Removing Private AS Numbers 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 all 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.
To have the local system strip private AS numbers from the AS path, include the remove-private statement:
For a list of hierarchy levels at which you can include this statement, see the statement summary section for this statement.
 | Caution: Changing configuration statements that affect BGP peerings, such as enabling or disabling remove-private or renaming a BGP group, resets the BGP sessions. Changes that affect BGP peerings 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. This 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.
