Hi
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.
Regards,
Baldur
Den 19/10/2016 kl. 18.27 skrev Martin T:
Hi,
I made a drawing of those two best solutions: http://i.imgur.com/7NQVgUH.png
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 <o...@delong.com> wrote:
On Oct 10, 2016, at 14:59 , Baldur Norddahl <baldur.nordd...@gmail.com> 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
