8.8 Issue with Using Private AS Numbers
8.8.1 Conserving AS numbers
To conserve AS numbers within the Internet, the Inter-NIC generally does not assign a legal AS number to customers whose routing policies are an extension of the policies of their provider. Thus, in the situation where a customer is single-homed or multihomed to the same provider, the provider generally requests that the customer use an AS number taken from the private pool of ASs (64512-65535). As such, all BGP updates the provider receives from its customer contain private AS numbers.

Private AS numbers cannot be leaked to the Internet because they are not unique. For this reason, Cisco has implemented a feature to strip private AS numbers out of the AS_path list before the routes get propagated to the Internet.

In the Figure, AS1 is providing Internet connectivity to its customer AS65001. Because the customer connects to only this provider and no plans to connect to an additional provider in the near future, the customer has been allocated a private AS number. If the customer later needs to connect to another provider, a legal AS number should be assigned.

Prefixes originating from AS65001 have an AS_path of 65001. Note prefix 172.16.220.0/24 in Figure 8-37 as it leaves AS65001. For AS1 to propagate the prefix to the Internet, it would have to strip the private AS number. When the prefix reaches the Internet, it would look like it has originated from the provider's AS. Note how prefix 172.16.220.0/24 has reached the network access point (NAP) with AS_path 1.

BGP will strip private ASs only when propagating updates to the external peers. This means that the AS stripping would be configured on RTC as part of its neighbor connection to RTE.

Private ASs should be connected only to a single provider. If the AS_path contains a mixture of private and legal AS numbers, BGP will view this as an illegal design and will not strip the private AS numbers from the list, and the update will be treated as usual. Only AS_path lists that contain private AS numbers in the range 64512 to 65535 are stripped.