R0106-HP MSR Router Series Layer 3 - IP Routing Configuration Guide(V7)

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a routing policy to control BGP route selection by modifying the AS_PATH length. For more
information about routing policy, see "Configuring routing policies."
{ Filter routes—By using an AS path list, you can filter routes based on AS numbers contained in
the AS_PATH attribute. For more information about AS path list, see "Configuring routing
policies."
• NEXT_HOP
The NEXT_HOP attribute may not be the IP address of a directly-connected router. Its value is
determined as follows:
{ When a BGP speaker advertises a self-originated route to a BGP peer, it sets the address of the
sending interface as the NEXT_HOP.
{ When a BGP speaker sends a received route to an EBGP peer, it sets the address of the sending
interface as the NEXT_HOP.
{ 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 at
tribute
• 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 with the same destination but with different next
hops, it considers the route with the smallest MED value the optimal route if other conditions are the
same. As shown in Figure 52, traffic from AS 10
to A
S 20 travels through Router B that is selected
according to MED.