BGP Overview 371
configured, the NEXT_HOP attribute will be modified. For load-balancing
information, refer to “BGP Route Selection” on page 372.
Figure 121 NEXT_HOP attribute
4 MED (MULTI_EXIT_DISC)
The MED attribute is exchanged between two neighboring ASs, each of which
does not advertise the attribute to any other AS.
Similar with metrics used by IGP, MED is used to determine the best route for
traffic going into an AS. When a BGP router obtains multiple routes to the same
destination but with different next hops, it considers the route with the smallest
MED value the best route if other conditions are the same. As shown below, traffic
from AS10 to AS20 travels through Router B that is selected according to MED.
Figure 122 MED attribute
In general, BGP compares MEDs of routes to the same AS only.
n
You can use the compare-different-as-med command to force BGP to compare
MED values of routes to different ASs.
5 LOCAL_PREF
AS 100
AS 200
AS 300
8.0.0.0
D=8.0.0.0
NEXT_HOP=1.1.1.1
EBGP
1.1.1.1/24
1.1.2.1/24
EBGP
D=8.0.0.0
NEXT_HOP=1.1.2.1
IBGP
D=8.0.0.0
NEXT_HOP=1.1.2.1
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
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