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. HTH > > Thanks Message Posted at: http://www.groupstudy.com/form/read.php?f=7&i=57263&t=57255 -------------------------------------------------- FAQ, list archives, and subscription info: http://www.groupstudy.com/list/cisco.html Report misconduct and Nondisclosure violations to [EMAIL PROTECTED]
