| In many cases, the routing policy that
is implemented in an AS is consistent with the ISP's policy. 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 connecting to two
ISPs, it is frequently necessary to use BGP. Some network
administrators connect their enterprise to different ISPs for
redundancy, load sharing, and lower tariffs at particular times
during the day or night. If you have a backup link for redundancy, you
can use a combination of static and default routes instead of BGP.
However, if both of these connections are active at the same time, BGP is required.
In addition, any time your policy requirements differ from
the policy of your ISP, BGP is required.
In the figure, router A is
advertising a default network into the AS through a local IGP, such
as 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: In general, it is
necessary to use BGP to connect to an ISP only when you have
different policy requirements than the ISP.
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