3Com Switch 8800 Advanced Software V5 Configuration Guide

BGP Overview 425
configured, the NEXT_HOP attribute will be modified. For load-balancing
information, refer to
“BGP Route Selection” on page 426.
Figure 137 NEXT_HOP attribute
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 AS10 to AS20 travels through Router B that is selected according to MED.
Figure 138 MED attribute
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
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