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Multihoming Without BGP

Even in a multihomed situation, BGP is not the only solution. If your goal is simply to provide internal users with access to the Internet, there's no need to run BGP at all. As long as the link layer protocol supports the exchange of keep-alive messages from router to router, link failure will be detected by the link layer protocol. Floating static routes can then reliably direct all outbound traffic to a working ISP link. Network Address Translation (NAT) is then used to send outbound packets with a source IP address associated by the ISP with that outbound link. Return traffic will automatically come back via the same working link, because that link is the only link servicing that address range.

Of course, this approach won't work if you're providing services to the outside world, as the addresses associated with the failed link will disappear from the Internet. Similarly, connections established over the link that failed will need to be reconnected. However, for many applications this impact is minor.

This approach is also sufficient to provide high-availability virtual private networks (VPN) across the Internet if you use a routing protocol to detect and route around failed IPSec tunnels.

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