H3C Technologies H3C S12500 Series Switches - Manual (Page 242)

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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