3Com MSR 50 Series Configuration Manual page 831

configured, the NEXT_HOP attribute will be modified. For load-balancing
information, refer to
Figure 237 NEXT_HOP attribute
AS 200
1.1.2.1/24
D=8.0.0.0
NEXT_HOP=1.1.2.1
4 MED (MULTI_EXIT_DISC)
The MED attribute is exchanged between two neighboring ASs, each of which will
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 AS 10 to AS 20 travels through Router B that is selected according to MED.
Figure 238 MED attribute
D=9.0.0.0
NEXT_HOP=2.1.1.1
MED=0
Router A
D=9.0.0.0
NEXT_HOP=3.1.1.1
MED=100
AS 10
In general, BGP compares MEDs of routes to the same AS only.
n
The current implementation supports using the compare-different-as-med
command to force BGP to compare MED values of routes to different ASs.
5 LOCAL_PREF
"BGP Route Selection" on page
D=8.0.0.0
NEXT_HOP=1.1.1.1
1.1.1.1/24
EBGP
EBGP
AS 300
IBGP
D=8.0.0.0
NEXT_HOP=1.1.2.1
MED=0
Router B
2.1.1.1
IBGP
EBGP
IBGP
EBGP
IBGP
3.1.1.1
Router C
MED=100
BGP Overview
832.
AS 100
8.0.0.0
9.0.0.0
Router D
AS 20
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