Giáo trình CCNA - Chương 3

Module 2 Single Area OSPFObjectives • Link State Routing Protocol • Single Area OSPF Concepts • Single Area OSPF Configuration... Link-state routing protocol • Collect routing informatio

CCNA – Semester3

Module 1 Introduction to Classless Routing

VLSM

Why VLSM?

• The addressing crisis

• Rapid increase in the size of routing tables

• IP address solutions:

– Short term extensions to IPv4

– Subnetting 1985

– Variable length subnetting 1987

– Classless Interdomain Routing 1993

– Private IP address

– Network Address Translation(NAT)

– IPv6

What is VLSM?

• With Variable-Length Subnet Masks (VLSM), a network administrator can use a long mask on networks with few hosts, and a short mask on subnets with many hosts

• In order to use VLSM, a network administrator

must use a routing protocol that supports it:

A waste of space

• It has become acceptable practice to use the first and

last subnets in a subnetted network in conjunction with VLSM

• With ip subnet-zero command, network has 8 usable subnets

Subnetting with VLSM

2.1 68 .13 6.0 /30

Route aggregation with VLSM

• When using VLSM, try to keep the subnetwork

numbers grouped together in the network to allow for aggregation

• The use of Classless InterDomain Routing

(CIDR) and VLSM not only prevents address waste, but also promotes route aggregation, or summarization

Route Summarization

Route Summarization

Route Summarization (Super network)

Route Summarization Rules

• A router must know in detail the subnet

numbers attached to it

• A router does not need to tell other routers about each individual subnet if the router can send one aggregate route for a set of routers

fewer entries in its routing table.

RIP version 2

RIPv1 Limitations

• It does not send subnet mask information in its updates

• It sends updates as broadcasts on 255.255.255.255

• It does not support authentication

• It is not able to support VLSM or classless interdomain routing (CIDR)

RIPv2

• RIP v2 is an improved version of RIP v1 and

shares the following features:

– It is a distance vector protocol that uses a hop count

metric

– It uses holddown timers to prevent routing loops – default

is 180 seconds

– It uses split horizon to prevent routing loops

– It uses 16 hops as a metric for infinite distance

RIPv2 vs RIPv1

Configuring RIP v2

• The router command starts the routing process The network command causes the

implementation of the following three functions:

– The routing updates are multicast out an interface

– The routing updates are processed if they enter that same interface

– The subnet that is directly connected to that interface is advertised

Configuring RIP v2

Verifying RIP Configuration

IP Routing Table

Troubleshooting RIP v2

Debug ip RIP output

Summary

• VLSM and the reasons for its use

• Subnetting networks of different sizes using

Module 2 Single Area OSPF

Objectives

• Link State Routing Protocol

• Single Area OSPF Concepts

• Single Area OSPF Configuration

Link State Routing Protocol

Link State and Distance Vector Routing

Link-state routing protocol

• Collect routing information from all other

routers in the network or within a defined area

of the network

• Each router independently calculates its best paths to all destinations in the network

• It is less likely to propagate incorrect

information provided by any of its neighboring routers

Link-state routing protocol functions

• Link-state routing protocols perform the

following functions:

– Respond quickly to network changes

– Send triggered updates only when a network change has occurred

– Send periodic updates known as link-state refreshes – Use a hello mechanism to determine the reachability of neighbors

Link State Routing

Link-state routing concepts

• Link-state routing uses the

– The resulting SPF tree

– A routing table of paths and ports

to each network to determine the best paths for packets

Advantages of link-state routing

• Support CIDR and VLSM

Disadvantages of link-state routing

• Require more memory and processing power than distance vector routers

• Require strict hierarchical network design

• They require good understanding of link-state routing

• Initial discovery process by flooding LSAs can

significantly decrease the capability of the network to transport data

Single Area OSPF Concepts

Single Area OSPF

• Open Shortest Path First

Large OSPF network

• Large OSPF networks use a hierarchical design

• Multiple areas connect to a distribution area, area 0, also called the

backbone

• Defining areas reduces routing overhead , speeds up convergence , confines network instability to an area and improves performance

OSPF terminology

OSPF terminology

OSPF terminology

OSPF terminology

OSPF terminology

OSPF terminology

OSPF terminology

OSPF terminology

Problem: Link state Updates

Shortest path algorithm

• Each node has a complete database of all the links so complete information about the physical topology is known

• All router link-state databases are identical

• The shortest path algorithm then calculates a loop-free topology

• OSPF routers determine which routers to become

adjacent to based on the type of network they are

connected to Once an adjacency is formed between neighbors, link-state information is exchanged

OSPF network types

• A neighbor relationship,

adjacent, is required for OSPF

routers to share routing

information which depends on

network type.

