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When BGP Is Mandatory

Only when you need to support a common IP address range using both ISPs do you need to run BGP. This will normally be the case any time your applications include providing services to Internet users. Before you turn on BGP, you must obtain a globally unique autonomous system number (ASN). The ASN is used by BGP to identify your organization as the source of routing advertisements for your network prefixes. You can then arrange with both ISPs to accept BGP advertisements of your IP address prefixes from you, and in turn to advertise those address prefixes to the rest of the Internet.

Getting your address prefixes advertised is usually not a problem, although you do have to use care in your configuration to ensure that you don't inadvertently advertise any other address prefixes. In particular, don't advertise yourself as a path between the two ISPs, or you could find your links consumed by transit traffic that's of no interest to you. More challenging is setting up advertisements so that incoming traffic is reasonably balanced between the ISP links. This is extremely implementation-dependent and in some situations may not even be possible.

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