What is the difference btw explicit and implicit acknowledgement in OSPF

[Deepak Sukumar]

Hi Guys,

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 .

Regards,

Deepak

[Latchum Naidu · Accepted solution]

Hi Deepak,

Implicit acks: send the same LSA back.
– Used when you would have sent it anyway.

Explicit acks: OSPF packet type 5.
– Carry only LSA header.

Please rate the helpfull posts.
Regards,
Naidu.

[Peter Paluch]

Hello Deepak,

Naidu is absolutely correct on this. Just to add an overview how the implicit and explicit ACKs would be used, think of this scenario:

1. A non-DR router on an Ethernet segment detects a topology change and needs to inform other routers about it. It sends the LSU packet to the DR and BDR using the 224.0.0.6 destination IP address.
2. Both DR and BDR receive the packet. Normally, you would expect that the DR would send an LSAck acknowledging the successful receipt of the LSU. However, the DR needs to propagate the LSU back to the segment using the 224.0.0.5 destination IP address anyway, so it just does exactly that, without sending a standalone LSAck.
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.
4. Other routers on the segment receive the LSU and acknowledge its receipt via an explicit LSAck message back to the DR. This is an explicit form of acknowledgement.

Best regards,

Peter

[Deepak Sukumar]

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.

Question

##########

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 .

Regards,

Deepak

[Latchum Naidu]

Hi Deepak,

When even a original routers receives its own update also it sends the implicit ack to the DR.

Please rate the helpfull posts.
Regards,
Naidu.

[Khaled.Kotp]

The example of Peter is very reliable
in case of the LSU which is destined from the DRothers need fast cooked LSACK from the DR/BDR to let the process of Dothers education let faster which is called implicit LSACK .

-When the Dothers received the LSU from DR/BDR this situation will be completed by sending normal LSACK which is called explicit LSACK .

BR
Khaled Kotp

[Bryan Middleton]

Hi,

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