642-902 Sample Question

QUESTION NO : 14

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.

Answer: F

Explanation:

The minimum configuration that guarantees you won't become a transit AS is shown in the following example:
router bgp 65000neighbor 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.