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CCNA 3 v3.1 Module 2

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CCNA 3 Module 2

Single Area OSPF

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Distance Vector Routing

• RIPv1, RIPv2, IGRP, EIGRP

• Interior gateway routing protocol

• Sends copies of routing tables to neighbors

View network from neighbors perspective

• Updates at specified times (30 secs,90 secs)

Slow convergence

High bandwidth usage

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Link State Protocols

• Examples

OSPF (Open Shortest Path First)

IS-IS(Intermediate System to Intermediate System

• Also known as Shortest Path First (SPF)

Dijkstra algorithm calculates loop free topology

• Interior gateway protocol

Sends routing updates to all routers within the autonomous systems

• All routers have a complete view of the network topology

Routers maintain a complex database of the topology

Link state database is identical on all routers

Routers have full knowledge of distant routers and how they interconnect

Require more memory and processing

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• Triggered updates

Contain changes only

Occur when topology change occurs

Updates sent to all routers

• Triggered updates result in

Fast convergence

Efficient use of bandwidth

• OSPF configuration

OSPF routing process must be enabled

Networks must be identified

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Link State Protocol• Send triggered updates

when a network change has occurred

periodic updates known as link-state refreshes

• Each router keeps track of

the state or condition of its directly connected neighbors by multicasting hello packets

all the routers in its network using link-state advertisements (LSAs)

• Builds database about the network using

hello information and LSAs

• Calculates shortest route to each network using

shortest path first (SPF) algorithm

• Stores this route information in its routing table

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Topological Database

• Every router advertises directly connected networks

via Link State Advertisements

• Every router has it’s own view of the network

it builds a ‘topological database’

• Router A is aware of 2 paths to 192.168.157.0

this provides redundancy should one of the routers fail

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Advantages of Link State Protocols

• Use cost metrics to choose paths

Cost metric reflects the capacity of the links

• Fast convergence because

Triggers flooded updates

• No routing loops - router knows network topology

• Routing decision based on up to date of information

LSAs are sequenced and aged

• Minimised Link-state database sizes

because of Dijkstra calculations and faster convergence

• Supports

Classless interdomain routing (CIDR)

Variable-length subnet masking (VLSM)

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Disadvantages of LSP

• More memory and processing power required

In particular during initial setup

• Require strict hierarchical network design

to reduce the size of the topology tables

• Administrator must have a good understanding of link-state routing

• Flooding of LSAs during the initial discovery process

decrease network transport capability

Uses the available bandwidth

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Compare Distance Vector and Link State

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OSPF• Non-Proprietary protocol

• Preferred to RIP because it is scalable

• Open Standard - IETF RFC 2328

• Link State routing protocol

• Interior Gateway Protocol for Autonomous systems

• Metric based on bandwidth

• Supports VLSM

• OSFP can use ‘areas’ for hierarchical design

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

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OSPF Terminology• Link

an interface on the router

• Link State

Status of a link

Relationship to neighbouring router

• Flooding

sends information out all ports, except the port the information was received

• Link State Database – topological database

List of information about all other routers

Shows the internetwork topology

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• Area

A collection of networks and routers

Each router in the area has the same link-state information

A router within an area is an internal router

Area 0 - Backbone

• Cost

Based on bandwidth

Transmission speed

• Routing table

Generated when SPF algorithm is run on link-state dbase

Unique to each router

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• Adjacent database

List of all directly connected routers

• Designated Router (DR) / Backup Designated Router (BDR)

a router elected by all others to represent the network area

All LSA sent to DR/BDR instead of to every single router

Multicast address 224.0.0.6

Reduces overhead of LSA updates

Standard on multi-access networks

DR is single point of failure – solution is BDR

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OSPF Network TypesMulti-access

Number of routers is unknownInfo sent to DR and BDR

Point-to-Point2 routersDR and BDR not elected

NBMAInfo sent to DR and BDR

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OSPF Hello Protocol• Hello protocol sends hello packets at

Router starts up

To initiate new adjacency

Regular intervals

ensure neighbor still functioning

Every 10 seconds on broadcast multi-access and point-to-point networks

Every 30 seconds to NBMA such as frame relay

• Hello packets have a L3 multicast address

224.0.0.5 - This address is all OSPF routers

• Hello packet has OSPF packet header

with a type field of 1

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Configuring OSPF

• Only 1 area

Number is 0

• Multiple areas

Number between 0 to 65,535

Must connect to area 0

• Router(config)#router ospf process-id (1 to 65,535)

• Router(config-router)#network network-address wildcard-mask area-id

• Router(config)#router ospf 1

• Router(config-router)#network 10.2.1.2 0.0.0.0 area 0

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Configure OSPF loopback address

• OSPF must have an active interface at all times

Not always possible if ethernet / serial interface are down

Create a loopback interface (logical interface) to overcome this

• OSPF uses the loopback interface address as the router ID (this overrides local IP address values)

• If there is more than one loopback interface

highest loopback IP address is taken as router ID

• Loopback configuration Commands

router(config)#interface loopback number

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

Subnet mask is always 255.255.255.255

• To turn off a loopback address

Router(config)#no interface loopback number

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Setting OSPF Priority

Priorities can be set from 0 to 255

0 prevents that router from being elected

Highest OSPF priority will win the election for DR

Configuring OSPF priority

Router(config)#interface fa0/0

Router(config-if)#ip ospf priority 50

Router(config0if)#end

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OSPF Cost Metric

• OSPF uses Cost metric for path selection

• Cost is based on bandwidth

• Default bandwidth is 1.544Mbps – cost is 64

• Cost can be 1 (100Mbps) to 65535

• Cost is 108 ÷ bandwidth

• ConfigurationRouter(config)#interface serial 0/0

Router(config-if)#bandwidth 64

Router(config-if)#ip ospf cost number

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OSPF Authentication

• To guarantee trust OSPF uses authenticatication

• Authentication key shared between routers

password can be up to eight characters

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

• After password is configured, enable authentication

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

This is sent in plain text – easy to decode

• To send encrypted authentication

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

MD5 keyword is the message-digest hashing algorithm to use

Encryption type is the type of encryption

where 0 means none and 7 means proprietary.

key-id 1 through 255

key is an alphanumeric password up to sixteen characters

Neighbor routers must use the same key identifier with the same key value

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

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OSPF Default Route

• Used to reach networks outside the domain

gateway of last resort

• Command

Router(config)#ip route 0.0.0.0 0.0.0.0 [interface | next-hop address]

• referred to as the quad-zero route

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What Causes a failure to establish a neighbor relationship

• 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.

• Other possibilities

All interfaces haven’t correct addresses or subnet mask

network area statements hasn’t correct wildcard masks

network area statements interfaces not in correct area

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Verifying OSPF• Show ip protocol

Displays, times, filters, metrics, networks

• Show ip route

Displays routes known and how they were learned

• Show ip ospf interfaces

Verifies interfaces configured in an area

• Show ip ospf

Displays how many times SPF has been executed

• Show ip ospf neighbor details

Displays a list of neighbors, priorities and states

• Show ip ospf database

Displays the contents of the topological database