Re: OSPF DBDs

Gents,

Not 100% sure that I'm correctly tracking the discussion here, but as
far as I can tell there's a slight muddling of two distinct concepts:

1. DR/BDR election
2. Master/Slave election

The former occurs only on a subset of network types (and even then
only in a subset of cases - namely when no DR/BDR already exists),
whereas the latter occurs every time two OSPF routers neighbor up. In
other words, on a p-t-p network (for example), although there won't be
a DR/BDR election, there will still be a Master/Slave election so that
the database synchronization process can move forward in an orderly
fashion.

In the below discussion, when it is said that "only the DR sets the
MASTER bit," it's would be more technically correct to say that "only
the Master sets the M/S Bit," as it has won the Master/Slave election
by virtue of its higher RID. Note that while the criteria for DR/BDR
election and Master/Slave election are essentially the same (highest
RID (assuming equal priority in the case of DR/BDR election)), it's
again important to note that DR/BDR election may or may not be
necessary (depending on both the network type and whether or not a DR/
BDR has already been elected), while Master/Slave election will always
be necessary in the formation of a new neighbor relationship,
regardless of network type and DR/BDR election status.

That last sentence was a doozy - hopefully it makes sense with a
careful reading or two. Heck, hopefully it makes sense at all! ;-)

Scott

____________________________________________
There are only 10 types of people in the world:
Those who understand binary and those who do not...

On May 16, 2011, at 2:14 , Paul Negron wrote:

> That's right. I forgot about the "INIT" bit.
>
> I'm not sure but....
> This may be an implementation issue. I see it as being a deterministic
> behavior thing since this still happens on a point-to-point link as
> well.
> Just no election of the DR.
>
> Where is Peter Lapukhov. He usually pretty good at explaining some
> of these
> details.
>
> Paul
> --
> Paul Negron
> CCIE# 14856 CCSI# 22752
> Senior Technical Instructor
> www.micronicstraining.com
>
>
>
>> From: <jnhdny_at_gmail.com>
>> Reply-To: <jnhdny_at_gmail.com>
>> Date: Mon, 16 May 2011 18:25:47 +0000
>> To: Paul Negron <negron.paul_at_gmail.com>, <ccielab_at_groupstudy.com>
>> Subject: Re: OSPF DBDs
>>
>> I think that's right Paul. Was looking at a capture and trying to
>> make sense
>> of it all. The INIT, MORE, and MASTER bit are set in the first two
>> DBDs (one
>> by each router). Subsequently, only the DR sets the MASTER bit.
>> Why is the election done at this time, when information needed to
>> decide
>> DR/BDR is contained in Hello packets?
>>
>>
>> Sent from my BlackBerry wireless device from MTN
>>
>> -----Original Message-----
>> From: Paul Negron <negron.paul_at_gmail.com>
>> Date: Mon, 16 May 2011 12:14:44
>> To: <jnhdny_at_gmail.com>; <ccielab_at_groupstudy.com>
>> Subject: Re: OSPF DBDs
>>
>> DBD's are also used to enforce the "Mastership" of an OSPF peering
>> relationship. During the "Exstart" phase of the peering, OSPF sends
>> DBD's to
>> enforce a Master and Slave state for peers that will Start the
>> Exchange.
>>
>> The "Master" bit and the "More Bit" are sent initially by both
>> peers in a
>> DBD but will negotiate based on the higher router-id. The peer with
>> the
>> higher router-id has its "More" bit and "Master" bit remain set.
>> The lower
>> router-id does not set the "Master" bit on the next DBD just prior to
>> exchanging LSA's.
>>
>> If this is seen as not being correct by another CCIE or CCIE
>> Candadite.
>> Please let me know. I am always looking for different explanations
>> or views
>> or correction if NEEDED.
>>
>>
>> Paul
>> --
>> Paul Negron
>> CCIE# 14856 CCSI# 22752
>> Senior Technical Instructor
>> www.micronicstraining.com
>>
>>
>>
>>> From: <jnhdny_at_gmail.com>
>>> Reply-To: <jnhdny_at_gmail.com>
>>> Date: Mon, 16 May 2011 17:45:33 +0000
>>> To: <ccielab_at_groupstudy.com>
>>> Subject: OSPF DBDs
>>>
>>> Please, does anyone know *why* DBDs have to be exchanged before
>>> actual LS
>>> requests and updates are sent? Why not just send all the LSAs in
>>> the database
>>> to the neighbour?
>>> Sent from my BlackBerry wireless device from MTN
>>>
>>>
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>
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Received on Mon May 16 2011 - 20:04:34 ART