One clarification below. On Mon, 2002-11-11 at 21:51, The Long and Winding Road wrote: > a couple of things - in line below > > > > ""bergenpeak"" wrote in message > news:200211120028.AAA03239@;groupstudy.com... > > Suppose I have several routers making up an iBGP mesh. Lets > > suppose I have two routers (R1 and R2) which are advertising the same > > set of networks: N1, N2, ... Nk. > > > > OSPF is running underneath BGP (assume area 0). All of the N > > networks are being advertised with a next-hop set to the respective > > loopback's from R1 and R2. > > > > Now consider some other BGP router in the network. It will have > > received a BGP announcement for each of N1, N2, .. Nk from R1 and R2. > > > > This third router will select one of the paths to N1, N2, etc. > > and insert it into the routing table. I'd expect to see something > > like: > > > > subnet next-hop > > --- --- > > N1 R1-lo0 > > N2 R1-lo0 > > ... ... > > Nk R1-lo0 > > > > R1-lo0 > > R2-lo0 > > > > Now, suppose R1 goes belly up. OSPF will quickly inform all > > other routers that R1 and its loopback no longer exist. I'm assuming > > that this will invalidate all the routes in the routing table which > > have R1-lo0 as next hop. This will therefore cause the removal of all > > occurences of routes to N1, N2, ... Nk from the routing table. > > > > The question is this: what event will trigger BGP to re-evaluate > > the routes it knows about and add in routes for N1, N2, ... Nk via > > R2-lo0? Will the removal of the N1 route from the routing table > > inform BGP to re-evaluate? Or will the BGP timers need to timeout > > and detect that R1 is dead before re-evaluating? > > > > detecting a link down, or dead timer expired. > > > > One other question-- does "no sync" in BGP have a role here or is that > > related only to determining when to advertise a route via eBGP? > > > iBGP will not install a route into the BGP table unless it can verify > reachability. I.e. unless there is a valid path to the advertiser in the > routing table. This is "synchronization. the "no synch" command allows BGP > to bypass this validation step. in the case you mention, with full mesh, and > full IGP connectivity, "no sync" is not not necessary.
Just wanted to clarify a point that might be ambiguous to some. BGP, sync or otherwise, must verify reachability to the BGP Next-Hop of each path advertised prior to considering a path valid. This is normal BGP blackhole prevention. With sync, BGP must verify that the actual NLRI for advertised in the path are otherwise reachable. > > HTH > > > > > > > Thanks Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57284&t=57255 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
