OSPF multi-level hierarchy: Necessary at all?

Alex P. Rudnev alex at Relcom.EU.net
Fri May 28 19:11:42 UTC 1999

I know. I meant _things changed and old estimations should be changed 
too_. Not more.

And then, if you don't include dialup interfaces into OSPF, but use 
static leased lines instead, why do you think the information changes 
frequently. Interface changes caused by failures, maintanance, 
reconfigurations. Ask any network admin how frequent are this events. 
Once/week or once/months. 

Through if we like to distribute information about dynamic dialup links - 
yes, you need some aggregation. If you are talking about the _huge_ 
events (such as cutting of the whole region from the flate network) - yes.

For example, if I exclude the OSPF_ASE announces of dialup addresses 
here, I'll see not more than 1 LSA change/1hour. Once a day some link can 
fail, or once/hour some engeneer can turn off some equipment. If there is 
100 POP's we'll see 100 such events/hour.

I am not saying flat network is good, not. But the troubles are 
exaggerating. If you simplyt get 5000 CISCO routers, configure them as

 router osfp 1
 network area 0
 interface Ethernet0/0
 ip address xxxx
 interface Serial1/0
 ip unnimbered Ethernet0/0
 interface Serial4/8
 ip unnimbered Ethernet0/0

connect all this routers and turn them on, it'll work. It's not the thing 
I recommend to do, but when someone say _you need areas for more than 100 
routers OSPF domain_, I am not sure it's true. If someone say _you need 
aggregation if you want to flood diaup addresses from 10,000 modem's POP 
over the 100 POP's network_ it's true, but the areas are not only 
aggregation mechod.

Through I like the idea of hierarchical routing system when someone can 
built AREA-0, then built another AREA-0 network, then interconnect them 
by AREA-1 backbone, etc etc. Btw, this means OSPF was not good designed 
because every AREA in it know _I am an area_, through it's better if it 
image itself as backbone (-:)). And there is administrating issues over 
it - it's important to allow different peoples control different parts of 
the network.

But please stop talking about _a lot of interface changes_ etc withouth 
referencing to the type of network. In case of leased-links network I can 
built the network of ANY size; and limitations depends of the link 
stability, not of the network size. 

And they depend of the administrative issues. If you have 3 branches, you 
(may be) have 3 sysadmins, and it's better if they don't bother about the 
whole network at all. And the back-bone admin don't like to bother about 
the areas. 

On Fri, 28 May 1999, Jessica Yu wrote:

> Date: Fri, 28 May 1999 14:51:40 -0400
> From: Jessica Yu <jyy at ans.net>
> To: "Alex P. Rudnev" <alex at Relcom.EU.net>
> Cc: Alex Zinin <zinin at amt.ru>, Vadim Antonov <avg at kotovnik.com>,
>     nanog at merit.edu
> Subject: Re: OSPF multi-level hierarchy: Necessary at all? 
> >Btw. Flat network...
> >
> >Routers now have - 128 or 256 MB RAM, 300 - 400 Mhz CPU. I guess you can 
> >built flat betwork with 5,000 routers withouth hard problems.
> With a network this large, probability of any router interface going down, 
> link flapping or rotuer failure is high so it can happen frequently.  With 
> one giant flat IGP area, any such incident occuring anywhere in the network 
> will cause network wide LSA flooding, SPF calculation and link database update.
> Lots of network resources including router CPU will be consumed.  Even one 
> has a lot of cpu power in super routers, it's still a good idea to prevent an 
> outage anywhere in the network from causing network wide event i.e. to 
> localize the problem.  A giant flat IGP area in a large network does not help 
> achieving fault isolation.
> 						--Jessica

Aleksei Roudnev, Network Operations Center, Relcom, Moscow
(+7 095) 194-19-95 (Network Operations Center Hot Line),(+7 095) 230-41-41, N 13729 (pager)
(+7 095) 196-72-12 (Support), (+7 095) 194-33-28 (Fax)

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