OSPF confusion

Answered Question
Apr 3rd, 2009

I read in a book about OSFP that on point-to-point interfaces the routers don't form adjacencies and there is no election of DR nor BDR. Also routers that do not form adjacencies do not send LSAs (routing info) to each other. So my question is how do these routers route if they do not exchange routing info?

Correct Answer by Jon Marshall about 7 years 10 months ago

Bart


On a ethernet segment with multiple routers running OSPF is where you will see this.


I modified the lab so you have 4 routers


R1 = 192.168.10.1

R2 = 192.168.10.2

R3 - 192.168.10.3

R4 = 192.168.10.4


All routers are running OSPF. From the attached screen capture you will see


R4 = DR (Designated Router)

R3 = BDR (Backup Designated Router)


All routers form a full adjacency with these 2 devices. BUt notice that R1 & R2 are in a 2WAY/DROTHER state with each other. They do not exchange LSA's with each other and therefore do not transition to the FULL state.


Jon

Correct Answer by Jon Marshall about 7 years 10 months ago

Bart


From lab -


R1 (s1/0 - 192.168.5.1/30) -> P2P -> (192.168.5.2/30 s1/0) R2


See attached screenshot


Jon



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Jon Marshall Fri, 04/03/2009 - 15:37

Bart


You are correct in that a DR/BDR is not elected but the 2 routers at either end of the P2P link will be become adjacent so i'm not sure what the book you read is referring to.


Jon

bsudol79p Fri, 04/03/2009 - 15:42

CCNA: Cisco Certified Network Associate Study Guide 5 Edition (640-801)by Tom Lammle

Jon Marshall Fri, 04/03/2009 - 15:44

Bart


I don't have that book but in the general case 2 routers on a P2P link will form an adjacency and exchange LSA's.


Jon

viyuan700 Fri, 04/03/2009 - 15:40

Book is right that there will be no election of DR & BDR as it is not required in point to point.


"OSFP that on point-to-point interfaces the routers don't form adjacencies "


Is this written under some specific case?

bsudol79p Fri, 04/03/2009 - 15:53

No this is not for a specific case. It states it in a general.OK so if they do not form adjacencies how do they exchange routing info? I am taking to CCNA soon and I would like to get a clarification. Thanks for your help in advance!!

Jon Marshall Fri, 04/03/2009 - 15:57

Bart


They do form adjancencies and that is how they exchange LSA.



Vishwamurti is, i believe, saying the same thing as me. He just quoted you from your original post.


Jon

bsudol79p Fri, 04/03/2009 - 15:59

OK, I will have to look in the book and find it exactly. I will post whatever it says. Thanks for the help.

bsudol79p Mon, 04/06/2009 - 07:39

Jon, I misread the book, what it said that just because the OSPF routers are neighbors it will not necessarily make them adjacent. Would you be able to give me a scenerio where this would be the case? Thanks again!!!

Correct Answer
Jon Marshall Mon, 04/06/2009 - 09:40

Bart


On a ethernet segment with multiple routers running OSPF is where you will see this.


I modified the lab so you have 4 routers


R1 = 192.168.10.1

R2 = 192.168.10.2

R3 - 192.168.10.3

R4 = 192.168.10.4


All routers are running OSPF. From the attached screen capture you will see


R4 = DR (Designated Router)

R3 = BDR (Backup Designated Router)


All routers form a full adjacency with these 2 devices. BUt notice that R1 & R2 are in a 2WAY/DROTHER state with each other. They do not exchange LSA's with each other and therefore do not transition to the FULL state.


Jon

bsudol79p Mon, 04/06/2009 - 10:02

Thanks for your help again. I wanted to make sure I understand this before taking the CCNA exam and thanks to you now I do.

lamav Fri, 04/03/2009 - 16:00

Hi:


Why dont you type what it says in the book on here so we can read it, too?

prakadeesh Mon, 04/06/2009 - 00:43

On P2P links the adjacency is formed, only DR/BDR elections dont take place. FULL adjacency means that LSA is exchanged.


The only difference is between HDLC and PPP encap using IP unnumbered interface.

PPP will inject the loopback on to as a directly connected route, whereas HDLC sees it as a route via OSPF.


This is my understanding, Please correct me if I am wrong.

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