Multihoming
Multihoming is a technique to increase the reliability of the internet connection of an IP network. multihoming is typically used to describe to a customer, rather than ISP network.
In order to be multihomed, a network must have an IP address range which can be advertised publiccally. 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 be routed automatically based on our awareness of the link failing. More importantly, other networks will be aware, via BGP updates of the multihomed networks routes to route incoming traffic via the correct 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 amoung BGP users and smaller prefixes may be filtered out. This will make multihoming fail.