Subject: RE: [Isis-wg] contents of the MIB object: isisManAreaAddr

From: Radia Perlman - Boston Center for Networking <[email-masked]>
Date: Fri, 7 Jan 2000 15:42:39 -0500 (EST)

> > To be honest, [isisSysMaxAreaAddresses] is 3.  
> > It is 3 in all existing implementations, and it is 
> > a show stopper if two implementations don't agree.   
> > 

Having this be a parameter is, in my opinion, really silly. It's
one of the things that the ISO committee added, along with ID size,
that seems like unnecessary flexibility, and as Henk pointed out,
everyone in the area has to have the same values or it won't work.

The reason for area address, if I recall, is so that you can detect
accidentally merging two areas, so they have different names. In CLNP
it also told level 1 routers what prefixes should be routed via
level 1 (look at the bottom 6 ...er ID_length... bytes) or sent to
a level 2 router.

The reason it's nice to allow more than one area address is to be able
to merge or split up areas while the net's running. To merge A and B,
add B as an allowable address to A, and A to B, and then when everyone
agrees on both names, then one by one turn off one of the addresses.
There's really no reason to need 3 -- 2 suffices but 3 seemed
like a good idea at the time, maybe so you could simultaneously merge
3 areas, or perhaps it allowed you to be lazy about removing an area
address for awhile, but certainly there's no reason to
need 107 area addresses!

It would be nice to leave it fixed at 3 and remove it from the MIB, I'd
think.

Radia

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From: John Harper <[email-masked]>
Date: Fri, 07 Jan 2000 21:45:40 +0000

And in fact, it was more or less deliberate perversity on our part at the time to
make it this way. We (those of us who worked on the original design at DEC
and subsequently took it into ISO) thought the idea of having this as 
a parameter was really stupid (as Radia says), and we certainly weren't
going to waste any of our neurones figuring out how to make misconfiguration
recover gracefully, as we generally tried to do in the parts of the protocol
that we cared about. 

Why anyone would see it as a feature to be able to set this to 8 boggles
my mind. 

	"I really like this Ferrari, but Caterpillar make vehicles with 96
	wheels."
	"OK signor, in V550.1 we'll add the 96 wheel feature".

    John

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From: Mike Shand <[email-masked]>
Date: Sat, 08 Jan 2000 10:41:49 +0000

>address for awhile, but certainly there's no reason to

It was to do with DECnet Phase IV compatibility Radia. Typically an area 
would have two area addresses, a Phaseiv compatible one, and a real OSI 
one. So that makes two. Now if you want to merge/split the OSI area you 
need another one. I think that was the argument anyway.

         Mike

ps. and yes I think a value other than 3 is silly too.