3.7.b Implement and troubleshoot IBGP and EBGP

3.7.b [iii] Private AS

Private autonomous system (AS) numbers which range from 64,512 to 65,535 are used to conserve globally unique AS numbers. Globally unique AS numbers (1 – 64,511) are assigned by InterNIC. These private AS number cannot be leaked to a global Border Gateway Protocol (BGP) table because they are not unique (BGP best path calculation expects unique AS numbers). It allows the stripping of private AS numbers out of the AS_PATH list before the routes are propagated to a BGP peer.

Generally customer networks and their routing policies are an extension of the respective Internet Service Providers (ISPs). When a customer network is large, the service provider may assign an AS number using a couple of different methods in order to manage the network and routing policies.

● One way is by permanently assigning an AS number in the range of 1 to 64511. This is done when a customer network connects to two different ISPs, such as multi-homing. This situation mandates that customer network should have a unique AS number so that it can uniquely propagate its BGP routes to a global BGP mesh via two ISPs.

● A second way is by assigning a Private AS number in the range of 64,512 to 65,535. This is done when a customer network connects to a single ISP (either single-homed or dual-homed to the same ISP) and the intention is to conserve the AS numbers. It is not recommended that you use a private AS number if you are planning to connect to multiple ISPs in the future.

When a private AS number is allocated to the customer network, the BGP updates from the customer network to ISP will have the private AS number in its AS_PATH list. When the ISP propagates its network information to the global BGP table (Internet), it should not propagate the AS_PATH with the private AS number of the customer to the Internet. To help the ISP remove the private AS number from its AS_PATH list, use the Cisco IOS remove-private-as command. To remove the private AS number, use the neighbor x.x.x.x remove-private-as router configuration command. The neighbor x.x.x.x remove-private-as per-neighbor configuration command forces BGP to drop the private AS numbers. You can configure this command for external BGP neighbors. When the outbound update contains a sequence of private AS numbers, this sequence is dropped.

Adam, Paul (2014-07-12). All-in-One CCIE V5 Written Exam Guide (Kindle Locations 3897-3905).  . Kindle Edition.

http://www.networkworld.com/article/2344837/cisco-subnet/how-to-remove-private-as-numbers-in-bgp.html