• OSPF interfaces recognize

three types of networks:

– Broadcast multi-access, such as

Ethernet

– Point-to-point networks

– Nonbroadcast multi-access

(NBMA), such as Frame Relay

• A fourth type,

point-to-multipoint, can be configured

on an interface by an

administrator

OSPF: Adjacency

• Too much overhead if n routers, n*(n-1)/2

adjacencies need to be formed

• The solution to this overhead is to hold an

election for a designated router (DR)

• All routers in a network form adjacency with DR

and BDR ( backup designated router ).

• Link-state information sent to OSDF routers

(DROther) at 224.0.0.5

• DROther sent link-state information to DR and

BDR at 224.0.0.6

OSPF Network Type

• On point-to-point networks only two nodes exist and no

DR or BDR is elected Both routers become fully

adjacent with each other

DR and BDR Receive RSAs

OSPF Hello protocol

• The hello packets are addressed to the multicast

address 224.0.0.5.

• OSPF routers use hello packets to initiate new

adjacencies and to ensure that neighbor routers are still functioning

• Hellos are sent every 10 seconds by default on

broadcast multi-access and point-to-point

networks

• On interfaces that connect to NBMA networks , such as Frame Relay, the default time is 30

seconds

OPSF Packet Header

• On multi-access networks the Hello protocol elects a

designated router (DR) and a backup designated router (BDR)

OSPF Hello Header

Exchange Process

172.16.5.1/24 E0

172.16.5.2/24

E1

Router B Neighbors List 172.16.5.1/24, int E1

I am router ID 172.16.5.1 and I see no one.

Down State

Init State

Exchange Process

172.16.5.1/24 E0

I am router ID 172.16.5.2, and I see 172.16.5.1.

172.16.5.2/24

E1

Router B Neighbors List 172.16.5.1/24, int E1

I am router ID 172.16.5.1 and I see no one.

I am router ID 172.16.5.2, and I see 172.16.5.1.

Router A Neighbors List 172.16.5.2/24, int E0

172.16.5.2/24

E1

Router B Neighbors List 172.16.5.1/24, int E1

I am router ID 172.16.5.1 and I see no one.

Down State

Init State

Two-Way State

Discovering Routes

E0 172.16.5.1

DR

E0 172.16.5.3

No, I will start exchange because I have a

higher router ID.

I will start exchange because I have router ID 172.16.5.1 Hello

afadjfjorqpoeru 39547439070713

Hello

afadjfjorqpoeru 39547439070713

DR

E0 172.16.5.3

No, I will start exchange because I have a

higher router ID.

I will start exchange because I have router ID 172.16.5.1 Hello

afadjfjorqpoeru 39547439070713

Hello

afadjfjorqpoeru 39547439070713

Exstart State

Discovering Routes

E0 172.16.5.1

E0 172.16.5.3 Thanks for the information!

DR

Discovering Routes

I need the complete entry for network 172.16.6.0/24.

Here is the entry for network 172.16.6.0/24.

Thanks for the information!

Loading State

E0 172.16.5.1

E0 172.16.5.3 Thanks for the information!

DR

Discovering Routes

Full State

I need the complete entry for network 172.16.6.0/24.

Here is the entry for network 172.16.6.0/24.

Thanks for the information!

Loading State

E0 172.16.5.1

E0 172.16.5.3 Thanks for the information!

DR

Maintaining Routing Information

Maintaining Routing Information

Maintaining Routing Information

No

Send LSU with newer information to source

Is seq # higher?

No

Yes

Is seq # the same?

Yes

Ignore LSA

Is entry in link-state database?

LSA

LSU

No

Run SPF to calculate new routing table

Configuring OSPF routing process

• OSPF routing uses the concept of areas

• Each router contains a complete database of link-states

• In multi-area OSPF networks, all areas are required to

connect to area 0 Area 0 is also called the backbone area

Enabling OSPF

• Each router contains a complete database of

link-states in a specific area

• An area in the OSPF network may be assigned

any number from 0 to 65,535

• A single area is assigned the number 0 and is

known as area 0

• In multi-area OSPF networks, all areas are

required to connect to area 0 Area 0 is also

called the backbone area

• To enable OSPF routing, use the global

configuration command syntax:

Router(config)#router ospf process-id

Configuring network

• IP networks are advertised as follows in OSPF:

Router(config-router)#network address wildcard-mask

area area-id

• The network address can be a whole network, a

subnet, or the address of the interface

• The wildcard mask represents the set of host

addresses that the segment supports

Basic OSPF Configuration

OSPF Router ID

• When the OSPF process starts, the Cisco IOS

uses the highest local active IP address as its OSPF router ID

• If there is no active interface, the OSPF process

will not start

• To ensure OSPF stability, loopback interface is

used, the highest loopback IP address is used

as router ID If there is no loopback interface, then other interface types are considered.

