Thats a good observation, and here's an explanation to why thats happening.
All OSPF routers send updates when there is a change in their own topology and expect an acknowledgement from their directly connected neighbors. Whenever an LSU is sent (in your case by R5 to the DR on address 126.96.36.199) it is stored in the link state restransmission list of R5 until acknowledgement is received from all neighbors. In case an acknowledgement is not received within the retransmit interval ( default 5 secs (can be changed with the command ip ospf retransmit-interval on a per interface level) on time from any neighbor another similar duplicate update packet is sent to the unresponsive neigbor, but this time on the neighbors unicast address not the multicast address, thats becasue only that particular neigbor did not respond and only he must receive the duplicate update packet not the others who have responded.
The second line in your ACL permits all IP packets and the retransmitted unicast packet matches the second ACL line and reaches R4 which then acknowledges the receipt and forwards the update to the other DRother router R6.
Hope this helps,and please let me know if you have further questions !
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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