We have a /21 and a /22 which we advertise out of our data centres in the UK via a variety of tier1's for redundancy. In the USA we use a subnet owned by CL, which we can only advertise via CL and thus have no carrier resiliency.
The current plan is to link up our UK and USA operations via a vpls/mpls, re-IP our USA operations out of a /24 contained within our /22.
What we dont want is for external traffic from the UK destined to our USA based /24 to carry across the vpls circuits as bandwidth is expensive and we can to keep the capacity for internal operations. So the plan was to amend what we advertise in the UK to be a /21, /24, and a /23, ommitting the /24 we want to use in USA. Then find an IP transit provider with whom we can peer with in the USA to advertise the /24 out to the world.
Been thinking about it though and the step of ammending our advertisements in the UK seems like a redundant step. If we advertise the /24 out of the USA and keep advertising the /22 in the UK, then externally traffic destined for the USA /24 so should go direct there via the transit provider anyway as its a more exact route.
Globally the internet will see a /22 reachable via GTT and a /24 reachable by whoever we choose in the USA, so traffic will go that way anyway?
Or ive got this totally wrong, and before more specific routes are even taken into consideration the BGP best path selection algorithm comes into play and the UK traffic will come via our UK DC due to the shorter AS path attribute?
Is my assumption correct or am i missing something out?
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