I have always read that OSPF has two types of LSack . ( Implicit ack and explicit ack ). I would like to know the difference in detail.
Please state the examples where implicit and explicit ack are used and in which scenarios are they sent .
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Naidu is absolutely correct on this. Just to add an overview how the implicit and explicit ACKs would be used, think of this scenario:
Hi Peter and Naidu,
Thanks for the clarification . My doubt in the 3 rd point is as follows
3.) The original router receives the same update from the DR it sent it a moment ago, and it considers it as an implicit acknowledgement. No further LSAck from the DR is expected.
Does the original router which recieves it own update packet needs to send an explicit ack to DR. To notify DR that it has recieved the update .
When even a original routers receives its own update also it sends the implicit ack to the DR.
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I realise this is a slightly older post now but just came across a couple of sections of the RFC which details the behaviour of acknowledgements pretty clearly: -
13.5. Sending Link State Acknowledgment packets Each newly received LSA must be acknowledged. This is usually done by sending Link State Acknowledgment packets. However, acknowledgments can also be accomplished implicitly by sending Link State Update packets
Actions taken on received update LSAs: if the received LSA is the same instance as the database copy * (a) If the LSA is listed in the Link state retransmission list for the receiving adjacency, the router itself is expecting an acknowledgment for this LSA. The router should treat the received LSA as an acknowledgment by removing the LSA from the Link state retransmission list. This is termed an "implied acknowledgment".
This table below is an excellent summary of behaviour with acknowledgements
Action taken in state Circumstances Backup All other states _________________________________________________________________ LSA has No acknowledgment No acknowledgment been flooded back sent. sent. out receiving in- terface (see Sec- tion 13, step 5b). _________________________________________________________________ LSA is Delayed acknowledg- Delayed ack- more recent than ment sent if adver- nowledgment sent. database copy, but tisement received was not flooded from Designated back out receiving Router, otherwise interface do nothing _________________________________________________________________ LSA is a Delayed acknowledg- No acknowledgment duplicate, and was ment sent if adver- sent. treated as an im- tisement received plied acknowledg- from Designated ment (see Section Router, otherwise 13, step 7a). do nothing _________________________________________________________________ LSA is a Direct acknowledg- Direct acknowledg- duplicate, and was ment sent. ment sent. not treated as an implied ack- nowledgment. _________________________________________________________________ LSA's LS Direct acknowledg- Direct acknowledg- age is equal to ment sent. ment sent. MaxAge, and there is no current instance of the LSA in the link state database, and none of router's neighbors are in states Exchange