LOCAL_PREF is a well-known discretionary attribute that shall be included in all UPDATE messages that a given BGP speaker sends to the other BGP speakers located in its own autonomous system. A BGP speaker shall calculate the degree of preference for each external route and include the degree of preference when advertising a route to its internal peers. The higher degree of preference should be preferred. A BGP speaker shall use the degree of preference learned via LOCAL_PREF in its decision process (see section 9.1.1).
A BGP speaker shall not include this attribute in UPDATE messages that it sends to BGP speakers located in a neighboring autonomous system. If it is contained in an UPDATE message that is received from a BGP speaker which is not located in the same autonomous system as the receiving speaker, then this attribute shall be ignored by the receiving speaker.