Introduction to BGP

Lessons Discussion
18

Jie,
I think you are asking two questions:

  1. unless you are assigned an ASN by an Internet authority, like ARIN, then it is common practice for ISPs to use a private BGP AS when they peer with you. These AS numbers are not allowed on the Internet, so the ISP will translate them into their own AS number before routes leave their AS to another ISP or organization. Think of it like BGP “NAT” where inside the ISP a private range is used, and gets translated to a publicly recognized range. Because of this, the customer will not have any problems with loop prevention with BGP

  2. It is also possible (and common) for ISPs to use the same private AS number among many of their customers. They can get away with this by use VRFs (virtual routing and forwarding). The setup of this feature can get a little complex because you have to use things called Route Distinguishers and Route Targets (both import and export). Here is a link where Cisco talks about how BGP works with VRF
    http://www.cisco.com/c/en/us/td/docs/ios-xml/ios/iproute_bgp/configuration/xe-3s/irg-xe-3s-book/per-vrf_assignment_of_bgp_router_id.html

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