Suppose the following example. ISP A has a router A1 in IXP1 and a router A2 in
IXP2; and ISP B has a routers B1 in IXP1 and a router B2 in IXP2. It is
possible that we have DIRECT link A1A2 and B1B2 to connnect two IXPs, but I
don't think there may be DIRECT link like A1B2 or A2B1. Since it should be much
cheaper and easier for ISP A and ISP B to be connnected in the same IXP using
links like A1B1 or A2B2. Am I right?
You're quite possibly wrong - for instance, if I'm ISB B, I *might* want
to have a direct peering session between A1 and B1 - but *also* have a
connection from B1 over to the *other* router A2, for several reasons:
1) I may know that the *next* router hop after A1 has questionable reliability,
and thus I want a fall-over to A2, which has better connectivity upstream.
2) I may be able to get a second link over to A2 for "essentially free" because
I have a connection to IXP2 because I peer with *another* provider C (who I
have to connect at IXP2 because C has no presence in IXP1). At that point,
I may be able to get B1-C2 for some cost - and then B1-A2 as a backup to the
B1-A1 is almost free at some IXPs - just one interconnect across the room.
3) Due to traffic balance quirks (maybe I'm content-heavy at IXP1 and
eyeball-heavy at IXP2), I may qualify for peering at one IXP but not the
other - so if my only peering is at IXP2, I have to haul traffic from
IXP1 to IXP2 and peer there. (Yes, that *would* be odd - but I've seen
stranger stuff happen with peering..
4) Traffic engineering may indicate that doing a cross-connect may be
faster/better - if you have a lot of traffic from your AS hitting router
A1, but the *other* end is just upstream of B2, you have 2 choices:
a) dump the traffic from A1 to B1 and let them haul it to B2 (hot potato
routing). This can suck if B1-B2 is congested...
b) Put in your own link from A1 to A2/B2 - this can win if your A1-B2 is
less loaded than B1-B2 is.
I'm sure that the guys who do the traffic engineering thing for a living
can come up with even more examples why it may be different...
So in your case, all the suppliers and peers DIRECTLY connected to any one of
your routers are located in the same IXP?
In our case, we have routers inside our AS1312 - and our two main next-hops
(Level3 and NetworkVirginia) are both at the other end of many miles of fiber.