196
Figure 78
MED attribute
D = 9.0.0.0
Next_hop = 2.1.1.1
MED = 0
D = 9.0.0.0
Next_hop = 3.1.1.1
MED = 100
MED = 0
Router B
Router A
Router C
Router D
2.1.1.1
3.1.1.1
MED = 100
AS 20
AS 10
9.0.0.0
EBGP
EBGP
IBGP
IBGP
IBGP
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.
5.
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
, traffic
from AS20 to AS10 travels through Router C that is selected according to LOCAL_PREF.
Figure 79
LOCAL_PREF attribute
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 (for example, determining whether to advertise the route and the scope for advertising the route)
