North American Network Operators Group Date Prev | Date Next | Date Index | Thread Index | Author Index | Historical Re: [nanog] Connections among ASes (fwd)
On Thu, 29 Nov 2007 20:49:13 CST, Chengchen Hu said: > 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. Attachment:
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