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9.6 | ![]() |
BGP Attributes | |
9.6.6 | ![]() |
AS_Path and private AS numbers |
In an effort to conserve AS numbers, customers whose routing policies
are an extension of the policies of their provider generally are not
assigned a legal AS number. Therefore, if 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, 64,512
to 65,535. As such, all BGP updates that the provider receives from
its customer contain private AS numbers.
Private AS numbers cannot be advertised 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 Figure
Prefixes originating from AS65001 have an AS_Path of 65001.
BGP will strip private AS numbers only when propagating updates to the external peers. This means that the AS stripping would be configured on RTB as part of its neighbor connection to RTC. Privately numbered autonomous systems should be used only when connected 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. BGP 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 64,512 to 65,535 are stripped. The example that follows demonstrates how BGP can be configured to prevent the leakage of private AS numbers into the Internet.
Notice how RTB is using the remove-private-as keyword in its neighbor connection to AS7.
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