OSPF Link State Acknowledgement

prakashj · ‎06-08-2008

I have a network with 7600 and 10720s in one OSPF Area. ABR is 7600 and all 10720s are internal routers. The DR is one of the 10720s. So let us say now 7600 sends an LSA update to this area. As it is a non-DR it will send to 224.0.0.6. Now the DR will pick up the update and send to other non-DR routers using 224.0.0.5.

Now I would like to know how ACK will work? The acknowledgement from non-DR will be send on 224.0.0.6 and will the DR router in turn send the ACK back to originating router on 224.0.0.5? Or will the originating router get ACK directly from Non-DR routers? What will be the approximate delay if it has to wait for ACK from DR routers? I am asking this as I can see some retransmissions happening in this area..the retransmissions are unicast..

Regards,

Prakash

riftjameel · ‎06-08-2008

Hi

I would say it is smarter if DR could send ack to orginating router. My idea is that it would be troubling if each an every non DR router sents the acknowlegement. I hope you got my point. Please let me know if it makes sense

Jon Marshall · ‎06-08-2008

Prakash

There are 2 types of acknowledgement.

Implict where the router receives a duplicate copy of the LSA in an update packet.

Explicit where the router explicitly acknowledges the receipt of an LSA by sending an acknowledgement.

When the DR receives the LSA from the ABR it will as you say send it out to the ALLSPFRouters multicast address 224.0.0.5. So all routers other than the ABR will receive this LSA and send an acknowledgement back to the DR ie. explicit acknowledgement.

But the ABR will also receive the LSA update from the DR because it too listens for all messages on 224.0.0.5. So the ABR will receive an implict acknowledgement from the DR because it receives a duplicate of the LSA it sent to the DR.

So the delay will be dependant on how long it takes for the DR to advertise out the LSA to ALLSPFRouters on that segment.

Hope this makes sense.

Jon

prakashj · ‎06-08-2008

Thanks for the replies...

But as I can see retransmission and normally retransmission is 5 seconds, can this overall activity take more than 5 seconds?

Regards,

Prakash

foxbatreco · ‎06-08-2008

The retransmission time ranges from 0-3600 secs..with the default being 5 secs.

Rate if this helps!!!

prakashj · ‎06-08-2008

Thanks...

I see this issue only in broadcast networks where there are DR and BDR...I dont see this issue in point-to-point networks...

So is this retransmission common in broadcast networks..

Regards,

Prakash

prakashj · ‎06-08-2008

Jon...if the ABR receives implicit ACK from DR then it should not retransmit the LSA right? For ABR it has already received an ACK from DR when DR sends the update on 224.0.0.5.

So the LSA retransmission (unicast) should only be send from DR to other routers if DR does not receive any ACK from the other routers.

Regards,

Prakash