How OSPF knows when link state change ? is that the Hello as well ?

[news2010a]

From reading the OSPF RFC, I have a question:

OSPF sends triggered updates(30 minutes) and LSA when a change in the topology occurs.

It is not clear to me how the OSPF network will "find out" that a change occurs. Let's say a link on interface of RouterF goes down the hill. Is it the Hello protocol (sent every 10 sec) the 'hearbeat' to find out that such link went down, and that would trigger routers to send an LSA to the DR, which in turn would flood the network with LSA ? Is that right ?

9.5. Sending Hello packets

"... Hello packets are sent out each functioning router interface. They

are used to discover and maintain neighbor relationships.[6] On

broadcast and NBMA networks, Hello Packets are also used to elect the

Designated Router and Backup Designated Router..."

[vijayasankar]

Hi,

I think you are referring to the LSRefreshtime definition in the RFC.

This is not related to triggered updates.

LSRefreshtime parameter states that even though there is no change in the content of the previously advertised LSA,

the router will again send that LSA, If LSA's Age had reached LSRefreshtime ( which is 30 minutes ).

This means, even though there is no change for a route advertisement, OSPF will still resend that information after 30 minutes.

This is by design of the protocol.

Now coming to your concern on how a state change is advertised, here's the answer.

Whenever a Change in routing information occurs, the router will generate a link-state advertisement.

This advertisement will contain the collection of all link-states on that router.

Upon receiving this information, the neighbour routers will rerun the SPF algorithm for the protocol to converge

Kindly have a look at the following URL to have an idea on how OSPF works.

http://www.cisco.com/en/US/tech/tk365/technologies_white_paper09186a0080094e9e.shtml#t3

Feel free to get back to us if you need further clarifications.

Hope this helps. Rate the post if you find it usefull

-VJ

[news2010a]

You said "whenever a change in routing information occurs, the router will generate a link-state advertisement". That was my question actually:Imagine that I just go to a router and remove it from the network. I understand that, but my question is, how does the neighbor router know that a change in information occurred ? Is that the Hello protocol sent out every 10 seconds (broadcast networks) which is going to be the hearbeat used to trigger the link-state advertisements ?

[sundar.palaniappan · Accepted solution]

Hi,

Yes, when you remove a router from the network the peering OSPF router will stop getting hellos and it removes the neighbor from the table and initate an LSA to update it's other neighbors. The process continues till every OSPF router within that area is made aware of the link state change.

HTH

Sundar