H3C Technologies H3C S7500E Series Switches - Manual (Page 210)

6-7
3) NEXT_HOP
Different from IGP, the NEXT_HOP attribute may not be the IP address of a directly connected router.
It involves three types of values, as shown in Figure 6-7 .
z When advertising a self-originated route to an eBGP peer, a BGP speaker sets the NEXT_HOP
for the route to the address of its sending interface.
z When sending a received route to an eBGP peer, a BGP speaker sets the NEXT_HOP for the
route to the address of the sending interface.
z When sending a route received from an eBGP peer to an iBGP peer, a BGP speaker does not
modify the NEXT_HOP attribute. If load-balancing is configured, the NEXT_HOP attribute will be
modified. For load-balancing information, see BGP Route Selection .
Figure 6-7 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 6-8 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