OSPF is an open standard link-state routing protocol that is widely used in large networks. It maintains three tables - routing, topology, and neighborship - and uses link state advertisements to exchange routing information. OSPF supports an unlimited number of routers and hop counts, uses cost metrics to determine the best path, and divides large networks into areas with Area 0 serving as the backbone area.

1. OSPF (Open Shortest Path First)
 It is an Open Standard Protocol
 It could be configured in all routers
 It is Link state Protocol
 It maintains three tables:-
 Routing Table
 Topology Table
 Neighbor ship table
 AD value is 110
 It has unlimited hop counts
 It supports unlimited no: of routers
 It is implemented in big networks

2. Areas
 A single autonomous system is divided into many Areas.
 Area 0 is the backbone area.
 ABR (Area Border Router) is used to communicate Routers in one Area to another.

3. Link
 According to OSPF the interface connected to a network or a Router is called a
Link

4. Router ID
 Address identified in each interface is called Router ID.
 Highest interface IP among the Network will be the Router ID.
 If there are Loopback interfaces configured in a network, then loopback interfaces
will be considered as Router ID.

5. Cost
 Metric of OSPF is called Cost
 Cost = 108/Bandwidth

6. Designated Router & Backup Designated
Router
 Designated router is one which controls whole network, which also has highest
priority.
 The second highest priority Router is Backup Designated router.
 One which has Highest router ID is called DR.
 If there are multiple interfaces and they are not needed, then the priority value is
made to 0 that they wont participate to find Router ID.
 Loop addresses will be Router ID if configured.

7. Wild Card Mask
 Reverse of Subnet Mask is called Wild card Mask.
 Network bits will be 0 and Host bits will be 1.
 Ex: 192.168.1.0/30
Broadcast = 255.255.255.255 –
Mask = 255.255.255.0
WCM = 0.0.0.255

8. OSPF Configuration
 R1#config t
 Enter configuration commands, one per line. End with CNTL/Z.
 R1(config)#router ospf 1
 R1(config-router)#network ‘Network Address’ ‘Wildcard Mask’ area 0

9. Verify OSPF
 Router#show ip protocols (Verify OSPF)
 Router#show ip route ospf (Routing Table)
 Router#show ip ospf interface (Router ID, Area ID)