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OSPF - Implicit ACK vs. Explicit ACK

I have read another thread and has a little confusion about the 3rd point below?

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  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.

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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

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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 ?

I think the original router will not notify DR with an LSAck if it receive the implicit ack. Can someone confirm it?

Regards,

Qixin

Everyone's tags (2)
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Accepted Solutions
Cisco Employee

OSPF - Implicit ACK vs. Explicit ACK

Hi Qixin,

I think the original router will not notify DR with an LSAck if it receive the implicit ack. Can someone confirm it?

You are right. Table 19 in RFC2328 talks about the same.

Circumstances            Backup                All other states
   _________________________________________________________________
  
________________
   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

HTH,

Nagendra

3 REPLIES
Cisco Employee

OSPF - Implicit ACK vs. Explicit ACK

Hi Qixin,

I think the original router will not notify DR with an LSAck if it receive the implicit ack. Can someone confirm it?

You are right. Table 19 in RFC2328 talks about the same.

Circumstances            Backup                All other states
   _________________________________________________________________
  
________________
   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

HTH,

Nagendra

New Member

OSPF - Implicit ACK vs. Explicit ACK

Hi Nagendra,

     Thanks for the response, I noticed the table too and need someone confirm it. However, if the original router doesn't notify DR with an LSAck, the question will be "will DR expect to receive LSAck after it send LSU to 224.0.0.5?". Look like all the DRother routers except the original one will send the LSAck back to DR, I am really cruious what is the expected behavior on DR's perspective. Can someone explain it?

Cisco Employee

OSPF - Implicit ACK vs. Explicit ACK

Hi Qixin,

This is the expected behaviour. DR will not expect any ack from the original router.

-Nagendra

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