3Com S7902E, Configuration Manual

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1-7
Figure 1-7 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 1-8 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.
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