| In many cases, the routing policy that
will be implemented in an AS is consistent with the policy
implemented by the ISP the AS will be connecting to. In these cases,
it is not necessary -- or even desirable -- to use BGP to exchange routing
information with the ISP. Instead, connectivity can be achieved
through a combination of static routes and default routes.
When you connect to two different
ISPs, it is frequently necessary to use BGP. Redundancy, load
sharing, and lower tariffs at particular times of the day or night
are reasons why some network administrators connect their enterprise
to two different ISPs. If you have a backup link for redundancy, you
can use a combination of static and default routes instead of BGP.
If both of these connections are active at the same time, however,
BGP is required. Also, any time your policy requirements differ from
the policy of your ISP, BGP is again required.
In the Figure, router A is
advertising a default network into the AS through a local IGP, such
as Routing Information Protocol (RIP). A static route affords
connectivity through router B to the ISP's AS. The ISP is running
BGP and is recognized by other BGP routers in the Internet.
Note: It is generally only
necessary to use BGP to connect to an ISP only when you have
different policy requirements than the ISP.
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