H3C Technologies H3C SecPath F1000-E - Manual (Page 332)

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The AS_PATH attribute can be used for route selection and filtering. BGP gives priority to the route with
the shortest AS_PATH length if other factors are the same. As shown in the above figure, the BGP router
in AS50 gives priority to the route passing AS40 for sending data to the destination 8.0.0.0.
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 7 .
• 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.
• 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.
Figure 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 Device B that is selected according to MED.