OSPF LSA Type 1 and 2

In the previous course, you learned that each area has its own LSDB.
But what is actually stored inside an LSDB?

Look at the topology again.
You can see three different areas:

• Area 1

• Area 0 (Backbone Area)

• Area 2

Figure 1 - OSPF Multi Area Topology

Now think about what is really happening behind the scenes.

• How do routers in Area 1 learn about the network inside Area 2?

• How does every router build an accurate and consistent view of the entire OSPF domain?

The answer is straightforward: OSPF uses different types of LSAs.
You already know LSAs from your CCNA: they are the messages OSPF routers exchange to build the LSDB.

The Five LSA Types You Must Know

For the CCNP ENCOR, you must clearly understand these five LSA types:

• Type 1 – Router LSA

• Type 2 – Network LSA

• Type 3 – Summary LSA

• Type 4 – ASBR Summary LSA

• Type 5 – External LSA

This course covers Type 1 and Type 2.
Type 3, 4 and 5 are covered in the next course.

Reading the LSDB

Connect to R1 and display its LSDB:

R1# show ip ospf database

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

                Router Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum Link count
1.1.1.1         1.1.1.1         512         0x80000004 0x00A1B2 1
2.2.2.2         2.2.2.2         498         0x80000003 0x00C3D4 1

                Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.1.12.1       1.1.1.1         512         0x80000001 0x00E5F6

                Summary Net Link States (Area 1)

Link ID         ADV Router      Age         Seq#       Checksum
10.0.23.0       2.2.2.2         480         0x80000001 0x001A2B
10.2.34.0       2.2.2.2         480         0x80000001 0x003C4D
10.2.100.0      2.2.2.2         475         0x80000001 0x005E6F

The LSDB is organized by LSA type.
Router Link States are the Type 1 LSAs. Net Link States are the Type 2 LSAs.
Let's look at them one by one.

A Type 1 LSA is generated by every OSPF router.
It describes the router's own links, interfaces, and their states inside the area.

Type 1 LSA Scope

• Type 1 LSAs remain inside the area.

• Type 1 → Intra-area scope

Figure 2 - Type 1 LSA

They are the foundation of the LSDB within an area.

Verifying a Type 1 LSA

We can verify a Type 1 LSA directly on a router using the following command:

R1# show ip ospf database router 

Router Link States (Area 1)

  Link State ID: 1.1.1.1
  Advertising Router: 1.1.1.1

    Link connected to: a Transit Network
     (Link ID) Designated Router address: 10.1.12.1
     (Link Data) Router Interface address: 10.1.12.1

Here, we can see that:

• The Link State ID matches the router ID (1.1.1.1).

• The Advertising Router is also 1.1.1.1.

• The output describes one of R1's interfaces (10.1.12.1).

You do not need to understand every detail of the output for the CCNP ENCOR exam.

What matters is the concept:

Every router describes itself and its links inside its own area using a Type 1 LSA.
These LSAs are then used by all routers in the same area to build the topology and run the SPF algorithm.

A Type 2 LSA is generated on a multi-access network segment.
It is created by the Designated Router (DR).

Type 2 LSA Scope

• Type 2 → Intra-area scope

• Type 2 LSAs remain inside the area.

They describe the multi-access network and list all routers attached to that segment.

Figure 3 - Type 2 LSA

Type 2 LSAs exist:

• Only in multi-access networks (Ethernet, for example)

• Not on point-to-point links

• Only one per segment (generated by the DR)

This reduces the number of adjacencies and simplifies the topology database.

Verifying a Type 2 LSA

We can verify a Type 2 LSA from R5, inside Area 2, where three routers share the same segment:

R5# show ip ospf database network 

Net Link States (Area 2)

  Link State ID: 10.2.100.2 (address of Designated Router)
  Advertising Router: 55.55.55.55

    Attached Router: 55.55.55.55
    Attached Router: 4.4.4.4
    Attached Router: 6.6.6.6

Here, we can see:

• The Link State ID corresponds to the IP address of the DR.

• The Advertising Router is 55.55.55.55 (R5's router ID).

• The Attached Router entries show all routers connected to that shared segment.

Conceptually, a Type 2 LSA represents the shared network itself and identifies which routers are connected to it.
Together, Type 1 and Type 2 LSAs build the complete intra-area topology used by the SPF algorithm.

Type 1 and Type 2 never leave their area.
The Summary section you saw in the LSDB is how networks are advertised between areas. That is the next course.