H3C S5800 Series Configuration Manual page 225

The current implementation supports using the compare-different-as-med command to force BGP to
compare MED values of routes received from different ASs.
5) LOCAL_PREF
The LOCAL_PREF attribute is exchanged between iBGP peers only, and thus 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 below, traffic
from AS20 to AS10 travels through Router C that is selected according to LOCAL_PREF.
Figure 6-9 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
6) 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 (determines whether to advertise the route and the scope for advertising the route, for
example) based on the COMMUNITY attribute values, thus to simplify routing policy usage and ease
management and maintenance. Well known community attributes involve:
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
6-8
Networks).