Hello Alexandr,

Note1:

if you refer to EIGRP autosummarization feature that is classful so it will:

summarize 10.x.x.x in 10/8 at major network boundary (Class A addresses first bit of first byte is 0 : 1-126)

summarize 172.16.x.x in 172.16/16 becuase it is a Class B major network (bits 10 in first byte)

summarize 192.168.x.0 in 192.168.x.0 because these are Class C major networks (first byte starts with binary 110...)

This comes from the first age of IPv4 where subnetting hadn't been introduced and networks were classified based on first byte value.

Note2:

you should be able to build summary routes of whatever prefix length out the interface where you configure it if you use the command for manual summarization.

You should also be able to perform supernetting by aggregating several Class C 192.168.x.0 in one entry but at least one entry has to be present in the EIGRP topology database and in the router routing table.

If you like you can provide more details on your issue in order to get better help.

Hope to help

Giuseppe