alaerte Vidali wrote: > > Thanks. > > The addresses are contiguous. > > Suppose a network with many ABRs, one in each city. Any big > city represents small cities. Could you use an area for each > ABR? (I am wondering if there is no limit in the number of > areas. I bet not).
Yes, you should use an area for each ABR. Well two areas per ABR, actually. As you probably know, OSPF networks have a two-layer hierarchy: the backbone and the attached areas. The topology is a logical star of areas. Think of a daisy with a center circle and petals surrounding it. Each ABR connects to Area 0 (the backbone) and at least one other area. (An ABR could connect to a couple other areas too, if that fits your needs. But an area shouldn't span 2 ABRs, which I think was what you were suggesting.) Areas are supposed to be contiguous (all in one connected piece). If links fail and render areas other than area 0 discontiguous, OSPF can handle this, but it's not how you are supposed to design it. Over the years, I have collected the following design recommendations which may be a bit old, and of course, the real answer is "it depends on router CPU and RAM, what else the router is doing, how stable routers and networks are, topological requirements, performance requirements, etc." * An area should contain no more than about 50 to 100 routers. * An OSPF autonomous system should contain no more than about 100 areas. * A router should be in at most about 3 areas. * A router should have fewer than about 60 adjacent neighbors. You could put the ABR in each large city and have each area represent a big city and the small, neighboring cities. Then your backbone could be the WAN between cities. A lot of experts recommend keeping the backbone small, fast, easy to manage, reliable, etc. however. And a large inter-city WAN might not meet those needs. So instead, make the backbone a set of routers at a central site connected via Gig Ethernet. Another alternative is just to have one area, Area 0. Are you sure you need multiple areas? > > What about put toghether two cities and form a bigger area with > two ABRs? Why? There's no reason to do this, is there? Here's a paper by Peter Welcher that might help: http://www.netcraftsmen.net/welcher/papers/ospf1.htm Also check Cisco's OSPF design guide, although the current versions has a tendency to say what you can do rather than what you should do and refuses to give any definte recommendations, saying instead things like, "For this reason, it's difficult to specify a maximum number of routers per area. Consult your local sales or system engineer for specific network design help." :-) http://www.cisco.com/warp/public/104/1.html Also check papers and books by Howard Berkowitz. In addition to 2 OSPF papers at CertificationZone, he also has a paper on scaling routing protocols. Priscilla > In that case, it would be necessary to connect these > ABR through they particular area and area 0, so the database > would be the same, right? > > Sorry about the graphics. It is difficult to draw with > characters. Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=72607&t=72587 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
