Page "Border Gateway Protocol" Paragraph 60

The LOCAL_PREFERENCE, WEIGHT, and other criteria can be manipulated by local configuration and software capabilities.

Such manipulation is outside the scope of the standard but is commonly used.

For example the COMMUNITY attribute ( see below ) is not directly used by the BGP selection process.

The BGP neighbor process however can have a rule to set LOCAL_PREFERENCE or another factor based on a manually programmed rule to set the attribute if the COMMUNITY value matches some pattern matching criterion.

If the route was learned from an external peer the per-neighbor BGP process computes a LOCAL_PREFERENCE value from local policy rules and then compares the LOCAL_PREFERENCE of all routes from the neighbor.