Subject: RE: [Ospf-wireless-design] Example comparing CDS approach to MPRapproach

From: "Henderson, Thomas R" <[email-masked]>
Date: Thu, 24 Mar 2005 12:51:35 -0800

> -----Original Message-----
> From: Richard Ogier [mailto:[email-masked]] 
> Sent: Wednesday, March 23, 2005 10:25 AM
> To: [email-masked]
> Subject: [Ospf-wireless-design] Example comparing CDS 
> approach to MPRapproach
> 
> Here is an example to show one advantage of using 
> (differential) Hellos to report 2-hop neighbor information, 
> and having routers select themselves as DRs, versus using 
> LSAs to report 2-hop neighbor information, and having routers 
> select neighbors as MPRs.
> This advantage has been discussed before, but I wanted to 
> present a clear, simple example.

Isn't this confounding two issues: i) how the 2-hop neighbor information
is propagated, and ii) whether you use SI-CDS vs. SD-CDS?  I know that
the draft-chandra uses LSAs instead of Hellos, but I believe that it
could be modified to use Hellos (according to Madhavi's recent post, it
can be).  I also understand that there may be certain advantages to
using LSAs, relating to how vendors implement the Hello and LSA
mechanisms already.

For example, googling on cisco+lsa+throttling will provide some
information on how the minLSInterval and arrival parameters can be tuned
(to values shorter than in the spec).

It seems to me that in a partial topology case, there may be an
advantage to using Hellos or other "link-local" packets, in that the
generation of new (non-link-local, or router) LSAs is more costly to the
network because they have to be propagated beyond the local
neighborhood.  That is, there may be changes in neighbor connectivity
that do not require the generation of a new router LSA if such neighbor
information can be locally propagated by other means.

Tom

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---

From: Acee Lindem <[email-masked]>
Date: Thu, 24 Mar 2005 16:39:34 -0500

>using LSAs, relating to how vendors implement the Hello and LSA
I agree that they are separate issues.

>neighborhood. 
Actaully we had modified the scope to say that upon reception the 
link-local LSA
should not be re-flooded other neighbors.

> That is, there may be changes in neighbor connectivity
We'll revisit the hello versus LSA issue.

Thanks,
Acee

---

From: Richard Ogier <[email-masked]>
Date: Thu, 24 Mar 2005 14:44:21 -0800

Acee Lindem wrote:

>>> From: Richard Ogier [mailto:[email-masked]] Sent: 
>>> Wednesday, March 23, 2005 10:25 AM
>>> Subject: [Ospf-wireless-design] Example comparing CDS approach to 
>>> MPRapproach
>>>
>>> Here is an example to show one advantage of using (differential) 
>>> Hellos to report 2-hop neighbor information, and having routers 
>>> select themselves as DRs, versus using LSAs to report 2-hop neighbor 
>>> information, and having routers select neighbors as MPRs.
>>> This advantage has been discussed before, but I wanted to present a 
>>> clear, simple example.
>>>   
>>
>>

Absolutely.  My example showed how the combination of these
two factors can increase delay, as I stated at the beginning
of my first email presenting this example.

>> (to values shorter than in the spec).

Sure. But as you (Tom) pointed out, advertising 2-hop neighbor
information in LSAs requires generating large, full-topology
LSAs that are flooded to the entire area.
So, as I pointed out, using a small MinLSInterval can increase
overhead a lot.  (But I see that you agree below that in a
partial topology case, using LSAs for 2-hop neighbor
information is costly.)

Richard

---

From: Acee Lindem <[email-masked]>
Date: Thu, 24 Mar 2005 18:01:25 -0500

Richard Ogier wrote:

>>>> select themselves as DRs, versus using LSAs to report 2-hop 
>>>> neighbor information, and having routers select neighbors as MPRs.

No - these have always been link-scoped LSAs and we added that
they should not be reflooed to neighbors. I'm not sure where the
notion of area scoping came from.

And as far as LSA size, how many relays, on the average, are
you figuring each node will have?

> partial topology case, using LSAs for 2-hop neighbor

I know you are responding to Tom but one of the advantages
of LSAs is that all the origination and backoff mechanisms are
already in place.