Some OSPF questions

Unanswered Question
Oct 16th, 2008

Hi all, can someone possibly answer these questions I have, I have been reading my cisco icnd 2 book, But I am still unsure of a few items.

1, When ospf starts, and forms a neighbour relationship on a lan with other routers, does each router only send an LSA to the DR about itself? and what kind of lsa is it, a router lsa or link lsa?

2, What does ospf define as a point to point link, if I have 2 ethernet ports directly connected but with a /30 address between them, will it class this as point to point?

3,How does the SPF algorithm work out the best way to a subnet, in that way i mean, how does it know to reach a certain subnet before a routing table is in place?

many thanks


I have this problem too.
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FiLeinster Thu, 10/16/2008 - 15:17

Hi Carl, Hopefully this will shed some light:

1. When a router starts, if it is on a LAN it will send hellos and all the routers will agree on the DR depending on the priority. This isn't strictly speaking true, as it wil be the first router to boot up on the LAN will be the DR as a higher priority router will not demote an existing DR. All communication from then on will be to the DR and the DR will pass on the information to other routers. A network LSA will be set for each attached network, and a router LSA will be sent per router.

2. All ethernet port default to broadcast. However, if you have a P2P ethernet interface it is best practice to manually set the type via "ip ospf network point-to-point". The default network type is set by interface type; ethernet, broadcast; Frame relay, non-broadcast; serial HDLC, point-to-point etc.

3. This is done with the OSPF database which is the collection of LSAs propgated throughout the area. As every router in the area has the same database each router has a full picture of the network topology.

carl_townshend Fri, 10/17/2008 - 02:08

Hi there

many thanks for your reply,

Hi there, so when you say network LSA, is called a link LSA? so the DR sends learns all routers by router LSA's sent from each router sent to, this then creates a link LSA and floods it out the routers on

2. what will be the benefit of making it P2P, will this mean that it sends link LSA's directly to each other?

lastly I hear that OSPF has a SPFholddown timer, what is this for, I gather its not the same as used for distance vector to avoid loops, is it to stop the SPF running too soon after it has just finished ?



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