What Is OSPF Router LSA? (Link ID, Routing Table OSPF 2.1)

Q1. What is Router LSA?

Answer: It is also called Type-1 LSA. A router running OSPF creates a Type-1 LSA to describe its interface IP addresses and link costs. When starting OSPF, the router creates a Router LSA to describe its links.

Q2. Why Type-1 LSA?

Answer. Hosts attached to routers need to learn each other's IP addresses and how to reach each other. One alternative is static routing. But that won't scale. OSPF let each router create its own Type-1 LSA and flood them when building Full Adjacency relationships. As a result, routers in an area are able to discover other routers' link addresses and know how to reach them.

Q3. What is Link ID in Router LSA?

Answer: Router LSA describes its links. Each link is specified by a Link ID, which is represented by Router ID, interface IP address, or subnet address based on the nature of the link.

For example, in this simulation, R3 has three interfaces.

- Link 1: R3 is connected to R11 by a serial link.

- Link 2: R3 is connected to R1 with an Ethernet link (a multi-access subnet.) R3 is elected as DR.

- Link 3: R3 is connected to a host subnet. No OSPF router is attached.

Now let's look at how to specify links:

Interface IP address Link ID Link type Description

#1 S0/0 1.1.1.1 1.1.1.1 Point-to-point Link ID is the Router ID of the connected router (R1)

#2 E0/0 10.1.1.1 10.1.1.1 transit network This subnet has OSPF routers. Link ID is DR interface IP.

#3 E0/1 192.168.1.1 192.168.1.1 host network This subnet has no OSPF router. Link ID is subnet address.

Q4. What are VEB bits in Type-1 LSA?

Answer: V=1 if this router uses virtual link to set up full adjacency with anther router.

E =1 if the Type-1 LSA is created by an ASBR.

B=1 if this router is an ABR.

Q5. What is Type-1 LSA's flooding scope

Answer: With the area.

This article is the FAQ of an interactive OSPF simulation: Router LSA. To check out this interactive simulation, go to the external link listed below.

1. OSPF Simulation: Router LSA (This article): http://pre.visualland.net/view.php?cid=916&protocol=OSPF&title=2.1%20Router%20LSA

2. More network protocol simulation: http://pre.visualland.net/

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