Configuring the as_path attribute – H3C Technologies H3C S12500 Series Switches User Manual
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Configuring the AS_PATH attribute
Permitting local AS number to appear in routes from a peer or peer group
In general, BGP checks whether the AS_PATH attribute of a route from a peer contains the local AS
number. If so, it discards the route to avoid routing loops.
However, in certain network environments (a Hub&Spoke network in MPLS L3VPN, for example), the
AS_PATH attribute of a route from a peer must be allowed to contain the local AS number. Otherwise, the
route cannot be advertised correctly.
To permit local AS number to appear in routes from a peer or peer group and specify the appearance
times.
Step Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Enter BGP view or BGP-VPN
instance view.
•
Enter BGP view:
bgp as-number
•
Enter BGP-VPN instance view:
a.
bgp as-number
b.
ipv4-family vpn-instance
vpn-instance-name
Use either method.
3.
Permit local AS number to
appear in routes from a peer
or peer group and specify the
appearance times.
peer { group-name | ip-address }
allow-as-loop [ number ]
By default, the local AS number is
not allowed to appear in routes
from a peer or peer group.
Disabling BGP from considering AS_PATH during best route selection
Step Command
Remarks
1.
Enter system view.
system-view
N/A
2.
Enter BGP view.
bgp as-number
N/A
3.
Disable BGP from considering
AS_PATH during best route
selection.
bestroute as-path-neglect
By default, BGP considers
AS_PATH during best route
selection.
Specifying a fake AS number for a peer or peer group
When Router A in AS 2 is moved to AS 3, you can configure Router A to specify a fake AS number of 2
for created connections to EBGP peers or peer groups. In this way, these EBGP peers still think Router A
is in AS 2 and need not change their configurations. This feature ensures uninterrupted BGP services.
To specify a fake AS number for a peer or peer group:
Step Command
Remarks
1.
Enter system view.
system-view
N/A