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Community Member

OSPF advertising process.


Hi there.

I'm wondering ospf advertising process.

R4 is DR and R5 and R6 area DRothers. there is no BDR.

as far as i know when some link changes occur in DRother, drother sends that change to DR using

Finally DR relized there was change and send LSU to DRother using

So i blocked that multicast in R4's incomming inerface.

I thought since R4 doesn't recieved from DRother, R4 doesn't know about that link change. and also doesn't advertise to other DRother.

But Still R4 recived that update and advertise to drother.

here is my configuration.


int fa0/0

ip add

ip ospf priority 255

access-list 101 deny ospf any host log (trying to block multicast from DRother)

access-list 101 permit ip any any

ip access-group 101 in


int f0/0

ip add

ip ospf priority 0


int f0/0

ip add

ip ospf priority 0

Neighbor status.

R4#sh ip ospf nei

Neighbor ID     Pri   State           Dead Time   Address         Interface           0   FULL/DROTHER    00:00:34         FastEthernet0/0           0   FULL/DROTHER    00:00:37         FastEthernet0/0

After configuring, I added Loopback interface and adversing on R5.

R5(config-if)#int lo0

R5(config-if)#ip add

R5(config-if)#ip ospf 1 area 0

I expected that R5 generate update using, but R4 block this multicast. So R4 doesnt' know about that change and also doesn't advertise to R6

But Still R6 know that change.

Why R4 knows about that change even though i blocked incomming multicast ? 

access-list is working on R4

R4#sh access-lists

Extended IP access list 101
    10 deny ospf any host log (31 matches)
    20 permit ip any any (429 matches)

Please correct me and explain and help me

Thanks in advance.

Community Member

Re: OSPF advertising process.

Hi ,

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  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 !

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