3Com S7906E - Switch, Configuration Manual

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1-6
Figure 1-6 AS_PATH attribute
8.0.0.0
AS 10
D = 8.0.0.0
(10)
D = 8.0.0.0
(10)
AS 20
AS 40
D = 8.0.0.0
(20,10)
AS 30
AS 50
D = 8.0.0.0
(30,20,10)
D = 8.0.0.0
(40,10)
In general, a BGP router does not receive routes containing the local AS number to avoid routing loops.
The current implementation supports using the peer allow-as-loop command to receive routes
containing the local AS number to meet special requirements.
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.
In some applications, you can apply a routing policy to control BGP route selection by modifying the
AS_PATH length.
By configuring an AS path filtering list, you can filter routes based on AS numbers contained in the
AS_PATH attribute.
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 1-7 .
z
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.
z
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.
z
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 Selection .