cancel
Showing results for 
Search instead for 
Did you mean: 
cancel
932
Views
5
Helpful
16
Replies

OSPF confusion

bsudol79p
Level 1
Level 1

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

Bart

From lab -

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

See attached screenshot

Jon

View solution in original post

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

View solution in original post

16 Replies 16

Jon Marshall
Hall of Fame
Hall of Fame

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

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

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
Level 5
Level 5

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?

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!!

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

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

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

Bart

From lab -

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

See attached screenshot

Jon

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!!!

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

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.

Bart

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

Jon

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

Jon

Getting Started

Find answers to your questions by entering keywords or phrases in the Search bar above. New here? Use these resources to familiarize yourself with the community: