NONTRANSIT AND TRANSIT ASs
An AS need not originate and terminate all traffic that flows through it. This AS is a multihomed transit network. The term multihomed means that the AS has more than one interface to other ASs. The term transit means the traffic has a source or destination address outside the AS. The term nontransit means that the AS does not permit traffic not destined for the AS (or not originated in the AS) to pass through it.
Some intranet routing domains permit transit operations, and some do not. The decision to permit transit traffic to pass through an organization's AS is based on the organization's routing policy.
Figure 7-5 shows an example of a multihomed non-transit AS, which is AS 1, and ISP A. AS 1 advertises addresses ...
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