Hosts are normally configured with a single default gateway. If that router fails, hosts lose all off-subnet connectivity even when a second router is sitting on the same subnet, because they have no way to switch gateways on their own. First hop redundancy protocols (FHRPs) solve this by letting two or more routers share a virtual IP address and a virtual MAC (media access control) address that hosts use as their gateway. One router actively forwards; another takes over if it fails, and hosts notice nothing because the gateway IP and MAC they already know stay the same.
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