HP HPE FlexNetwork 7500 series, Configuration Manual

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The ORIGIN attribute specifies the origin of BGP routes. This attribute has the following types:

IGP —Has the highest priority. Routes generated in the local AS have the IGP attribute.

EGP —Has the second highest priority. Routes obtained through EGP have the EGP
attribute.

INCOMPLETE —Has the lowest priority. The source of routes with this attribute is unknown.
Routes redistributed from other routing protocols have the INCOMPLETE attribute.
•
AS_PATH
The AS_PATH attribute identifies the ASs through which a route has passed. Before
advertising a route to another AS, BGP adds the local AS number into the AS_PATH attribute,
so the receiver can determine ASs to route the message back.
The AS_PATH attribute has the following types:

AS_SEQUENCE —Arranges AS numbers in sequence. As shown in Figure 51 , the number
of the AS closest to the receiver's AS is leftmost.

AS_SET —Arranges AS numbers randomly.
Figure 51 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 51 , 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 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.
8.0.0.0
AS 10
D = 8.0.0.0
AS_PATH = 10 D = 8.0.0.0
AS_PATH = 10
AS 20
AS 40
D = 8.0.0.0
AS_PATH = 20, 10
AS 30
AS 50
D = 8.0.0.0
AS_PATH = 30, 20, 10
D = 8.0.0.0
AS_PATH = 40, 10