[Still trying to get through the filters]
"Autonomous System," as used in IGPs, does not really match its
definition in current exterior routing documents. The definitive
definitions are in exterior routing documents:
RFC 1930 (Guidelines for Use of an AS) describes an AS as a set of
addresses (and, by association, routers), under one or more
administrations, that present a common routing policy to the internet.
RFC 1771, the BGP specification, says
"the classic definition of an Autonomous System is a set of routers under
a single technical administration, using an interior gateway protocol
and common metrics to route packets within the AS, and using an
exterior gateway protocol to route packets to other ASs. Since this
classic definition was developed, it has become common for a single
AS to use several interior gateway protocols and sometimes several
sets of metrics within an AS. The use of the term Autonomous System
here stresses the fact that, even when multiple IGPs and metrics are
used, the administration of an AS appears to other ASs to have a
single coherent interior routing plan and presents a consistent
picture of what destinations are reachable through it."
OSPF and EIGRP consider an AS a set of addresses and routers under a
single administration. More current practice tends to call this a
routing realm or a routing domain. Cisco training materials does not
necessarily use the current usage. (E)IGRP has the notion that even
though routing domains may be under the control of the same
administrator, the domains will not automatically exchange routing
information unless they have the same AS number.
OSPF and ISIS are not explicitly concerned with AS numbers, but both
have a concept of external routes. External routes are any that are
not generated by the local dynamic routing domain (i.e., a backbone
and a set of subordinate ares). OSPF and ISIS need to know what is
external to them, but not what number is associated with the
esternals.
You can have more than one OSPF routing domain on a Cisco router; the
purpose of the process ID is to keep the domains separate except for
explicit redistribution. All the domains, incidentally, do submit
routes to the main routing table.
OSPF and ISIS do have additional concepts of scopes of internal
routes. Conceptually, an internal route stays in its own area. In
actuality, OSPF intra-area routes will propagate outside the
originating area to other non-stub areas and area 0.0.0.0.
Standard ISIS level 1 routes only propagate into the backbone. There
are new extensions that allow controlled leakage of ISIS level 2
routes into nonbackbone areas, but otherwise, ISIS nonbackbone areas
behave like OSPF totally stubby areas.
>
>On 7 Nov 2000 11:47:41 -0500, [EMAIL PROTECTED] ("Shaw,
>Winston Mr.") wrote:
>
> >The concept of an Autonomous System is carried in the AREA(S) used by OSPF.
OSPF's external relationships are not particularly associated with
areas. You can reasonably argue that OSPF assumes that an AS is a
set of areas (i.e., a single area, or an area 0.0.0.0 and some number
of nonzero areas), which exchange routing information. Externals
come from redistribution or default information origination.
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