> > I do understand the reasoning behind preferring customer routes. > However in the case where a customer of a customer also connects to > you directly via peering doesn't it make sense to prefer the direct > connection? or at least not prefer the customer learned routes.
So from my experience of working at transit providers over more years than I care to contemplate I can assure you what may seem to make sense as a customer does not necessarily translate to how IP routing works. IP has no concept of Customer or Peer it is simply designed to hand the packet to a valid next hop as determined by policies. As such routes are normally divided into customer, peer or further upstream transit if you are not one of the tier 1 providers. A peer provides you no income, a customer (a customer of a customer is largely the same thing as being a direct customer). Take the example of the customer buying a transit service on a 95th percentile basis. So as a transit provider I get paid based on how much traffic I hand to that port(s) and in turn I provide connectivity to all my peers, customers and upstream transits all over the world. I can't do this for free as then I can't pay for my network and I am a company not a charity. Customer X advertises his lets say a /22 to me, all good. But then customer X advertises his /22 and some disaggregated /24's to a local peering exchange that I am also connected to. If I do not both prioritise customer X's customer port routes AND drop any more specific routes learnt from customer X then I will end up handing all customer X's outgoing traffic to them over the peering instead of the revenue generating port. I have seen customers do this both through innocent and malicious intent. Sure, there are a lot of complex policies that I might apply to accept local traffic in one area and hand other traffic via the transit port but why on earth would I do that and likely cause all sorts of other potential routing issues while reducing the revenue I am entitled to?
