Hello all...
Please is there any other reference (better if there is an rfc) which could explain better this...
When a classful protocol such as RIP v1 advertises redistributed routes, it must make assumptions about the subnet mask of the redistributed route, since this information is not carried in the advertisement. How each route is advertised to other RIP-speaking routers depends on how the interfaces on the border router are configured. If the router has an interface configured for a subnet of the major (classful) network of the redistributed route, the route will be advertised on only those interfaces that have the same subnet mask as the redistributed route. If there are no interfaces with the same subnet mask, the route will not be redistributed. If the router does not have any interfaces on a subnet of the major (classful) network, only the major network is advertised, not the subnet being redistributed.
Regards