Type 2 Network LSAs

Network LSAs are generated only by designated routers (DRs) and describe the set of routers attached to a particular nonbroadcast multiaccess (NBMA) or broadcast network.

106 Chapter 3: OSPF Communication

The purpose of the network LSA is to ensure that only one LSA is generated for the NBMA or broadcast network (as opposed to one from each attached router). This is a form of internal OSPF summarization. Specifically, Type 2 LSAs describe all routers that are attached to a multiaccess network. This information is an indication of all routers that are connected to a particular multiaccess segment, such as Ethernet, Token Ring, FDDI, and Frame Relay (as well as NBMA). Figure 3-4 shows that Type 2 LSAs are flooded in the area that contains the network.

The link-state ID is the IP interface address of the DR. Figure 3-5 shows the structure of each network LSA packet.

Figure 3-5 Network LSA Packet Format

0 12 3 01234567890123456789012345678901

LS Age



Link State ID

Advertising Router

LS Sequence Number

LS Checksum


Network Mask

Attached Router


Type of service (TOS) is no longer used in OSPF, so TOS fields remain for clarity in the packet structure.

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