You can create policies that control routes, work with default routing, control specific and aggregated routes, and manipulate BGP path attributes.
Four categories of BGP path attributes exist:
Well-known mandatory attributes must be in every BGP update message.
Well-known discretionary attributes can be in a BGP update message.
Optional transitive attributes are accepted and passed to other BGP peers.
Optional non-transitive attributes can be either accepted or ignored, but must not pass along to other BGP peers.
Border routers that utilize built-in algorithms or manually configured polices to select paths use path attributes. BGP uses the following path attributes to control the path a BGP router chooses:
origin (well-known mandatory)
AS_path (well-known mandatory)
next hop (well-known mandatory)
MED attribute (optional non-transitive)
local preference (well-known discretionary)
atomic aggregate (well-known discretionary)
aggregator (optional transitive)
community (optional transitive)
For more information about path attributes in BGP updates, see Path Attributes.