OSPF multi-level hierarchy: Necessary at all?

Jessica Yu jyy at ans.net
Thu May 27 18:45:25 UTC 1999

The main purpose of doing hierarchical IGP (OSPF or ISIS), in my view,
is to scale IGP routing and achieve fault isolation in large scaled 
networks.  Most of the large networks exit today (especially those using
IS-IS as IGP) are still having one flat IGP routing area since IS-IS simply 
does not have adequate 2-level support.  The protocol issues are being 
addressed in the IETF. I can see a move to 2-level hierarchy in the near 
future but do not see more than 2 levels would be needed soon.  In addition,
some large ISPs are moving away from ATM overlay model to MPLS, this will 
reduce IS-IS/OSPF adjacency greatly (no more full mesh topology from IGP's
point of view) thus alleviate IGP scaling issue.  Further, the complexity
of protocol and operation management will increase with the number of
levels added to the hierarchy.  In sum, the use of more than 2-level
of IGP hierarchy is less likely, at least not in the near future.

The following Internet-draft include some discussions of the topic in 
discussing routing scaling issues in general.

        Title           : Scalable Routing Design Principles
        Author(s)       : J. Yu
        Filename        : draft-yu-routing-scaling-00.txt
        Pages           : 24
        Date            : 15-Apr-99

A URL for this Internet-Draft is:


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Subject: OSPF multi-level hierarchy: Necessary at all?
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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
irc: //EFNET/#cisco, //irc.msn.com/#NetCisco [Ustas]

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