Full form: Open Shortest Path First
Also known as: Open Shortest Path First
Quick Definition
A link-state routing protocol that calculates the best path using cost based on bandwidth.
OSPF is an open-standard link-state routing protocol (RFC 2328) used for routing within an autonomous system. Each OSPF router builds a map of the entire network (the Link State Database) by flooding Link State Advertisements (LSAs) and then runs the Dijkstra algorithm to calculate the shortest (lowest-cost) path to each destination. OSPF metric (cost) is calculated as 100 Mbps / link bandwidth. OSPF forms neighbour adjacencies through Hello packets and elects a Designated Router (DR) and Backup Designated Router (BDR) on multi-access networks.
router ospf 1 network 10.0.0.0 0.0.0.255 area 0 show ip ospf neighbor show ip ospf database show ip route ospf
On a 1 Gbps link, OSPF cost = 100,000,000 / 1,000,000,000 = 0.1 → rounded to 1. On a 100 Mbps link, cost = 1. On a 10 Mbps link, cost = 10.
OSPF uses cost (bandwidth-based) as its metric. All links faster than 100 Mbps have a cost of 1 unless the auto-cost reference-bandwidth command is used to change the reference. Also, OSPF neighbours must agree on Hello/Dead intervals, area ID, authentication, and subnet — these are common reasons adjacencies fail.
A virtual IP address on a router that is always up and never tied to a physical interface.
The elected OSPF router on a multi-access network that collects and distributes LSAs on behalf of all neighbours.
The OSPF router elected to take over as DR if the current DR fails.
The information packets OSPF routers flood to share their link state with all other OSPF routers.
A 32-bit value that uniquely identifies an OSPF router within an OSPF domain.
A value (0–255) that tells a router how trustworthy a routing source is when multiple sources advertise the same prefix.
The value a routing protocol uses to measure the desirability of a route to the same destination.
OSPF is an open-standard link-state routing protocol (RFC 2328) used for routing within an autonomous system. Each OSPF router builds a map of the entire network (the Link State Database) by flooding Link State Advertisements (LSAs) and then runs the Dijkstra algorithm to calculate the shortest (lowest-cost) path to each destination. OSPF metric (cost) is calculated as 100 Mbps / link bandwidth. OSPF forms neighbour adjacencies through Hello packets and elects a Designated Router (DR) and Backup Designated Router (BDR) on multi-access networks.
OSPF uses cost (bandwidth-based) as its metric. All links faster than 100 Mbps have a cost of 1 unless the auto-cost reference-bandwidth command is used to change the reference. Also, OSPF neighbours must agree on Hello/Dead intervals, area ID, authentication, and subnet — these are common reasons adjacencies fail.
On a 1 Gbps link, OSPF cost = 100,000,000 / 1,000,000,000 = 0.1 → rounded to 1. On a 100 Mbps link, cost = 1. On a 10 Mbps link, cost = 10.
OSPF falls under the Routing domain of the 200-301 exam. Understanding it in context with related terms like dr and bdr is essential for answering scenario-based questions correctly.