OSPF Basics (FAQ from an interactive simulation 1.1)

Q1. What is OSPF?
Answer: OSPF is a link state routing protocol. OSPF routers exchange their Link State Advertisement (OSPF LSA) to learn interface IP addresses from each other. Each router saves its LSAs in its Link State Database (LSDB.) Neighboring nodes synchronize their LSDBs. With consistent LSDBs, OSPF routers are able to calculate shortest paths to reach destinations.

Q2. What does OSPF do?

Answer: Roughly, OSPF is going through the following stages:

1) Each OSPF node creates a Router LSA to describe its interface IP addresses. This LSA is the most basic LSA.

2) Routers discover neighbors by flooding Hello. When two neighbors find each other, they are in 2-way state.

3) Neighbors synchronize their LSDBs in three steps:

i. Send DD to get LSDB catalog from each other.

ii. Send Request to ask for missing LSAs.

iii. Send Update to transmit LSAs requested.

When two neighbors have the same LSDB, they are in Full Adjacency state.
4) With the same LSDB, routers have consistent knowledge of the network topology and interface IP addresses. They can calculate routing paths independently and update their routing tables. The collection of routing tables from OSPF routers provides the shortest paths between destinations.

Q3. What is Router LSA?
Answer: Each OSPF router creates a Router LSA to describe its interfaces' IP addresses. In a very simple OSPF network, only Router LSAs are needed to calculate shortest paths.

Q4. What does Router LSA do?
Answer: In a simple OSPF network (no LAN, no area), neighboring nodes exchange their Router LSAs and learn how many links its neighbor has, and their interface IP addresses. After neighbor propagation, all OSPF nodes have the same set of Router LSAs in their LSDBs. Now they have the same knowledge of network topology.

Q5. How can a router use LSDB to calculate routing paths?
Answer: From LSDB, an OSPF node learns the complete topology of the network: the number of routers being connected; Individual router's interfaces and their IP addresses; Interface link costs (OSPF metric). With such detail information, OSPF calculates shortest paths to reach all destinations found in LSDB.

For example [a], in the OSPF simulation, R1's LSDB contains two Router LSAs:

1) R1 originated LSA contains two IP addresses: 192.168.1.0/24,192.168.3.0/30.
2) R2 originated LSA contains two IP addresses: 192.168.2.0/24,192.168.3.0/30.
After LSDB synchronization, R1 and R2 both have these two LSAs. And they know that R1 and R2 are connected by the link 192.168.3.0. Now R1 can calculate routing path to reach remote destination 192.11.68.2.2 and adds an entry (192.168.2.0/24, S1/0) to its routing table.

[a]: This example refers to a companion OSPF simulation. You can play this simulation interactively at the external link listed below.

access point vs router