ospf on ethernet

Answered Question
Mar 22nd, 2009
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Hi everybody!

1)Let say we have four router on ethernet, r1,r2,r3,r4

r1 is dr and r2 bdr.

Does neighbor relation ship exists between r3 and dr,bdr only or also between r3 and r4?

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My book shows following example about ospf routers on ethernet.

Ra is in down state, sends hello packets at 224.0.0.5

All routers receives the hellp packets and add ra as their neighbor in their neighbor list.This init state.

All routers then send hello packet at ra unicast address.

ra receives thses packets and adds them in its neighbor list.

the process continues.


2)My question once the stability is reached i.e that is routers are adjacent to dr and bdr. Do they send hello packet at unicast address to maintain neighbor relationship or they send it at 224.0.0.5 ?



thanks a lot and have a nice weekend!


Correct Answer by lamav about 8 years 5 days ago

Sarah, a DROTHER will establish and maintain adjacencies with the DR and BDR and will also track neighborships with other DROTHERS. On broadcast and point-to-point network types, Hellos are multicast to the ALLSPFRouters address of 224.0.0.5.


HTH


Victor

Correct Answer by Richard Burts about 8 years 5 days ago

All of the routers on the Ethernet send Hello packets to destination address 224.0.0.5. And all of the routers on the Ethernet become neighbors with every other router on that segment.


Victor said it and I believe that it is an important concept: there is a difference between becoming a neighbor and becoming adjacent. I believe that sometimes we talk about becoming OSPF neighbors when what we really mean is becoming adjacent. r3 and r4 in your example become neighbors with each other. But they do not become adjacent with each other.


HTH


Rick

Correct Answer by Giuseppe Larosa about 8 years 5 days ago

Hello Sarah,

OSPF terminology is very articulated.

We need to distinguish between:


- adjacency:


adjacencies are formed only with DR and BDR, this means complete DB exchanged and in sync.


-neighborship:


between two routers that are both DRother the neighbor state machine is stopped at the Two way state:

this means both DRother nodes see each other OSPF RID listed in the other node's OSPF hello.

But no DB exchange happens between them.


It is important to note this, because almost in all other routing protocols neighborship and adjacency are used as the same concept, but here in OSPF it is different.


Hope to help

Giuseppe


Correct Answer by lamav about 8 years 5 days ago

Sarah, because Im a sweetheart of a guy, I am going to answer the question too, not just give you that great link. :-)


1. In a multi-access network, each DROTHER, which is a non-DR/BDR router, establishes an adjacency with the DR and BDR. Moreover, the DR and BDR establish adjacencies with each other.


By the way, keep in mind that there is a difference between establishing a neighbor relationship by exchanging Hellos and actually establishing an adjacency and exchanging DDB packets as a result.


2. The DROTHERS communicate with the DR and BDR on 224.0.0.6, and the DR and BDR send data to the DROTHERS on 224.0.0.5.


HTH


Victor

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Correct Answer
lamav Sun, 03/22/2009 - 12:25
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Sarah, because Im a sweetheart of a guy, I am going to answer the question too, not just give you that great link. :-)


1. In a multi-access network, each DROTHER, which is a non-DR/BDR router, establishes an adjacency with the DR and BDR. Moreover, the DR and BDR establish adjacencies with each other.


By the way, keep in mind that there is a difference between establishing a neighbor relationship by exchanging Hellos and actually establishing an adjacency and exchanging DDB packets as a result.


2. The DROTHERS communicate with the DR and BDR on 224.0.0.6, and the DR and BDR send data to the DROTHERS on 224.0.0.5.


HTH


Victor

sarahr202 Sun, 03/22/2009 - 13:26
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HI Victor!


thanks for your reply.

"2. The DROTHERS communicate with the DR and BDR on 224.0.0.6, and the DR and BDR send data to the DROTHERS on 224.0.0.5."


Does it mean hello messages are not exchanged between drothers on ethernet?


Drother maintain adjacency with dr and bdr. But drother also maintain neighbor relationship with others drothers and dr and bdr on ethernet. Is it a correct concept?


thanks a lot!


Correct Answer
Giuseppe Larosa Sun, 03/22/2009 - 13:44
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Hello Sarah,

OSPF terminology is very articulated.

We need to distinguish between:


- adjacency:


adjacencies are formed only with DR and BDR, this means complete DB exchanged and in sync.


-neighborship:


between two routers that are both DRother the neighbor state machine is stopped at the Two way state:

this means both DRother nodes see each other OSPF RID listed in the other node's OSPF hello.

But no DB exchange happens between them.


It is important to note this, because almost in all other routing protocols neighborship and adjacency are used as the same concept, but here in OSPF it is different.


Hope to help

Giuseppe


Correct Answer
Richard Burts Sun, 03/22/2009 - 13:45
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All of the routers on the Ethernet send Hello packets to destination address 224.0.0.5. And all of the routers on the Ethernet become neighbors with every other router on that segment.


Victor said it and I believe that it is an important concept: there is a difference between becoming a neighbor and becoming adjacent. I believe that sometimes we talk about becoming OSPF neighbors when what we really mean is becoming adjacent. r3 and r4 in your example become neighbors with each other. But they do not become adjacent with each other.


HTH


Rick

Correct Answer
lamav Sun, 03/22/2009 - 13:53
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Sarah, a DROTHER will establish and maintain adjacencies with the DR and BDR and will also track neighborships with other DROTHERS. On broadcast and point-to-point network types, Hellos are multicast to the ALLSPFRouters address of 224.0.0.5.


HTH


Victor

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