HP 6125G Configuration Manual page 131

ORIGIN is a well-known mandatory attribute that defines the origin of routing information (how a
route became a BGP route). This attribute has the following types:
IGP—Has the highest priority. Routes added to the BGP routing table using the network
command have the IGP attribute.
EGP—Has the second highest priority. Routes obtained via EGP have the EGP attribute.
INCOMPLETE—Has the lowest priority. The source of routes with this attribute is unknown,
which does not mean such routes are unreachable. The routes redistributed from other routing
protocols have the INCOMPLETE attribute.
• AS_PATH
AS_PATH is a well-known mandatory attribute. This attribute identifies the autonomous systems
through which routing information carried in this Update message has passed. When a route is
advertised from the local AS to another AS, each passed AS number is added into the AS_PATH
attribute, so the receiver can determine ASs to route the message back. The number of the AS
closest to the receiver's AS is leftmost, as shown in
Figure 48 AS_PATH attribute
D = 8.0.0.0
(10)
AS 20
D = 8.0.0.0
(20,10)
D = 8.0.0.0
(30,20,10)
AS 30
Generally, a BGP router does not receive routes containing the local AS number to avoid routing
loops.
NOTE:
The current implementation supports using the peer allow-as-loop command to receive routes containing
the local AS number in order to meet special requirements.
Use the AS_PATH attribute 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
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.
8.0.0.0
AS 10
D = 8.0.0.0
(10)
AS 40
D = 8.0.0.0
(40,10)
AS 50
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Figure 48:
Figure 48, the BGP router in