We've received a /48 allocation from ARIN, and want to advertise smaller blocks from different sites in our organization. I tried advertising a /56 from each site, but ISP is telling me they won't advertise anything smaller than a /48. So each site would have to have its own /48.
Has anyone else run into a similar issue with their ISP?
Yes. Nobody will advertise any network smaller than a /48. The same thing is in IPv4; no ISP will advertise any network smaller than a /24.
The problem is the sheer number of routes that would generate on the Internet by advertising networks smaller than a /48. Even at /48 with the current global address range of 2000::/3, that translates to 35,184,372,088,832 possible routes.
In fact, IANA had a rule that nobody (even individuals) should get networks smaller than /48. The ISPs eventually overruled that for individuals, giving them /56. Each site is supposed to have at least a /56, and a company gets at least a /48 which the company can advertise. If you wish to advertise individual sites, they each need a /48 (or a /56 out of separate /48s).
You can go to the RIR and get a smaller mask. For instance, we got a /32 from each RIR.