OSPF multi-level hierarchy: Necessary at all?

Alex Zinin zinin at amt.ru
Thu May 27 16:57:11 UTC 1999

Hey, Alex:

At 20:23 27.05.99 +0400, you wrote:
>First of all, which 'WG' do you mean?

I mean IETF's OSPF WG, sorry :)

>Then. I can't understand from your message which kind of hierarchy do you 
>mean. OSPF have a few of different hierarchy issues - (1) two types of 
>metrics, inter and intra-area metrics (type 1 and type 2); (2) there is 2 
>level hierarchy of AREA/BACKBONE with the summarisation on the area 

OK, lemme put it the way it is now:

1. From intra-domain routing standpoint we have two levels: intra-area
    and inter-area. All routers within an area know the whole topology
    of it. On the area borders the topology is hidden, routing info summarized 
    and internal routers see summaries from all areas.

2. For routing to destinations outside an OSPF domain we have Type5-LSAs
    which can carry type 1 or type 2 routes (this is what you meant, I
    depending on whether your external metric is comparible with the OSPF 
    one or not.

>If talking about the first, I hardly imagine the situation when someone 
>is not satisfied by 2 existing metric types (except he can be unsatisfyed 
>by the calculation scheme). 


>If about the second - may be, not for ISP 
>(ISP don't use complex OSPF routing, they have a lot of headache with 
>IBGP instead),

I've heard some of them do have big enough OSPF networks :)

> but for the corporate networks. Really, why can't I have 
>any-level hierarchy for the OSPF zone - area 0, area 0.1, area 0.1.1, for 
>example (this mean - I built area-0 part; then I add some area 0.1 part - 
>first is _backbone_ in existing terms, second _area_), then if I'd like 
>to add some big part to the area 0.1, I prefere to create sub-area 0.1.1 
>(for example) instead of building virtual links and using some other 

Yeah, in this case the sub-area 0.1.1's topology would be hidden
from the 0.1's routers and vice versa.

> (moreobver, VL can't be used with CISCO's at all because CISCO 
>don't allow to control router-id directly and you can't build VL withouth 

Well, you do know that you can create loopback interfaces and the router-ID
will be the highest one among them. Say you do:

int lo 0
 ip address

It will hardly be overriden by another loopback.

>knowing router-id; it's amazing why for a few years CISCO can't implement 
>one simple command
> router ospf 111
> router-id

Actually there is a DDTS on the wish-list, but it is still not implemented for
some reason, let's hope it will be....
Derek, are you reading us ? ;))

> router-id Ethernet0
>Through I think the problem of building complex ara schemas is not 
>important for the ISP. More important is the problem of import/export - I 
>can installl BGP routing with the customer and control announces by the 
>route-map or distribute-lists; I can use RIP (I can't, but it's not 
>important) and control announces by the distribute-lists; why can't I 
>connect the customer's OSPF area (this is area-0 for HIM) to my OSPF 
>network and name his _AREA with the strict filtering on the 

I was thinking about it as well. One could configure some area range
as a "discard" one, effectively saying that all routes dropping into the
range should be ignored instead of announced in a summary-LSA.

>This is reason why ISP don't like OSPF and such protocols - they can be 
>used for the inter-router routing, but they can't be used to connect with 
>the customers (no, I can run 10 different OSPF processes and re-advertise 
>routes - one more headache for the network admins).

Actually, you can use NSSA, but doesn't allow for filtering either.

>PS. From ISP's point of view. What I'd like.

[snip: got your wish, Alex]

>3) Moreover, why can't I determine different BGP AS numbers for the boths 
>ISP and CUST OSPF zones. 

who said you can't ? or I'm missing something?


>On Thu, 27 May 1999, Alex Zinin wrote:
>> Date: Thu, 27 May 1999 19:36:13 +0400
>> From: Alex Zinin <zinin at amt.ru>
>> To: nanog at merit.edu
>> Subject: OSPF multi-level hierarchy: Necessary at all?
>> Hello, 
>> We're currently discussing necessity of multi-level hierarchy
>> in OSPF on the WG mail list.
>> The idea is to implement SPF-based interarea routing
>> with more than two levels of topology abstraction and
>> route aggregation (we have two levels in OSPF at the
>> moment level1 being intra-area routing and level2 being
>> the inter-area one).
>> I have some thoughts on how this could be done,
>> but the main question is whether there is a demand
>> for it or not.
>> Everyone is really welcome to share opinions.
>> Thanks in advance,
>> ------------------------------------------------------------------
>> Alex D. Zinin, Consultant
>> CCSI #98966
>> CCIE #4015
>> AMT Group / ISL 
>> Cisco Systems Gold Certified Partner
>> http://www.amt.ru
>> irc: //EFNET/#cisco, //irc.msn.com/#NetCisco [Ustas]
>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)
Alex D. Zinin, Consultant
CCSI #98966
CCIE #4015
AMT Group / ISL 
Cisco Systems Gold Certified Partner
irc: //EFNET/#cisco, //irc.msn.com/#NetCisco [Ustas]

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