question asked by cisco person

[arvenugopal]

hi

can ospf act as a distance vector protocol, i havent seen in cisco documentation or books, is it true can any one tell me the right answer

[Georg Pauwen]

Hello,

as a true link-state routing protocol, OSPF cannot be configured to act as a distance vector protocol. What exactly is the requirement ?

Regards,

GP

[arvenugopal]

the question was asked by cisco employee, he says that ospf can behave like Dist vect protocol

[Harold Ritter]

From a theorical standpoint, OSPF is a true LS protocol within the scope of a single area. If you look at the intre-area behavior, OSPF acts as a DV protocol.

Hope this helps,

[arvenugopal]

thanks for hitting the naul on then head,

[thisisshanky]

Harold, just curious abt this statement..

"If you look at the intra-area behaviour, OSPF acts as a DV protocol"...

As far as i understand OSPF with in an area (say a lan segment) does not send regular updates like a DV protocol does every X number of seconds. (Though it does do a database refresh every 30 mins or so). Also DVPs dont have concepts of DR and BDR, which are truly designed to reduce the number of link state updates required between N number of routers in a segment. So could you justify your statement about the behaviour of OSPF within an area ?

[Richard Burts]

As I understand the distinction: a link state protocol has complete information about all of the links and can draw an accurate map of the topology of the network and select the best path.

This is true for OSPF within an area. However in OSPF when you cross area boundaries you translate type 1 and type 2 LSAs and create type 3 summary LSAs. Routers within remote areas do not have complete link information and can not draw an accurate map of the topology with type 3 LSAs. They can only determine the best path to the border router who created the summary LSA. This is more of a DV behavior than Link State.

HTH

Rick

[Richard Burts]

I would like to follow up my post with another observation. The important distinction between Link State and Distance Vector protocols does not have much to do with whether they use broadcast or multicast to propagate updates, and not much to do with whether they do periodic updates, and not much to do with whether they have DR and BDR. The important distinction has to do with what kind of information they use and how they make decisions about what is the best forwarding path.

In a DV protocol a router receives updates from a neighbor that reflect the contents of the neighbors table and contain a forwarding vector (distance) and metric information. The router records that information and makes its decision based on that. The router is deciding which neighbor represents the best path without requiring any understanding of the topology of the network.

In a Link State protocol we get updates from neighbors that report information about the network. Based on these updates the router creates a map that represents its understanding of the topology of the network and chooses the best forwarding path based on its understanding of the topology not on which neighbor seems to have the best path.

In that sense OSPF inter-area is more like a DV than an LS protocol.

HTH

Rick

[thisisshanky]

That makes sense..I was mistaken by the term "intre" and i thought he meant "intra" instead of "inter"...