HP 6125G Configuration Manual page 133

In general, BGP compares MEDs of routes received from the same AS only.
NOTE:
The current implementation supports using the compare-different-as-med command to force BGP to
compare MED values of routes received from different ASs.
LOCAL_PREF
•
The LOCAL_PREF attribute is exchanged between IBGP peers only; therefore, it is not advertised to
any other AS. It indicates the priority of a BGP router.
LOCAL_PREF is used to determine the best route for traffic leaving the local AS. When a BGP router
obtains from several IBGP peers multiple routes to the same destination, but with different next
hops, it considers the route with the highest LOCAL_PREF value as the best route. As shown
in Figure 51, traffic from AS20 to AS10 travels through Router C that is selected according to
LOCAL_PREF.
Figure 51 LOCAL_PREF attribute
Local_pref = 100
EBGP
8.0.0.0
2.1.1.1
Router A
EBGP
3.1.1.1
AS 10
Local_pref = 200
COMMUNITY
•
The COMMUNITY attribute is a group of specific data. A route can carry one or more
COMMUNITY attribute values (each of which is represented by a four-byte integer). The receiving
router processes the route (for example, determining whether to advertise the route and the scope
for advertising the route) based on the COMMUNITY attribute values. This simplifies routing policy
usage and facilitates management and maintenance. Well-known community attributes are as
follows:
INTERNET—By default, all routes belong to the Internet community. Routes with this attribute
can be advertised to all BGP peers.
NO_EXPORT—After received, routes with this attribute cannot be advertised out the local AS or
out the local confederation, but can be advertised to other sub-ASs in the confederation. For
confederation information, see
NO_ADVERTISE—After received, routes with this attribute cannot be advertised to other BGP
peers.
NO_EXPORT_SUBCONFED—After received, routes with this attribute cannot be advertised out
the local AS or other ASs in the local confederation.
Router B
IBGP
Next_hop = 2.1.1.1
Local_pref = 100
Router D
IBGP
D = 8.0.0.0
Next_hop = 3.1.1.1
IBGP
Local_pref = 200
AS 20
Router C
"Settlements for problems in large scale BGP
123
networks."