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
A URL for this Internet-Draft is:
http://www.ietf.org/internet-drafts/draft-yu-routing-scaling-00.txt
--Jessica
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