R21xx-HP FlexFabric 11900 Layer 3 IP Routing Configuration Guide

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Figure 44 AS_PATH attribute
BGP uses the AS_PATH attribute to implement the following functions:
{ Avoid routing loops—A BGP router does not receive routes containing the local AS number to
avoid routing loops.
{ Affect route selection—BGP gives priority to the route with the shortest AS_PATH length if other
factors are the same. As shown in Figure 44,
the BGP router in AS 50 gives priority to the route
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. 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
poli
cies."
• 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."