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Aggregation replaces several consecutive prefixes with one shorter prefix. Combining 2^b equal-size networks shortens the mask by b bits, and the aggregate only advertises correctly if the first network sits on a boundary that is a multiple of the aggregate size. Any addresses inside the aggregate that you have not actually allocated are advertised anyway, which is why alignment and waste both matter.
Aggregate prefix
b = ceil(log2(member count)); aggregate prefix = member prefix - b; aggregate network = first network AND aggregate mask
The aggregate silently covers addresses that belong to someone else, which can black-hole traffic. Always verify that the first network equals the aggregate network address.
Not with a single prefix. Round up to the next power of two and accept the extra coverage, or advertise two or more aggregates of different lengths.