hello,
just some more details to what Edi already said.
auto-summary is used by default by Distance Vector routers and happens at a major network transition. If you have a router that have the DV IGP running on interfaces with ip addresses belonging to two different major networks (example 10.0.0.0/8 and 172.16.0.0/16) when preparing the updates to send out an interface that is part of major network 2 it will send out the classful major network instead of the known subnets in major network 1 and viceversa out of an interface thar is part of major network 1 it will send out a single entry major_net2.
This behaviour can be a problem if you have used subnets of a major network on different remote sites (for example you use some 10.x.y.0 prefixes on the client lan and you use on the wan links subnets in class 172.16.x.y) and you want from the central site to know the details for correct forwarding.
RIPv2 and EIGRP allows to disable auto-summary feature.
Old protocols like RIPv1 and IGRP have the problem that they do not send the prefix mask in their updates.
Hope to help
Giuseppe
