Multihoming

Multihoming is the practice of connecting a computer network to more than one Internet service provider (ISP). The primary aim is to improve fault tolerance and availability by avoiding dependence on a single upstream link. Multihomed networks are common among organizations of various sizes, data centers, and hosting providers that require continuous access to the Internet and resilience against outages or degraded performance from any single provider.

Implementation commonly relies on the Border Gateway Protocol (BGP). A multihomed site may obtain its own autonomous system number and either provider-independent (PI) address space or provider-assigned (PA) space. It contracts with two or more ISPs, establishes BGP sessions with each, and configures routing policies to control how traffic enters and leaves the network. Inbound traffic largely follows the prefixes announced by each ISP, while outbound traffic can be balanced across links using ECMP or policy-based routing. Active/active configurations may use multiple links in parallel for load sharing, while active/standby configurations use a primary link with a backup link that takes over on failure. Multihoming can also be achieved at higher layers with SD-WAN or similar technologies that manage multiple transport paths.

Trade-offs include increased complexity, higher operational cost, and the need for BGP expertise. Suboptimal routing can occur if policies are not carefully designed, and address management is a consideration when using PA versus PI space. Security considerations include mitigating route leaks and hijacks, for example with prefix filtering and RPKI.