OSPF Network LSA Demystified

We know that the Type-1 LSA describes the link type connected to the router, the neighbor router and the subnet number.

In this topology, assume we dont have a Type-2 LSA, so each router will create its own Type-1 LSA, the Type-1 LSA will describe the neighboring router and the subnet connected to.

Each router will create its own Type-1 LSA with 5 links, inside each Type-1 LSA 4 Link Type 1 to describe who is connected to me, and 1 Link Type 3 to describe the subnet number shared with the neighboring router, in this case all routers share the same subnet number

Each router will list 4 neighboring routers. Now imagine we have 100 routers for example, each routers will have 99 Link Type 1 in the Body of its own Type-1 LSA.

The first issue is that each Router creates a big Type-1 LSA with a lot Links Type 1, the flooding of these big LSAs Type-1 between the routers will cause a chaotic situation for the network.

The second issue is that the Link Type 3 (which describes the subnet number) is repeated uselessly.

Let s imagine a router in the same area but not attached to the multiaccess segment, then imagine we have 50 routers in this segment. If each attached router will carry the shared subnet. The router which is not attached to the segment will receive 50 LSA Type-1 , inside each LSA the same shared subnet This is not efficient. One router is enough to inform it about this subnet instead of 50 routers. This is why the information of this special subnet was moved to the DR. No imagine the same drother is connected to multiple segments. Let’s say 50 segments in the same area. Therefore it will generate a single Type-1 LSA with 50 links in the body of this LSA increasing the size of the Type-1 LSA with the risk to go beyond the MTU value 1500 and causing ip fragmentation, from the design ‘s perspective this is not desirable.

The creators of OSPF optimized ospf behavior on multiaccess by:

-first introducing the concept of DR

-second introducing the Network LSA.

After that to optimize LSA flooding. The DR has the responsability to carry two informations:

-the attached routers so the drother routers will discover them

-the shared subnet of the multiaccess.

So that other routers NOT ATTACHED to this subnet will learn this subnet through LSA Type-2 instead of the LSA Type 1 of each attached routers.

Representation of OSPF Multiaccess Without the concept of DR and Network LSA


Representation of OSPF Multiaccess With the concept of DR and Network LSA


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