The OSPF Type-2 LSA is one of the misunderstanding LSA among all the popular LSAs in OSPF , most people learns that this kind of LSA (Type-2) is generated by DR the Designated Router in a broadcast segment, for example when two or more than two routers are connected to an ethernet link, but the famous question is why we need the Type-2 LSA to describe some special intra-area informations since there a Type-1 LSA which has the same purpose, here you will find in this post, why we need the Type-2 LSA.
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 14 links, 7 Link Type 1 to describe who is connected to me, and 7 Link Type 3 to describe the subnet number shared with the neighboring router, in this case all routers share the same subnet number 192.168.1.0/24.
For example for R1, the Type-1 LSA will look like this:
Link State ID: 0.0.0.1
Advertising Router: 0.0.0.1
Number of Links: 14
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.2
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.3
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.4
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.5
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.6
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.7
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
Link connected to: another Router (point-to-point)
(Link ID) Neighboring Router ID: 0.0.0.8
Link connected to: a Stub Network
(Link ID) Network/subnet number: 192.168.1.0
(Link Data) Network Mask: 255.255.255.0
The first issue is that each Router creates a big Type-1 LSA with a lot Links Type, the flooding of these big LSA Type-1 between the routers will cause a chaotic situation for the network. the second issues is that the Link Type 3 (which describe the subnet number) is repeated uselessly.
This is why the creators of OSPF have introduced the DR concepts and the Type-2 LSA, the purpose of the DR is to manage the adjacencies between the routers in the same broadcast domain, by suppressing the Link Type 1 (neighboring router) and the Link Type 3 (Subnet Number), instead each non-DR router will create the Type-1 LSA with a new Link Type 2, this Link Type 2 will identify the DR with the IP address only, now a non-DR router does not see the other non-DR routers in its Type-1 LSA, instead it sees only the DR, and the DR sees the non-DR routers through the Type-2 LSA and identifies the non-DR routers with the router-ID, also the DR is now responsible of advertising the Subnet Number in order to avoid the second issues described above.
If R1 is the DR. For example for R2, the Type-1 LSA will look like this:
Link State ID: 0.0.0.2
Advertising Router: 0.0.0.2
Number of Links: 1
Link connected to: a Transit Network
(Link ID) Designated Router address: 192.168.1.1
(Link Data) Router Interface address: 192.168.1.1
We have only one link carried by each LSA Type-1. The size of the Type-1 LSA is reduced greatly, and it does not describe the neighboring routers and the subnet number, this is done by the Type-2 LSA originated by the DR.
The Type-2 of the DR R1 looks like this:
Link State ID: 192.168.1.1 (address of Designated Router)
Advertising Router: 0.0.0.1
Network Mask: /24
Attached Router: 0.0.0.2
Attached Router: 0.0.0.3
Attached Router: 0.0.0.4
Attached Router: 0.0.0.5
Attached Router: 0.0.0.6
Attached Router: 0.0.0.7
Attached Router: 0.0.0.8
Now the DR R1 is simply responsible to tell to all non-DR Routers who is connected to the segment and which subnet is shared between them.
Note: Since the IP prefix of the transit multi-access network is not carried in the Type-2 LSA, rather it should be computed by other routers that receive this LSA; by ANDing the IP address of the DR 192.168.1.1 with the network mask /24 included in the Type-2 LSA.
