Path Attributes - HPE 5800 Series Configuration Manual

Path attributes

BGP path attributes are a group of parameters carried in update messages. They give detailed route
attributes information that can be used for route filtering and selection.
•
ORIGIN
The ORIGIN attribute identifies the origin of routing information (how a route became a BGP
route). 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 via EGP have the EGP attribute.
INCOMPLETE—Has the lowest priority. The source of routes with this attribute is unknown.
The 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 to reach the
destination. 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 AS_PATH attribute has the following two types:
AS_SEQUENCE—Arranges AS numbers in sequence. As shown in
of the AS closest to the receiver's AS is the first one listed in the AS_PATH.
AS_SET—Arranges AS numbers randomly.
Figure 57 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.
Performs route selection—BGP gives priority to the route with the shortest AS_PATH
length if other factors are the same. As shown in
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.
Figure
169
Figure 57, the number
57, the BGP router in AS 50 gives