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

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

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

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

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

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

Evaluation of Link State Routing

Review – Link State & Distance Vector

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

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

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

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 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

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

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

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

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