This document discusses the removal of the private autonomous system numbers in BGP.
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Private autonomous system (AS) numbers which range from 64512 to 65535 are used to conserve globally unique AS numbers. Globally unique AS numbers (1 - 64511) 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; see BGP Best Path Selection Algorithm for more information on BGP path selection). For this reason, a new feature was added in Cisco IOS® Software release 10.3 and later, which 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 multihoming. 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 64512 to 65535. 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.
The following conditions apply:
You can only use this solution with external BGP (eBGP) peers.
If the update has only private AS numbers in the AS_PATH, BGP removes these numbers.
If the AS_PATH includes both private and public AS numbers, BGP doesn't remove the private AS numbers. This situation is considered a configuration error.
If the AS_PATH contains the AS number of the eBGP neighbor, BGP does not remove the private AS number.
If the AS_PATH contains confederations, BGP removes the private AS numbers only if they come after the confederation portion of the AS_PATH.
For an configuration example, refer to Sample Configuration for Removing Private AS Numbers in BGP.
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