| An internetwork is a group of smaller,
independent networks. Each of these smaller networks may be owned
and operated by a different organization: a company, a university, a
government agency, or some other group. The Internet is one example
of a single, albeit immense, internetwork.
Not surprisingly, the operators of
these individual networks desire autonomy, or self-administration,
over their own systems. Because the routing and security policies of
one organization may conflict with the policies of another,
internetworks are divided into domains, or autonomous systems. Each
AS typically represents an independent organization and applies its
own unique routing and security policies. EGPs facilitate the
sharing of routing information between autonomous systems. (See the
Figure).
An AS is any set of routers that
share similar routing policies and operate within a single
administrative domain. An AS can be a collection of routers running
a single IGP, or it can be a collection of routers running different
protocols all belonging to one organization. In either case, the
outside world views the entire AS as a single entity.
Each AS has an identifying number,
assigned by an Internet registry or a service provider, between 1
and 65,535. AS numbers within the range, 64,512 through 65,535 are
reserved for private use (similar to RFC 1918 IP addresses). Because
of the finite number of available AS numbers, an organization must
present justification of its need before it will be assigned an AS
number.
Today, the Internet Assigned Numbers
Authority (IANA) is enforcing a policy whereby organizations that
connect to a single provider and share the provider's routing
policies use an AS number from the private pool (64,512 to 65,535).
These private AS numbers appear only within the provider's network
and are replaced by the provider's registered number upon exiting
the network. Thus, to the outside world, several individual networks
are advertised as part of one service provider's network. In
principal, this process is similar to NAT (see Chapter 2, IP
Addressing).
During the early days of the
Internet, an EGP called EGP version 3 (EGP3 - not to be confused with
EGPs in general) was used to interconnect autonomous systems.
Currently, BGP4 is the accepted standard for Internet routing and
has essentially replaced the more limited EGP3.
The following sections detail the
different types of autonomous systems: single-homed, multihomed
nontransit, and multihomed transit. In addition to defining these
three types of systems, these sections will examine BGP's role in
connecting each type of AS to an ISP.
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