nested prefixes in Internet

Baldur Norddahl baldur.norddahl at
Wed Oct 19 20:29:29 UTC 2016


Solution B is what happens by default and requires no changes by any 
party. A, B and C just do what they would do in any transit relation. 
The default BGP shortest AS path length first algorithm will make sure 
that traffic is delivered correctly.

Solution A requires that ISP A actively filter the /24 announcement and 
that would have severe negative impact. It would mean that you would not 
receive any traffic at all through that link unless the link to ISP B is 
down. Not even traffic from A to B (that would go A -> C -> B because of 
the more specific). Maybe you meant that they would only filter the /24 
announcement on the peering link between A and C, but that would have no 
effect and therefore makes no sense.

Remember that ISP A is only originating their own /19. The /24 route 
announcement is received from ISP B and merely propagated (not 
originated) to ISP As uplink and peers. The moment that the link between 
A and B is down, the /24 route announcement will gone as well - instead 
A will be receiving the /24 route announcement via C.



Den 19/10/2016 kl. 18.27 skrev Martin T:
> Hi,
> I made a drawing of those two best solutions:
> As much as I understand, both solutions require no special changes
> from "ISP C". Only advantage of solution B over solution A, that I can
> see, is that at the time when link between "ISP C" and "ISP B" is up,
> the traffic from Internet towards "ISP B" prefers the "ISP C"
> connection.
> In case the link between "ISP A" and "ISP B" goes down, then traffic
> from "ISP A" addressed to this /24 will use a private peering link
> between "ISP A" and "ISP C" so the transit costs are not an issue.
> thanks,
> Martin
> On Wed, Oct 12, 2016 at 1:58 AM, Owen DeLong <owen at> wrote:
>>> On Oct 10, 2016, at 14:59 , Baldur Norddahl <baldur.norddahl at> wrote:
>>> Den 10/10/2016 kl. 22.27 skrev Owen DeLong:
>>>> Not true… There are myriad reasons that the /24 might not reach a network peered with ISP-A, including the possibility of being a downstream customer of a network peered with or buying transit from ISP-A. In the latter case, not an issue, since it’s paid transit, but in the former (peered, not transit), again, ISP-A is probably not super excited to carry traffic that someone isn’t paying them to carry.
>>> But ISP-A is in fact being paid to carry the traffic. Supposedly ISP-B has a paid transit relation to ISP-A. In the case the transit link is down ISP-A might have to transport the traffic through a less profitable link however.
>> Which isn’t really in the agreement between ISP-B and ISP-A unless it was specifically (and unusually) negotiated.
>> Also, you’re assuming that the leased space came with a transit agreement. In many cases, address leases don’t, so consider the additional scenario where ISP-B leases addresses from ISP-A, but has transit contracts with ISP-C and ISP-D but no connection at all to ISP-A.
>>> I know that if ISP-A was my network I would be making money even with the transit link down. Yes I might have to transport something out of my network through one of my transits, but outbound traffic is in fact free for us because we are heavy inbound loaded.
>> Yes, but it doesn’t help if it also came in on a transit link. Any traffic you both receive and transmit on transit costs you money pretty much no matter who you are.
>> Owen

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