Private AS numbers 64512 65535 can be used for customers connected to a single service provider

© 2004 Cisco Systems, Inc. All rights re

BGP requires the use of AS numbers. When BGP is configured, the AS number is mandatory information. However, public AS numbers are a scarce resource. Customers should use public AS numbers only when they are required. A customer that uses BGP to exchange routing information with only one ISP does not require a public AS number. This customer can use a private AS number.

An ISP network that is running BGP with some of its customers must determine whether a public or a private AS number is required for each customer. When the customer can use a private AS number, the ISP must allocate one from the range of private AS numbers (6451265535). The ISP must make sure not to assign any of the private AS numbers to more than one customer.

When the ISP receives BGP routes from the customer, the ISP routers see the private AS number in the AS path and treat the private number as any other AS number. However, before the ISP propagates any of these routes to the rest of the Internet, it must remove the private AS numbers from the AS path, because the same AS number may be in use by someone else. After the private AS number is removed, the route appears as belonging to the public AS of the ISP.

Copyright © 2004, Cisco Systems, Inc. Customer-to-Provider Connectivity with BGP

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AS Number Allocation—Multihomed Customers

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