Configure Loopback Interface

• To create and assign an IP address to a

loopback interface use the following

commands:

Router(config)#interface loopback number

Router(config-if)#ip address ip-address subnet-mask

• This loopback interface should be configured

with an address using a 32-bit subnet mask of

255.255.255.255.

DR Election

• The interface reporting the highest priority for a router will ensure that it becomes the DR A router with the

second highest priority will be the BDR

• When OSPF priorities are the same, the OSPF election

for DR is decided on the router ID The highest router ID

• If the network type of an interface is broadcast, the

default OSPF priority is 1

• The priorities can be set to any value from 0 to 255

• Modify the OSPF priority by entering interface

configuration:

Router(config-if)#ip ospf priority number

• To verify OSPF interface priority:

Router#show ip ospf interface type number

Show ip ospf interface

Modifying OSPF cost metric

• Cost is calculated using the formula 10^8/bandwidth , where bandwidth is expressed in bps

• It is essential for proper OSPF operation that the correct

interface bandwidth is set

Router(config)#interface serial 0/0

Router(config-if)#bandwidth 64

• The cost number can be between 1 and 65,535

• Use the following interface configuration command to

set the link cost:

Router(config-if)#ip ospf cost number

Cisco IOS Default IOS Path Cost

Configuring OSPF authentication

• The password can be up to eight characters

Use the following command syntax to configure OSPF authentication:

Router(config-if)#ip ospf authentication-key password

• After the password is configured, authentication

must be enabled:

Router(config-router)#area area-number authentication

Encrypted authentication

• With simple authentication, the password is sent as

plain text This means that it can be easily decoded if a packet sniffer captures an OSPF packet

• To send encrypted authentication information and to ensure greater security , the message-digest keyword is used

• The MD5 keyword specifies the type of message-digest hashing algorithm to use, and the encryption type field refers to the type of encryption, where 0 means none and 7 means proprietary

Configure encrypted authentication

• To send encrypted authentication information and to ensure greater security , the message-digest keyword is used

• Use the interface configuration command mode syntax:

Router(config-if)#ip ospf message-digest-key key-id md5

encryption-type key

• The key-id is an identifier and takes the value in the

range of 1 through 255 Neighbor routers must use the same key identifier with the same key value

• The following is configured in router configuration

mode:

Router(config-router)#area area-id authentication

message-digest

Configuring OSPF timers

• OSPF routers must have the same hello intervals and the same dead intervals to exchange information

• By default, the dead interval is four times the value of the hello interval

• On broadcast OSPF networks, the default hello interval

is 10 seconds and the default dead interval is 40

seconds

• On nonbroadcast networks, the default hello interval is

30 seconds and the default dead interval is 120 seconds

Configuring OSPF timers

• The default values result in efficient OSPF

operation and seldom need to be modified

unless to improve performance

• To configure the hello and dead intervals on an

interface, use the following commands:

Router(config-if)#ip ospf hello-interval seconds

Router(config-if)#ip ospf dead-interval seconds

OSPF, propagating a default route

• The following configuration statement will

propagate the default route to all the routers in a normal OSPF area:

Router(config-router)#

default-information originate

Propagating a default route

Common OSPF configuration issues

• Failure to establish a neighbor relationship is caused by

any of the following reasons:

– Hellos are not sent from both neighbors

– Hello and dead interval timers are not the same

– Interfaces are on different network types

– Authentication passwords or keys are different

– Router neighbors have duplicate IP addresses

– Router interfaces have not been up

Verifying the OSPF configuration

Debug and clear commands

Summary

• The features of link-state routing

• OSPF terminology

• OSPF network types

• The OSPF Hello protocol

• The basics steps in the operation of OSPF

• OSPF configuration

• OSPF verification

Module 3 EIGRP

EIGRP Concepts

EIGRP Overview

• Cisco released EIGRP in 1994 as a scalable,

improved version of its proprietary distance vector routing protocol, IGRP

• EIGRP improves the convergence properties and the operating efficiency significantly over IGRP

EIGRP and IGRP

• IGRP and EIGRP are compatible with each other

This compatibility provides seamless

interoperability with IGRP routers

• EIGRP scales the metric of IGRP by a factor of

256

• IGRP has a maximum hop count of 255 EIGRP

has a maximum hop count limit of 224

EIGRP and IGRP route redistribution

• Redistribution, the sharing of routes, is automatic

between IGRP and EIGRP as long as both processes use the same autonomous system (AS) number