This document discusses the removal of the private autonomous system
numbers in BGP.
There are no specific requirements for this document.
This document is not restricted to specific software and hardware
For more information on document conventions, refer to the
Cisco Technical Tips
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
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)
If the update has only private AS numbers in the AS_PATH, BGP removes
If the AS_PATH includes both private and public AS numbers, BGP
doesn't remove the private AS numbers. This situation is considered a
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.