Multihoming is a technique to increase the reliability of the internet connection of an IP network. As an adjective, it is typically used to describe to a customer, rather than an ISP network.

IPv4 multihoming

In order to be multihomed, a network must have a public IP address range. Then a connection to two (or more) separate ISPs is established. The routing over these connnections is normally controlled by a BGP enabled router.

In the case where one outgoing link from the multihomed network fails, outgoing traffic will automatically be routed via one of the remaining links. More importantly, other networks will be notified, through BGP updates of the multihomed network routes, of the need to route incoming traffic via another ISP and link.

A key pitfall in multihoming is that two apparently independent links, from completely different ISPs may actually share a common transmission line. This will form a single point of failure and considerably reduce the reliability benefits from multihoming.

Another problem to look out for is that multihoming too small a network may not be effective since route filtering is very common among BGP users and smaller prefixes may be filtered out. This will make multihoming fail.

IPv6 multihoming

Multihoming in the next-generation IPv6 protocol is not yet standardized, as discussions about the various possible approaches to multihoming are still unresolved.

External links

IPv4 multihoming:

• O'Reilly article on BGP Multihoming (http://www.oreillynet.com/pub/a/network/2002/08/12/multihoming.html)
• Cisco multihoming configuration example (http://www.cisco.com/warp/public/459/27.html)

IPv6 multihoming:

• IETF IPv6 multihoming working group (http://ops.ietf.org/multi6/)
• RFC 3582: Goals for IPv6 Site-Multihoming Architectures (http://rfc3582.x42.com/)
• Internet-Draft: Analysis of IPv6 Multihoming Scenarios (http://www.consulintel.euro6ix.org/ietf/draft-palet-multi6-scenarios-00.txt)