OSPF network LSAs (or type 2 LSA) are generated for NBMA (non broadcast multiaccess) and transit broadcast networks that resides in an area. As a transit broadcast network could serve an Ethernet network where two or more routers are connected via ethernet links and share the same broadcast domain like in the picture below.
In the picture you see an OSPF network, where there is a broadcast domain, which consists of R2 with R1 and ABR connected through a ethernet switch. Unlike router LSAs, which are generated by every router, network LSAs (type 2 LSAs) are generated only by DR (Designated Router) for the multiaccess network. In current topology, ABR router, is at the same time Designated Router (because of highest router id – 18.104.22.168) for ethernet segment between R1-R2-ABR, so, ABR is in charge for generating type 2 LSA to represent this multiaccess network in area 0. Let’s have a look at some output from “
show ip ospf database“.