OSPF Network LSA (Type 2 LSA)

1. OSPF NETWORK LSA (TYPE
2 LSA)
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2. OFPF NETWORK LSAS (OR TYPE 2 LSA) ARE GENERATED FOR
NBMA (NON BROADCAST MULTI-ACCESS) 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 TOPOLOGY BELOW.

3. In the Topology we can 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 multi-access network.
In current topology, ABR router, is at the same time Designated
Router (because of highest router id – 4.4.4.4) for Ethernet segment
between R1-R2-ABR, so, ABR is in charge for generating type 2 LSA
to represent this multi-access network in area 0.

4. HAVE A LOOK AT SOME OUTPUT FROM “SHOW IP
OSPF DATABASE“:
R1#sh ip ospf database
OSPF Router with ID (1.1.1.1) (Process ID 1)
Router Link States (Area 0)
... ouptut omitted ...
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.0.1 4.4.4.4 1420 0x80000001 0x00E11D

5. R3#sh ip ospf database
OSPF Router with ID (3.3.3.3) (Process ID 1)
Router Link States (Area 0)
... output omitted ...
Net Link States (Area 0)
Link ID ADV Router Age Seq# Checksum
10.0.0.1 4.4.4.4 1463 0x80000001 0x00E11D

6. R5#sh ip ospf database
OSPF Router with ID (5.5.5.5) (Process ID 1)
Router Link States (Area 1)
Link ID ADV Router Age Seq# Checksum Link count
4.4.4.4 4.4.4.4 1527 0x80000001 0x00F365 2
5.5.5.5 5.5.5.5 1521 0x80000002 0x008EC5 2
Summary Net Link States (Area 1)
Link ID ADV Router Age Seq# Checksum
10.0.0.0 4.4.4.4 1512 0x80000002 0x00F027
10.0.1.0 4.4.4.4 1522 0x80000001 0x00F3F0
10.0.3.0 4.4.4.4 1512 0x80000002 0x004097
R5#

7. From the output, highlighted in bold we can see network LSAs on R1
and R3. Advertising router is ABR in this case, which has router id
4.4.4.4.
The Link-State ID used by network LSA is the IP address of the DR
which you can see in the excerpts in previous slides.
Please, be aware that in this topology DR (Designated Router) and
ABR (Area Border Router) is the same router is just a coincidence. A
router could be the DR for a network segment, however not the ABR
for the area or vice versa.
Type 2 LSAs are flooded within area, but they don’t leave the area in
which they are generated which you might have noticed in the output
from router R5.

8. FOR A DETAILED VIEW OF NETWORK LSAS USE “SH IP OSPF
DATABASE NETWORK” OR ONE OF ITS VARIATIONS BY PROVIDING
LINK-STATE ID OR ADVERTISING ROUTER-ID.
Link State ID: 10.0.0.1 (address of Designated
Router)
Advertising Router: 4.4.4.4
LS Seq Number: 80000003
Checksum: 0xDD1F
Length: 36
Network Mask: /24
Attached Router: 4.4.4.4
Attached Router: 1.1.1.1
Attached Router: 2.2.2.2
R3#
R3#sh ip ospf database network
OSPF Router with ID (3.3.3.3) (Process
ID 1)
Net Link States (Area 0)
Routing Bit Set on this LSA
LS age: 207
Options: (No TOS-capability, DC)
LS Type: Network Links

9. From this view you we get precious information like attached routers
and subnet mask. One more variation of this command that we would
pay attention to, is “show ip ospf database network self-originate“.
When you issue this command on a router, we will get network LSA
generated by the router itself.

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