Multihomed

Multihomed is a configuration describing a host computer with two or more network addresses. However, the connections may vary. For example, the host computer may have two or more network connections to the same type of network, a connection to a network and a serial line, or connections to two separate LAN segments or other networks such as those used by an Internet service provider (ISP), which do not (or are not allowed to) communicate with each other.

Multihoming also references the process or technique of connecting a single network to two ISP networks, or the process of connecting a single computer to two or more network or network segments. It is generally used to eliminate single point of failure (SPOF) network problems, which means providing alternate data transmission paths in the event any one transmission path should fail.

Multihomed host computers, or multihoming a network to two or more networks, may increase the performance of the host computer or increase the reliability of connections by providing an alternate data transmission route in the event of a transmission line fault (called fault tolerance), or both.

There are numerous variants for multihoming. These include a single link with two or more IP addresses and multiple links with multiple IP addresses. Other important variants are: multiple interfaces with a single IP address per interface and multiple links with a single IP address. All four of these are discussed and described below.

However, there are certain limitations or caveats to eliminating SPOF network problems. Here are four general situations to consider:

• Multiple Upstream Links: These are useful in the event one upstream network, such as an ISP, fails. However, if all these data transmission lines pass through the same channel, and the channel is cut (possibly by construction equipment, e.g. a backhoe), there is no redundancy. Therefore, a wise design would physically separate these channels to avoid a single incident from damaging all channels.
• Positioning of Routers and Switches: These must be designed to eliminate a SPOF. Avoid the common situation where multiple Internet links from ISPs converge on a single router, which then would be the SPOF.
• Host Duplication: A reliable host computer is helpful. But with it multiple network interfaces should be used, each having a separate router or switch. However, a better design would use multiple hosts, or duplicating one host on each of multiple computers, and each of these having a separate router or switch.
• Server Name Resolution: The host computer must be accessible. Therefore, the network design must assure that no single element (switch, router, hub, transmission line, etc.) failure will block users from resolving the domain name system, or IP addresses, server name.

Other considerations for multihoming to avoid data routing failures relate to IPv4 (Internet Protocol version 4) and the protocol backing the Internet routing decisions, such as the Border Gateway Protocol, which makes routing decisions based on determined policies and rules. Thus, it is more accurately referred to as a reachability protocol, meaning that a given route is permitted or not permitted based on set policies and rules.