OSPF multi-level hierarchy: Necessary at all?


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,


First of all, which 'WG' do you mean?

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

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), 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
tricks (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
knowing router-id; it's amazing why for a few years CISCO can't implement
one simple command

router ospf 111


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

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).

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

1) There is some network. They run OSPF over it. Router CUST1 is AREA-0

2) There is some other network. They run OSPF over it. Router ISP1 is
AREA-0 router for this network too (or it is AREA-xx, not important).

I'd like to communicate this two OSPF networks by OSPF protocol, with the
distribute-list restrictions (I define router blocks I can receive and
minimal network size I'd like to receive from the CUST1), and (from my,
ISP, point of view) this customer looks as usial OSPF area. For the
customer, my AREA-0 looks as some other AREA-1. And we can do filtering
on the board and control strictly which networks can be injected - from
ISP to CUST, from CUST to ISP.

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