What Is OSPF Network LSA? (Link ID, Attached Routers, 2.1)

Q1. What is DR? Answer: DR stands for Designated Router. In a multiacces subnet (E.g., Ethernet), one routers with higher priority/router ID is elected as DR. DR is responsible to maintain a Full Adjacency relationship with each non-DR routers. That is, DR synchronizes LSA with non-DR routers. It acts as the center of a star network for all routers in the subnet.

Q2. Why DR? Answer: The short answer is to reduce the amount LSA flooding traffic. In a multiacess subnet, all routers are neighbors. They are directly connected via a layer two device such as switch or hub. However, synchronize their LSA is complicated and costly when the number of routers increases. For example, in a 5 routers subnet, there are 10 full relationships to maintain (5*4/2). When the number of routers increases to 10, there are 45 full relationships (10*9/2). With DR acting as a the hub, the number of full relationships is reduced to 9 (10-1).

Q3. What is Network LSA? Answer: It is also called Type-2 LSA. It describes attached OSPF routers in a subnet.

Q4. How is Type-2 LSA being created? Answer. It is created by DR. Since DR keeps a full relationship with each non-DR routers. Therefore, DR knows exactly who are in the subnet.

Q4. What is the content of Type-2 LSA? Answer. This LSA contains 3 fields: Type: 2. LSA type. Link State ID: This is the IP address of DR's interface connected to the subnet. Attached routers: This is the router ID list of all routers connected to this subnet (including DR).

Q5. What is the flooding scope of Type-2 LSA? Answer: Within an Area.

Q5. How is Network LSA being used? Answer: The short answer to clean up LSA when a router is down. In a multi-access subnet (like a LAN), a subnet is up, the list of router IDs of the attached routers to the subnet is flooded to the rest of network. This router list is specified by Network LSA. Together with Router LSA of the attached routers, routers outside the subnet can figure out the subnet part of the network topology, calculate routes and update their routing tables. When an attached router is down, it is notified to the rest of the OPSF network by reflooding Network LSA. When routers outside the subnet receive the updated attached router list, they update their topology, recalculate their routes, and remove destination addresses associated with the failing router. (This example becomes more clear when playing the interactive simulation listed below)

This article is the FAQ of an interactive OSPF simulation: Network LSA. The simulation is listed in External links below.

/view.php?cid=816&protocol=OSPF&title=2.2 Network LSA

access point vs router