On Wed, Oct 3, 2012 at 2:03 PM, Russ White <[email protected]> wrote: > >>> No more than if the longer prefix were filtered for any other reason >>> --such as if the provider is originating the shorter prefix, and decides >>> to filter the longer prefix for whatever local policy reason. The only >>> way to see this as "losing AS Path information," is to see the filtering >>> of any possible route, including routes lost because they aren't the >>> best path, as "losing AS Path information." >> >> With the occasional exception of leaf ASes, the longer prefix *isn't* >> filtered for any reason today. Precisely because it breaks the routing >> system. Some folks tried some filtering for a while, filtering at /21 >> based on RIR minimum allocation sizes within particular /8's but it >> didn't work out. Operationally, the current expectation is that any >> prefix /24 or shorter will be propagated throughout the system. > > There's a vast difference between filtering out everything longer than a > /21 and suppressing a /24 when an overlapping route that takes the > traffic through the same path available. Completely different concepts.
Hi Russ, How do you know that the overlapping route takes traffic through the same path? AS path != routing path and BGP is a distance-vector protocol. Your router has no reliable knowledge of the routing path more than 1 hop away. Even if you could know the routing path was the same, consider: ISP 88 10.1.0.0/16 via 2 3 10.1.1.0/24 via 2 3 (suppressed) ISP 99 10.1.0.0/16 via 4 12 62 16 2 3 10.1.1.0/24 via 4 8 9 5 3 (not suppressed) ISP 111 (customer of 88 & 99): 10.1.0.0/16 via 88 2 3 10.1.1.0/24 via 99 4 8 9 5 3 Best route was lost. But 111 can't safely suppress the /24 route because he has no knowledge of whether the /24 is reachable via 88 1 2 3. To the best of 111's knowledge, AS3 might look like this right now: 2 - 3A 5 - 3B No connection between 3A and 3B. That's right, 111 doesn't know that AS3 is a contiguous network right now. 111 doesn't even know that it's *intended* to be a contiguous network ever. The most he knows about AS3 is that it's the same organization from an administrative standpoint. So, 111 is stuck using the badly suboptimal route because 88 suppressed a route that 111 needed to choose a reasonable next hop. This is if you require exact path matching to suppress a route, which your draft does not. Trivial scenarios following your draft lose connectivity entirely. Regards, Bill Herrin -- William D. Herrin ................ [email protected] [email protected] 3005 Crane Dr. ...................... Web: <http://bill.herrin.us/> Falls Church, VA 22042-3004 _______________________________________________ GROW mailing list [email protected] https://www.ietf.org/mailman/listinfo/grow
