 255.255.255.192

ip rip send version 1

ip rip receive version 1

• FastEthernet0/2 has no special

configuration and therefore

sends and receives version 2

by default.

RIPv2

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