Not sure what you mean here concerning 'unroutable' prefixes, but the issue with obtaining an allocation for one of the upstream provider's CIDR block when multihomed *does* have its drawbacks, at least from the end-user perspective. If said prefix (let's say a /24) is announced in the 'allocating' provider's aggregate, and the more specific is announced via the 'other' provider, the more specific will always be preferred.
This is brain damaged. Given AS1 ----- Sprint | | | | AS2 ----- anything else not Sprint You can not announce a bit of Sprint space AS1->AS2->MCI as a fallback (note the 'extra' AS hop) because Sprint aggregates your announcement and the longer prefix is announced to the world via <anything else>. Use Sprint space, bye bye fallback. To the best of my knowledge (which ain't that hot), all other providers have discovered suppress-map. randy