H3C S7500E Series Operation Manual page 409

Operation Manual – IPv4 Routing
H3C S7500E Series Ethernet Switches
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.
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, refer to
BGP Route
Figure 5-7 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 5-8 MED attribute
Selection.
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Chapter 5 BGP Configuration