On Fri, 08 Nov 2002 01:55:03 +0530, alok said:
take a simple scenario AS-1 , AS-2 and AS-3 and as-4
AS-2 and as-3 in the middle, as-1 and as-4 multihome on them and are on either side of as-2 and as-3..they dont peer with each other ...(though as-2 and as-3 mebbe)
as-1 advertises a network x.y.z.w via as-2 only. as-4 sees this and knows that to go back to x.y.z.w he has to go via as-2
Ahh.. but in your example, all 4 as have *SOME* route. So loose RPF would still work. Now let's consider this example: AS-1 advertises to *ONLY* as-2, and as-3 filters as-2's announcement, so they have *no* route to as-1. as-4 gets a route to as-1 via as-2. as-1 packets come in to as-3 *anyhow* on their way to as-4, and return packets go 4-2-1. This still works, as long as as-3 doesn't do loose-RPF because they'll drop the packets due to lack of a route. Of course, if any customer of as-3 wants to actually talk to as-1, you're going to be opening a trouble ticket. -- Valdis Kletnieks Computer Systems Senior Engineer Virginia Tech