R3303-HP HSR6800 Routers Layer 3 - IP Routing Configuration Guide

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passing AS 40 for sending data to the destination 8.0.0.0. In some applications, you can apply
a routing policy to control BGP route selection by modifying the AS_PATH length.
{ Filter routes—By configuring an AS path filtering list, you can filter routes based on AS numbers
contained in the AS_PATH attribute. For more information about routing policies and AS path
filtering lists, see "Configuring routing policies."
• NEXT_HOP
The NEXT_HOP attribute is not necessarily 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 information about load balancing, see "BGP load
balanc
ing."
Figure 60 NEXT_HOP attribute
• MED (Multi-Exit-Discriminator)
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 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
from different EBGP peers, it considers the route with the smallest MED value the best route given
that other conditions are the same. As shown in Figure 61, traffic
from AS 10 to AS 20 travels
through Router B that is selected according to MED.