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Question on link-state changes and flooding

Answered Question
Oct 10th, 2006
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Hi, can you clarify this:


An exercise states:


"Imagine one of the WAN links on an OSPF

network has gone down, causing a topology

change. What's the reaction

to maintain route consistency

when OSPF notices a change in the

network ?


Answer: When a link changes state, a link-state advertisement (LSA) concerning that link (route)

is created by the device that detected the change and propagate to all neighboring devices using a special multicast address. This is

called flooding."



My question:


Does the Router which notices the link state change actually floods the network, or in reality the router sends the new LSA to the DR; then the DR would send out the LSA to all other routers in the network ? Thanks for your great help !



Correct Answer by sundar.palaniappan about 10 years 10 months ago

Yes, the LSA will be flooded out all OSPF interfaces on detecting link state change. One of the requirements of OSPF is all the routers within an OSPF area has to have an identical database.


HTH


Sundar

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royalblues Wed, 10/11/2006 - 02:05
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  • Green, 3000 points or more

Hi marlon,


The DR & BDR are elected only in a multiaccess network to reduce the number of adjacencies to be formed.


So whenever there is a link state in a multiaccess network, the router would send the LSA to the ALLDRouters (224.0.0.6) which would then flood them to all its neighbor.


On the other hand if the network is not a Multiaccess network the LSA are flooded by using the multicast address of 224.0.0.5


HTH, rate if it does

Narayan

news2010a Wed, 10/11/2006 - 06:02
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Very good observation about the Multiaccess vs NBMA environment.


In addition to the sending the LSA to ALLDRouters (224.0.0.6) upon link state change, doesn't the router also send the LSA to all its interfaces in order to notify adjancent routers ? Isn't that right ?

Correct Answer
sundar.palaniappan Wed, 10/11/2006 - 10:06
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  • Green, 3000 points or more

Yes, the LSA will be flooded out all OSPF interfaces on detecting link state change. One of the requirements of OSPF is all the routers within an OSPF area has to have an identical database.


HTH


Sundar

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