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EIGRP is a CISCO proprietary protocol and a type of Interior Gateway Protocol, which replaced now obsolete Interior Gateway Routing Protocol. This protocol like every other interior routing protocol, is designed to work within a same "Autonomous System". If that is the case, why it has concept of configuration on multiple Autonomous systems with their Autonomous System Number (ASN)?

Isn't that violates it's core principal?

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  • The core principle of EIGRP is to share routing information with neighboring routers. Other IGPs also have the same AS concept, e.g. OSPF.
    – Ron Maupin
    Dec 19 '19 at 8:37
  • @RonMaupin trying to think of a routing protocol that doesn’t, IS-IS is a bit different but most people dont know it well, I just happen to love it :) Dec 19 '19 at 12:56
  • I think you are confusing the AS concept with the ASes assigned by the RIRs for BGP. A company may not be assigned an AS by its RIR, but it could have ASes inside its network. The AS being a routing domain, so it could have separate routing domains for different purposes than the ASes assigned by the RIR for different companies for BGP on the Internet.
    – Ron Maupin
    Dec 19 '19 at 18:36
  • @RonMaupin not confusing them at all, I was merely commenting that most routing protocol use AS concepts apart from IS-IS Dec 20 '19 at 4:53
  • @MattDouhan, not you, the OP.
    – Ron Maupin
    Dec 20 '19 at 7:50
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Not really. EIGRP can use multiple processes to talk to companies you have acquired, to limit updates to spread across to big areas, etc.

There are many reasons you might want to use multiple AS numbers some admin related others performance related, its just another tool in the tool box of the network engineer to know when to use them.

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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 network."(LINK).

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.

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  • Yes, but you can look at other RFCs, such as RFC 2328, OSPF Version 2, where it discusses ASes in terms of the IGP.
    – Ron Maupin
    Dec 20 '19 at 8:30
  • @RonMaupin, when you configure ospf on cisco router command is "#router ospf process-id", where process id is an integer. OSPF never advertise it as ASN, but process-id
    – some_guy
    Dec 20 '19 at 8:41
  • @RonMaupin, Also in the abstract itself of OSPF RFC it is mentioned- "It is designed to be run internal to a single Autonomous System."
    – some_guy
    Dec 20 '19 at 8:43
  • Now, you are confusing the process ID with an ASN. The collection of OSPF areas connected to a backbone area is the AS, and you can have multiple ASes in your own network. EIGRP, OSPF, IS-IS, etc. use ASes. You can always connect the ASes together to exchange routes between the ASes, but the administration and policies in one AS are separate from the other ASes, even if they belong to the same company and they exchange routes between the ASes.
    – Ron Maupin
    Dec 20 '19 at 8:47
  • EIGRP AS numbers must match in the same AS, but OSPF process IDs are only locally significant, and they do not need to match between routers in the same AS. EIGRP is not as structured as OSPF or IS-IS, which is why it uses the ASN to determine the AS, where OSPF and IS-IS use the backbone topology to distinguish between ASes.
    – Ron Maupin
    Dec 20 '19 at 8:50

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