After lot of digging in documentation and RFC's I found this-
Term “Autonomous System” was coined by inventors of very first network ARPANET. ARPANET's whole routing architecture revolved around this word "Autonomous System". Which they described as "Collection of networks controlled by a single authority"(LINK). Actually ARPANET network was the very first one which designed and implemented RIP protocol. This is why RIP don’t have any concept of multiple autonomous systems. At that time they referenced "Autonomous System" as single entity which can't be further divided.
Time passes, and there was a growing need of a protocol which addresses requirement of "multiple routing systems in same network or Autonomous System". Cisco called this concept "Autonomous(independently working) regions within an autonomous system". But to avoid confusion with the traditional meaning of autonomous system they mentioned in the documentation-
The IP implementation of IGRP has additional structure. First, the
update message is identified by an autonomous system number. This
terminology comes out of the Arpanet tradition, and has specific
meaning there. However, for most networks what it means is that you
can run several different routing systems on the same
When BGP was introduced, it provided method for Inter-Autonomous System routing. But I think they knew that there is a confusion about the meaning of "Autonomous System" in the general community. That is why in the RFC (section 1.1 Autonomous System LINK) they mentioned-
The classic definition of an Autonomous System is a set of routers
under a single technical administration, using an interior gateway
protocol (IGP) and common metrics to determine how to route packets
within the AS, and using an inter-AS routing protocol to determine how
to route packets to other ASes. Since this classic definition was
developed, it has become common for a single AS to use several IGPs
and, sometimes, several sets of metrics within an AS. The use of the
term Autonomous System stresses the fact that, even when multiple IGPs
and metrics are used, the administration of an AS appears to other
ASes to have a single coherent interior routing plan ,and presents a
consistent picture of the destinations that are reachable through it.
So, when cisco mention ASN, they are not exactly autonomous systems but merely two different networks, governed by different rules within an actual single Autonomous system. That is why in EIGRP, it is required that two ASN must have a common router to “redistribute” routing information. Because they are just two different OS processes running different instances of EIGRP and they can only communicate on OS level so there should exist a single common router.
In reality all Interior Routing Protocols works within same Autonomous System. The true multi ASN is what we see in BGP.