Supported Platforms
Understanding Private AS Number Removal from AS Paths
By default, when BGP advertises AS paths to remote systems, it includes all AS numbers, including private AS numbers. You can configure the software so that it removes private AS numbers from AS paths. Doing this is useful when any of the following circumstances are true:
- A remote AS for which you provide connectivity is multihomed, but only to the local AS.
- The remote AS does not have an officially allocated AS number.
- It is not appropriate to make the remote AS a confederation member AS of the local AS.
Most companies acquire their own AS number. Some companies also use private AS numbers to connect to their public AS network. These companies might use a different private AS number for each region in which their company does business. In any implementation, announcing a private AS number to the Internet must be avoided. Service providers can use the remove-private statement to prevent advertising private AS numbers to the Internet.
In an enterprise scenario, suppose that you have multiple AS numbers in your company, some of which are private AS numbers, and one with a public AS number. The one with a public AS number has a direct connection to the service provider. In the AS that connects directly to the service provider, you can use the remove-private statement to filter out any private AS numbers in the advertisements that are sent to the service provider.
 | Caution: Changing configuration statements that affect BGP peers, such as enabling or disabling remove-private or renaming a BGP group, resets the BGP sessions. Changes that affect BGP peers should only be made when resetting a BGP session is acceptable. |
The AS numbers are stripped from the AS path starting at the left end of the AS path (the end where AS paths have been most recently added). The routing device stops searching for private ASs when it finds the first nonprivate AS or a peer’s private AS. If the AS path contains the AS number of the external BGP (EBGP) neighbor, BGP does not remove the private AS number.
 | Note: As of Junos OS 10.0R2 and later, if there is a need to send prefixes to an EBGP peer that has an AS number that matches an AS number in the AS path, consider using the as-override statement instead of the remove-private statement. |
The operation takes place after any confederation member ASs have already been removed from the AS path, if applicable.
The software is preconfigured with knowledge of the set of AS numbers that is considered private, a range that is defined in the Internet Assigned Numbers Authority (IANA) assigned numbers document. The set of AS numbers reserved as private are in the range from 64,512 through 65,534, inclusive.
