OSPF Type 4 is the response to a link-state request sent by R1 to R2 to asking for more information about each LSA. These packets contain the number of LSAs and full information for each LSA.

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Here is the LS Update packet which sent from R2 to R1.

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In every OSPF packet, we have an OSPF header. You can see the Message Type in the OSPF header is LS Update Type-4.

Look at the inside of the packet: the number of LSAs is two which is the total number of LSAs, inside of the packet. It has all of the information for LSA-type 1(Router LSA) and LSA-type 2(Network LSA).

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Link-State Update sends extra information for LSAs, these information doesn’t send inside of the DBD packets to R1.

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We have Number of Links that shows all of the links that connect with R2. We have two Loopback and two Fast Ethernet interface. Transit link is the link between R2 and R1.

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You can open each of the links and see all the information for that link.

The update packet is a reliable packet. It means if the router doesn’t receive any LSAck it starts to retransmit again the packet. OSPF uses a link-state update packet for flooding the packets