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Chapter 2 – Single Area OSPF

Single Area OSPF

Link State Routing

Single Area OSPF Concepts

Single Area OSPF Configuration

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Chapter 2 – Single Area OSPF

Identify Distance Vector & Link State Routing Characteristics

Periodic updates

Topology changes trigger updates

Updates sent to neighbours

Updates contain changes only

Updates contain entire routing table

Increased memory & processing requirements

Updates consume significant bandwidth

Rapid convergence

Updates sent to all routers Support CIDR/VLSM

Slow convergence

CCNA3: Switching Basics and Intermediate Routing v3.0

CISCO NETWORKING ACADEMY PROGRAM

Chapter 2 – Single Area OSPF

Identify Distance Vector & Link State Routing Characteristics

Periodic updates

Topology changes trigger updates

Updates sent to neighbours

Updates contain changes only

Updates contain entire routing table

Increased memory & processing requirements

Updates consume significant bandwidth

Rapid convergence

Updates sent to all routers Support CIDR/VLSM

Slow convergence

CCNA3: Switching Basics and Intermediate Routing v3.0

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Chapter 2 – Single Area OSPF

Summary of Link State Features

• Responds quickly to network changes• Use ‘hellos’ to discover neighbours• Send updates when a there has been a change

in the network topology • Updates contain changes not whole routing table• Calculates shortest path to each route in the

network

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Chapter 2 – Single Area OSPF

Link State Operation

• Routers are aware of directly connected networks known as ‘links’

• Routers send ‘hellos’ to discover neighbours• Routers send Link State Advertisements to other routers

informing them of their links• All routers add Link State Advertisements to their

topological database• Shortest Path algorithm calculates best route to each

network• When link states change LSA update sent to all routers

which recalculate their routes

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Chapter 2 – Single Area OSPF

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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Chapter 2 – Single Area OSPF

Evaluation of Link State Routing

CCNA3: Switching Basics and Intermediate Routing v3.0

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Chapter 2 – Single Area OSPF

Review – Link State & Distance Vector

CCNA3: Switching Basics and Intermediate Routing v3.0

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Chapter 2 – Single Area OSPF

OSPF Overview

• Preferred to RIP on larger networks• 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’ to allow hierarchical

design

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Chapter 2 – Single Area OSPF

OSPF Key Words

Adjacencies database• Directly connected routers

Topological Database• Routes to every network

Routing table– Best path to each network

Designated Router• a router elected by all others to

represent the network area

Area 0• backbone

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Chapter 2 – Single Area OSPF

Designated Router/Backup DR

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

• Reduces overhead of LSA updates

• Standard on multi-access networks

• DR is single point of failure – solution is BDR

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Chapter 2 – Single Area OSPF

OSPF basic commands

Router(config)#router ospf process-id

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

EXAMPLE

Router(config)#router ospf 2

Router(config-router)#network 172.16.10.0 0.0.0.255 area 2

NOTES• process-id can be a value between 0 and 65,535• Wildcard mask NOT subnet mask used with network command

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Chapter 2 – Single Area OSPF

OSPF Loopback Address

• For OSPF to function there must always be an active interface

• Physical interfaces e.g. serial/Ethernet may not always be active – routing would fail

• Configure virtual “loopback” interface as solution• Subnet mask will always be 255.255.255.255

Router(config)#interface loopback number

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

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Chapter 2 – Single Area OSPF

DR/BDR selection

• To suit the topology used the network administrator will want to choose DR/BDR

• DR/BDR election based on OSPF priority• Lowest priority=DR• 2nd lowest priority=BDR

Router(config-if)#ip ospf priority number

Router#show ip ospf interface type number

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Chapter 2 – Single Area OSPF

OSPF Cost

• Cost is the OSPF metric used in 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• Gigabit Ethernet will require change to default cost –

why?

Router(config)#interface serial 0/0

Router(config-if)#bandwidth 64

Router(config-if)#ip ospf cost number

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Chapter 2 – Single Area OSPF

Additional Configuration

Network administrators can also configure

• LSA update authentication• ‘Hello’ & ‘Dead’ interval timers• Default route to routers outside the

area/autonomous system