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New Member

OSPF confusion

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?

2 ACCEPTED SOLUTIONS

Accepted Solutions
Hall of Fame Super Blue

Re: OSPF confusion

Bart

From lab -

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

See attached screenshot

Jon

Hall of Fame Super Blue

Re: OSPF confusion

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

16 REPLIES
Hall of Fame Super Blue

Re: OSPF confusion

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

New Member

Re: OSPF confusion

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

Hall of Fame Super Blue

Re: OSPF confusion

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

Silver

Re: OSPF confusion

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?

New Member

Re: OSPF confusion

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!!

Hall of Fame Super Blue

Re: OSPF confusion

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

New Member

Re: OSPF confusion

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

Hall of Fame Super Blue

Re: OSPF confusion

Bart

Have a look at this doc. This covers P2P with OSPF. If you look at the Verify section you will see that the 2 routers form a FULL adjacency -

http://www.cisco.com/en/US/tech/tk365/technologies_configuration_example09186a0080094057.shtml

Jon

Hall of Fame Super Blue

Re: OSPF confusion

Bart

From lab -

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

See attached screenshot

Jon

New Member

Re: OSPF confusion

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!!!

Hall of Fame Super Blue

Re: OSPF confusion

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

New Member

Re: OSPF confusion

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.

Hall of Fame Super Blue

Re: OSPF confusion

Bart

No problem and good luck with your CCNA when you take it.

Jon

Hall of Fame Super Blue

Re: OSPF confusion

Apologies, forgot to attach the screen capture - here it is.

Jon

Blue

Re: OSPF confusion

Hi:

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

New Member

Re: OSPF confusion

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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