At 1:44 PM +0000 12/2/02, Peter van Oene wrote: > >A general concept in routing is to always prefer information from the >most accurate source. In Link State routing, a given router always has >the most accurate information about the area itself, and thus will >always prefer information derived from there. This mechanism also >prevents loops. > > >Issues like these often occur in OSPF. Pat Murphy, in his NSSA drafts, >refers to this phenomenon as "hijacking". It is good to keep in mind >that this only produces sub-optimalities, not routing instabilities. >However, all routing impementations can be prone to sub-optimal routing >if you do not optimally design the topology. BGP confederations often >suffer from this as the length of the AS-Confed-Sequence is not used in >the BGP path selection algorithm. > > > >> Given the topology of area 0, little might be possible in avoiding > > the sub-optimal routing > > > >As hopefully I've pointed out, there really isn't a way in OSPF to iron >out all the potential for sub-optimality that a given topology might >present. It is incumbent upon the designer to understand and design >around, or live with these issues. > >
Good points, Peter. I think one real-world nuance that doesn't come through in most routing classes is that people often worry too much about suboptimal routing and too little about instability. With today's transmission technologies and router speeds, you can often solve the first by throwing bandwidth at the problem and "neutralizing" the suboptimality. Instability, however, can kill your network, and, if on the Internet, other networks. It's my impression that the CCIE test strategy overemphasizes optimality and underemphasizes stability. Certainly, some of the major reasons to use hierarchical redistribution, blackhole routes, minimizing information sent to routers, etc., are stability -- and these techniques are usually forbidden in the lab. Sometimes I wonder if there needs to be a book or seminar, "OK, now you are a CCIE. This is what you need to unlearn." ;-) Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=58397&t=57990 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
