194
ï‚¡ When a BGP speaker sends a received route to an EBGP peer, it sets the address of the
sending interface as the NEXT_HOP.
ï‚¡ When a BGP speaker sends a route received from an EBGP peer to an IBGP peer, it does
not modify the NEXT_HOP attribute. If load balancing is configured, BGP modifies the
NEXT_HOP attribute for the equal-cost routes. For load balancing information, see "BGP
load balancing."
Figure 51 NEXT_HOP attribute
• MED (MULTI_EXIT_DISC)
BGP advertises the MED attribute between two neighboring ASs, each of which does not
advertise the attribute to any other AS.
Similar to metrics used by IGPs, MED is used to determine the optimal 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 as the optimal route. As shown in
Figure 52, traffic from AS 10 to AS 20 travels through Router B that is selected according to
MED.
Figure 52 MED attribute
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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