Network LSA for Beginners (The FAQ From An Interactive OSPF Simulation 2.2)

This is the FAQ from an interactive OSPF Simulation.
1. What is Network LSA?
Answer: It is also called Type-2 LSA. It describes attached routers in a subnet. It is created by DR.
2. What is Type-2 LSA structure?
Answer: There are 3 fields in this LSA:
Type: 2. This a Network LSA.
Link State ID: DR interface's IP address.
Attached routers: A list of router IDs of routers attached this subnet.
3. What is the flooding scope of Type-2 LSA?
Answer: Within an Area.
4. Why do we need Network LSA?
Answer: When a node on the subnet is down, its Router LSA links should be removed from other routers' LSDB. Type 2 LSA is a list of router ID (RID) attached to the subnet. Routers use a subnet's RID list to match its links and remove them from LSDB.
Consider this topology: R1, R2, R3, R4 are OSPF routers attached to subnet-1 192.168.1.0. R3, R4, R5, R6 are attached to a different subnet-2 192.168.2.0. R3, R4 have two RID lists for subnet-1 and subnet-2. Now R3's link connected to subnet-1 is down. R4 is able to remove R3's links on subnet-1, not R3's link fon subnet 2. Without Type 2 LSA, R4 will be confused.
This article is the FAQ of an interactive OSPF simulation: Network LSA. Its focus is on common, simple but important questions that are too abstract to grasp by reading text. Read it together with playing interactive simulation that is case-specific may help learners visualize concepts. See external links below.

External links for network simulation:
1. OSPF Simulation: Network LSA (This article): http://pre.visualland.net/view.php?cid=816&protocol=OSPF&title=2.2%20Network%20LSA
2. More network protocol simulation: http://pre.visualland.net/

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