Comments below....
---- On Tue, 5 Mar 2002, Hunt Lee ([EMAIL PROTECTED]) wrote: > I believe someone might have mentioned this already but since I'm > studying > it right now I thought I'd ask again... It would be greatly appreciated > if > someone can shed some light on this. > > For OSPF, I understand that a "flapping" subnet will cause LSAs to be > flooded throughout the internetwork at each state transition. However, > my > question is: > > TCP / IP Vol1 by Jeff Doyle says if a subnet is summarized by a summary > address, the subnet's instability will no longer be advertised. But if > this > is the case, then what happens if:- > > e.g. Router A advertised a summary route (advertising subnet > 172.20.10.0 > /24 to Router B. Now if a host in that subnet (say 172.20.10.1 is > bouncing) - if this instability is hidden by the summary route, does it > mean > that Router B wouldn't realized that 172.20.10.1 is flapping, and > continues > to forward packets to it? That's exactly right. Router B has no knowledge whatsoever of any hosts in this case. It is only aware of the existence of the /24 being advertised by router A. This isn't quite what Doyle is referring to, though. Let's use a different example. 172.16.1.0/24\ 172.16.2.0/24 --- RA ---(172.16.0.0/22) --- RB 172.16.3.0/24/ Not a great drawing, but here's what's going on. Router A is aware of three /24 networks and it summarizes them to a single /22 before advertising them to Router B. Typically, as long as any one of those /24 prefixes is up Router A will advertise the aggregate. Unless all three routes go down the aggregate--or summary-- gets announced, thus making Router B blissfully unaware of any flapping of individual routes. And you're right, since it would not be aware of the state of any given /24, it would continue to forward traffic for that prefix to Router A. HTH, John > > Please help... > > Best Regards, > Hunt Lee [EMAIL PROTECTED] Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=37234&t=37228 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
