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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.
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