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LSA 3 – Summary LSA

As we know OSPF uses concept of areas for scalability. But as we discussed in LSA 1 (Router LSA) and LSA 2 (Network LSA) are used for propagating Router links and Multiaccess segment info within an area only. 
So LSA 1 and LSA 2 generated into a area are confined to that area only. So how does OSPF shares prefix information between areas? Well, the answer is LSA 3 (Summary LSA).
Using LSA 3, OSPF will share the prefix information between areas.
Q :: But why we do not use LSA 1 or LSA 2 for propagating information between areas?
A :: Because of below reasons :-
      1. LSA 1 and LSA 2 are used in SPF computation, so if a link goes down all routers in OSPF domain will run SPF (if LSA 1/2 are used between areas also). But by limiting LSA 1/2 to within area only, only routers within that area will run SPF.
       2. OSPF is a link state protocol but it behaves as a Distance Vector protocol between areas.
       3. We can not summaries LSA 1 and LSA 2 but we can summaries LSA 3.

LSA 3 is always generated by ABR Router. Number of LSA 3 = Number of prefixes need to share between areas. Yes, that’s correct – 1 LSA 3 per prefix unless summarized.

LAB TOPOLOGY :-

So we run OSPF 100 in R1, R2 and R3. As per our configuration R2 becomes ABR.
Lets see if R1 receives the R3 routes via ABR router R2

R1#sh ip route
...
      1.0.0.0/32 is subnetted, 1 subnets
C        1.1.1.1 is directly connected, Loopback0
      2.0.0.0/32 is subnetted, 1 subnets
O        2.2.2.2 [110/2] via 12.12.12.2, 00:37:18, GigabitEthernet0/0
      3.0.0.0/32 is subnetted, 1 subnets
O IA     3.3.3.3 [110/3] via 12.12.12.2, 00:37:17, GigabitEthernet0/0
      12.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
C        12.12.12.0/30 is directly connected, GigabitEthernet0/0
L        12.12.12.1/32 is directly connected, GigabitEthernet0/0
      23.0.0.0/24 is subnetted, 1 subnets
O IA     23.23.23.0 [110/2] via 12.12.12.2, 00:37:18, GigabitEthernet0/0
R1#

As we can see, R1 recives the R3 routes. Now lets check the OSPF database on R1 and R3
On R3
So from above snipps we do know that LSA 3 contains below fields :-
1. Link State ID : – This is prefix which is carried by LSA 3
2. Adv router – This is router ID of ABR.

Metric – Cost of prefix from the ABR (Notice Metric is 2 for 1.1.1.1 on R3, for 1.1.1.1 metric is 2 on R2 also –     1.1.1.1 [110/2] via 12.12.12.1, 00:53:42, GigabitEthernet0/0 )

LSA 3 Capture in Wireshark :-
SO this was all about LSA 3. If you have any doubt please leave a comment.

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