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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.


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
... is subnetted, 1 subnets
C is directly connected, Loopback0 is subnetted, 1 subnets
O [110/2] via, 00:37:18, GigabitEthernet0/0 is subnetted, 1 subnets
O IA [110/3] via, 00:37:17, GigabitEthernet0/0 is variably subnetted, 2 subnets, 2 masks
C is directly connected, GigabitEthernet0/0
L is directly connected, GigabitEthernet0/0 is subnetted, 1 subnets
O IA [110/2] via, 00:37:18, GigabitEthernet0/0

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 on R3, for metric is 2 on R2 also – [110/2] via, 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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