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RE: OSPF multi-level hierarchy: Necessary at all?
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: http://www.ietf.org/internet-drafts/draft-yu-routing-scaling-00.txt --Jessica - ------- Forwarded Message Return-Path: [email protected] Received: from mail.nyp.ans.net (mail.nyp.ans.net [126.96.36.199]) by cannes.aa.ans.net (8.8.5/8.8.7) with ESMTP id LAA08768 for <[email protected]>; Thu, 27 May 1999 11:40:17 -0400 (EDT) Received: from segue.merit.edu (segue.merit.edu [188.8.131.52]) by mail.nyp.ans.net (8.8.7/8.8.7) with ESMTP id LAA29164; Thu, 27 May 1999 11:40:11 -0400 (EDT) Received: by segue.merit.edu (Postfix) id 7BB1044439; Thu, 27 May 1999 11:38:43 -0400 (EDT) Received: by segue.merit.edu (Postfix, from userid 56) id 5371344445; Thu, 27 May 1999 11:38:43 -0400 (EDT) Received: from amtsun.amt.ru (amtsun.amt.ru [184.108.40.206]) by segue.merit.edu (Postfix) with ESMTP id 881D144439 for <[email protected]>; Thu, 27 May 1999 11:38:35 -0400 (EDT) Received: from zinin (amtsun.amt.ru [220.127.116.11]) by amtsun.amt.ru (8.8.8/18.104.22.168) with SMTP id TAA14815 for <[email protected]>; Thu, 27 May 1999 19:38:21 +0400 (MSD) Delivered-To: [email protected] Message-Id: <[email protected]> X-Sender: [email protected] X-Mailer: Windows Eudora Pro Version 2.2 (32) Mime-Version: 1.0 Date: Thu, 27 May 1999 19:36:13 +0400 To: [email protected] From: Alex Zinin <[email protected]> Subject: OSPF multi-level hierarchy: Necessary at all? Sender: [email protected] Precedence: bulk Errors-To: [email protected] X-Loop: nanog Content-Type: text/plain; charset="us-ascii" Content-Length: 764 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] - ------- End of Forwarded Message ------- End of Forwarded Message