1. Single Area OSPF Link State Routing Single Area OSPF Concepts
Single Area OSPF Configuration

2. Identify Distance Vector & Link State Routing Characteristics
Updates contain entire routing table
Slow convergence
Updates consume significant bandwidth
Updates contain changes only
Increased memory & processing requirements
Updates sent to all routers
Topology changes trigger updates
Support CIDR/VLSM
Updates sent to neighbours
Rapid convergence
Periodic updates

3. Identify Distance Vector & Link State Routing Characteristics
Updates contain entire routing table
Slow convergence
Updates consume significant bandwidth
Updates contain changes only
Increased memory & processing requirements
Updates sent to all routers
Topology changes trigger updates
Support CIDR/VLSM
Updates sent to neighbours
Rapid convergence
Periodic updates

4. 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

5. 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

6. 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 – this provides redundancy should one of the routers fail

7. Evaluation of Link State Routing

8. Review – Link State & Distance Vector

9. 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

10. 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

11. 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

12. OSPF basic commands NOTES
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 area 2
NOTES
process-id can be a value between 0 and 65,535
Wildcard mask NOT subnet mask used with network command

13. 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
Router(config)#interface loopback number
Router(config-if)#ip address ip-address subnet-mask

14. 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

15. 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

16. Additional Configuration
Network administrators can also configure
LSA update authentication
‘Hello’ & ‘Dead’ interval timers
Default route to routers outside the area/autonomous system