H3C Technologies H3C SR8800 - Manual (Page 216)

200
Figure 78 MED attribute
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, and 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 79 , traffic from AS20 to AS10 travels through Router C that is selected according to
LOCAL_PREF.
Figure 79 LOCAL_PREF attribute
• 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
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
EBGP
Router B
Router A
Router C
Router D
D = 8.0.0.0
Next_hop = 3.1.1.1
Local_pref = 200
IBGP
IBGP
IBGP
EBGP
2.1.1.1
8.0.0.0
Local_pref = 100
Next_hop = 2.1.1.1
Local_pref = 100
Local_pref = 200
3.1.1.1
AS 20
AS 10