>Hi,
>
>I am repeating my question, since I didn't hear from anybody, so I
>assume that I didn't ask it the right way.
>
>Here it is: If you want to connect AS 100 to AS 300 or multiple
>AS's what are different ways that we can do it? We can use BGP. But
>I saw this question asking that know "other ways" to make connection
>between different AS rather using BGP. What they could be if they
>are not EGP? I saw this question under the new objectives for
>Routing 2.0. I would be really thankful if anybody can clear my
>understanding for this Routing question.
>
>Best regards,
>Maria
>
As Bill Clinton might say, that depends on how you define "autonomous system."
If you look at RFC 1930 and the general interpretation by the
registries that assign AS numbers, a registered AS:
-- is a set of addresses (and routers)
-- under one or more administrations
-- presents a common routing policy to the Internet
-- announces that policy with BGP-4
(Additional practical guidelines from the registries)
-- connects (or will connect in 1 month) to 2 or more other AS
-- upstream connections of at least T1 speed
So, in what I think is the most pure definition, BGP-4 is the only
way to interconnect true AS.
A special case of interconnection is from a customer network with a
private AS number to an upstream provider with a public AS number.
Such connections use BGP-4, and the Internet properly sees only
announcements originating from the public AS.
Another special case is a BGP confederation, in which multiple AS
with private AS numbers communicate with BGP, and, if there are any
announcements to the general Internet, remove the private AS numbers
and show the origin as the registered AS.
Now, there are older usages of the term AS, and you will see them
associated with Cisco discussions of interior routing protocols. The
older definition essentially considers an AS:
-- a set of addresses (and routers)
-- under one administration
-- uses common internal metrics and routing policy
Under this definition, IGRP and EIGRP can create AS that do not
exchange routes without explicit redistribution. By putting IGRP and
EIGRP in the same AS number, you can get automatic redistribution.
OSPF also uses an older definition of AS, which really means the set
of addresses and routers that are in, or connect to, a single area
0.0.0.0. Even if a route comes in from another OSPF process, it must
come in through an "ASBR".
The OSPF, and generally (E)IGRP, usages of "AS" are much closer to
what the IETF would call a "routing realm" or "routing domain".
Historically, Cisco courses and tests do not use this term, although
you'll see it in Cisco contributions to the IETF.
A final comment -- an enterprise that connects to a single service
provider using static/default routing on its side is part of the
provider's AS.
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