Typical IP-networked hosts are configured with a single default route. For single-homed hosts the default route defines the first destination for packets addressed outside of the local subnet; but for multi-homed hosts the default route also implicitly defines a default interface to be used for all outbound traffic. Specific subnets may be accessed using non-default interfaces by defining static routes; but the single default route remains a “single point of failure” for general access to other and Internet subnets. The Linux kernel, together with the iproute2 suite, supports the definition of multiple default routes distinguished by a preference metric. This allows alternate networks to serve as failover for the preferred default route in cases where the link has failed or is otherwise unavailable.
