What technique should be used on BGP edge routers to prevent a multi-homed autonomous
system from becoming a transit system?

A.
Advertise with a high MED value all networks that are discovered via external BGP.

B.
Set the no-export community attribute on all networks that are advertised externally.

C.
Remove the AS-Path information on all routes in the BGP table prior to advertising
externally.

D.
Set the origin code to incomplete for all networks that are discovered via external BGP.

E.
Only advertise networks externally if they have been discovered via internal BGP.

F.
Use an outgoing distribution list to filter all networks not originating from inside the
autonomous system.

Explanation:
The minimum configuration that guarantees you won’t become a transit AS is shown in the
following example:
router bgp 65000
neighbor 10.1.1.1 filter-list 1 out
!
ip as-path access-list 1 permit ^$In this example, the outgoing filter list says that all traffic not
sourced from the local AS, should not be advertised. This will prevent the announcement of
routes that originated from other Autonomous Systems, and prevent traffic destined for
networks in another AS from going through your AS. This is useful real-world information to
know and if you configure your network with BGP in a multi-homed environmet, odds are
good that you will want to implement this kind of filter.