LOCAL_PREF

LOCAL_PREF is a well-known discretionary attribute which a BGP speaker sends to other speakers within its own AS. The LOCAL_PREF attribute ranks external routes from an AS, so that internal neighbors can decide which route to take out of the AS. BGP speakers use routes with higher local preferences. The LOCAL_PREF attribute is not forwarded to BGP speakers outside the AS.

The external routers of C each assign a value for the LOCAL_PREF for routes to B. Other BGP speakers in C will choose the route with the higher local preference, in this case the one with 15, for external routes through B.