OSPF Questions

Studying OSPF and I think I'm going crazy, I reading the Cisco Press book and the links off the Cisco.com site. Here are some questions that I have after reading the material:

  1. I keep reading that OSPF will flood the area with LSA's after an update or change and then is says that it will only send LSA's to the DR and BDR. Which is it?? Does it send LSA to neighbors or only to DR's?

"OSPF uses flooding to exchange link-state updates between routers. Any change in routing information is flooded to all routers in the network."

  1. What is exactly in an LSA(doesn't matter which type)? If someone has some info on this I would really like to read it, along with other OSPF packet types (DBD, LSR, LSU, LSA TYPES, etc...) I haven't found to much information on them.

  1. The cost of an interface is based on the bandwidth; does the router use the bandwidth command to retrieve this information?

  2. Now for the question that I'm having the biggest issue with. When routers are on a multi-access network and they have multiple neighbors do they stop at a 2WAY state with all other neighbors except the DR and BDR which they are full? And by doing this they will only create adjacencies with the DR and BDR? So when a router boots up for the first time its not going to send its directly connected neighbors its topology table since it has to go through the DR?

Please Help,

Thanks Matt

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hi matt,

seems u r fedup with OSPF...which is one of the best routing protocols...

Qus.1. There are different states in an ospf router.

1 down - initial state whn the router is connected to an OSPF network 2 init - whn the router boots up , it starts sending hello packets to directly connected neighbors to establish adjacency, which is a multicast.
  1. two way state - in this state the router ( router A) sees its router id in a hello packet send by any directly connected router(router B). whn the router(A) sees its id in a hello packet, it forms adjacency with that router (B) once this has been done, if it is a multiaccess network, like ethernet LAN, they select a DR and BDR for every network. In that case further communication is done between DR/BDR. if it is not a multiaccess network, OSPF uses multicast to exchange link-state updates between routers.Any change in routing information is multicast to all routers in the network.

---------------- After this process router continuous to other states....which are

  1. exstart
5.exchange 6.loading 7.full state ============================================

qus.2 LSA's are Link state Advertisements DBD, LSR, LSU etc are examples of LSA types

Qus.3. cost of ospf link is proportional to "inverse" bandwidth. So u can manipulate the default BW by changing the bandwidth of interface by the bandwidth command.

qus.4 if DR is present it will send topology table to DR only. But establish two way state with others

pls let me know whether my reply was usefull to u

regards, sony.

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Thanks alot for your help, the fog is starting to lift. So upon start up a router won't broadcast its LSDB to it neighbors like EIGRP does? It will form adjaceny with the other routers and then pick a DR or BDR? Does it form adjaceny with other router or only the DR and BDR? And will it send the LSA to both the DR and the BDR, or only the DR? Any articles I can read that will help me with the LSA's?

Thanks in Advance,


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You may find the following Cisco papers useful:

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- Free CCNA and CCNP Study Guides

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