I was wondering if there were any issues to consider in using a single AS to advertise different subnets from various sites around the globe. e.g. say you had AS 1888 with address block 184.108.40.206/24 advertised from London office, AS 1888 220.127.116.11/24 advertised from Paris, AS 1888 18.104.22.168/24 advertised from Barcelona. Each site with its own ISP. Are there any downsides or special considerations to that approach as opposed to using a different AS for each site?
You'll only face one problem, which is that for example the routes of the Paris site will travel over the internet to reach Barcelona site, but the problem is that Barcelona site will drop the routes since your AS is in the AS_PATH (since you are using the same AS at all sites), but this can be worked around simply by doing iBGP over GRE between your sites (plus of course the eBGP with the local provider), another option would be to use the bgp bestpath as-path ignore command but i don't think that it is recommended as it might introduce routing problems.