04-03-2009 03:30 PM - edited 03-06-2019 04:59 AM
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?
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04-06-2009 02:56 AM
Bart
From lab -
R1 (s1/0 - 192.168.5.1/30) -> P2P -> (192.168.5.2/30 s1/0) R2
See attached screenshot
Jon
04-06-2009 09:40 AM
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
04-03-2009 03:37 PM
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
04-03-2009 03:42 PM
CCNA: Cisco Certified Network Associate Study Guide 5 Edition (640-801)by Tom Lammle
04-03-2009 03:44 PM
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
04-03-2009 03:40 PM
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?
04-03-2009 03:53 PM
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!!
04-03-2009 03:57 PM
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
04-03-2009 03:59 PM
OK, I will have to look in the book and find it exactly. I will post whatever it says. Thanks for the help.
04-03-2009 04:16 PM
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
04-06-2009 02:56 AM
04-06-2009 07:39 AM
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!!!
04-06-2009 09:40 AM
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
04-06-2009 10:02 AM
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.
04-06-2009 10:03 AM
Bart
No problem and good luck with your CCNA when you take it.
Jon
04-06-2009 09:41 AM
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