CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Single Area OSPF
Link State Routing
Single Area OSPF Concepts
Single Area OSPF Configuration
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Identify Distance Vector &
Link State Routing Characteristics
Slow convergence
Updates contain
changes only
Updates sent to all
routers
Updates sent to
neighbours
Chapter 2 – Single Area OSPF
Updates contain entire
routing table
Increased memory &
processing requirements
Topology changes
trigger updates
Updates consume
significant bandwidth
Support CIDR/VLSM
Rapid convergence
Periodic updates
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Identify Distance Vector &
Link State Routing Characteristics
Slow convergence
Updates contain
changes only
Updates sent to all
routers
Updates sent to
neighbours
Chapter 2 – Single Area OSPF
Updates contain entire
routing table
Increased memory &
processing requirements
Topology changes
trigger updates
Updates consume
significant bandwidth
Support CIDR/VLSM
Rapid convergence
Periodic updates
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Evaluation of Link State Routing
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Review – Link State & Distance Vector
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
OSPF Overview
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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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
OSPF Cost
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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
Chapter 2 – Single Area OSPF
CISCO NETWORKING ACADEMY PROGRAM
CCNA3: Switching Basics and Intermediate Routing v3.0
Additional Configuration
Network administrators can also configure
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LSA update authentication
‘Hello’ & ‘Dead’ interval timers
Default route to routers outside the
area/autonomous system
Chapter 2 – Single Area OSPF