CCNP ROUTE: 4.OSPF internal LSAs: LSA Type 2 - Network LSA

-When a multiaccess data link exists — for instance, a LAN — OSPF must somehow model that LAN so that the topology represents nodes and links between only a pair of nodes.
-To do so, OSPF uses the concept of a Type 2 Network LSA.
-OSPF routers actually choose whether to use a Type 2 LSA for a multiaccess network based on whether a designated router (DR) has or has not been elected on an interface.
-The elected DR in a subnet creates the Type 2 LSA for that subnet. The DR identifies the LSA by assigning an LSID of the DR’s interface IP address in that subnet.
-The type 2 LSA also lists the DR’s RID as the router advertising the LSA.

In the R1-R2-R3 LAN, R3 is chosen as DR, R2 as BDR and R1 as DROther, based on the highest RID (interface priorities are equal: default 1):

R3#show ip ospf neighbor 

Neighbor ID     Pri   State           Dead Time   Address         Interface
1.1.1.1           1   FULL/DROTHER    00:00:35    192.168.0.1     FastEthernet0/0
2.2.2.2           1   FULL/BDR        00:00:38    192.168.0.2     FastEthernet0/0

As mentioned above, R3 (the DR) created the Type 2 LSA for that subnet and identified it by assigning the LSID of its interface IP address in that subnet (192.168.0.3).
Also, as seen below, R3 sets itself as the advertising router (RID 3.3.3.3):
R3#show ip ospf database 
...
                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.3     3.3.3.3         381         0x80000002 0x006E3D

This Type 2 LSA is advertised to other routers in the area having the same LSID and RID:

R1#show ip ospf database 
...
                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.3     3.3.3.3         602         0x80000002 0x006E3D

R2#show ip ospf database 
...
                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.3     3.3.3.3         585         0x80000002 0x006E3D

R9#show ip ospf database 
...
                Net Link States (Area 0)

Link ID         ADV Router      Age         Seq#       Checksum
192.168.0.3     3.3.3.3         623         0x80000002 0x006E3D

R1#show ip ospf database router 3.3.3.3

            OSPF Router with ID (1.1.1.1) (Process ID 1)

                Router Link States (Area 0)

  LS age: 909
  Options: (No TOS-capability, DC)
  LS Type: Router Links
  Link State ID: 3.3.3.3
  Advertising Router: 3.3.3.3
  LS Seq Number: 80000002
  Checksum: 0x455E
  Length: 48
  Number of Links: 2

    Link connected to: a Stub Network
     (Link ID) Network/subnet number: 192.168.13.13
     (Link Data) Network Mask: 255.255.255.255
      Number of TOS metrics: 0
       TOS 0 Metrics: 1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 192.168.0.3
     (Link Data) Router Interface address: 192.168.0.3
      Number of TOS metrics: 0
       TOS 0 Metrics: 10

-Each router’s Type 1 router LSA lists a connection this pseudonode, often called a transit network, which is then modeled by a Type 2 network LSA.

Note: OSPF can model all the topology inside a single area using Type 1 and 2 LSAs.