1. A multi-router system comprising: a plurality of routers; and a physical interface installed within a first one of the plurality of routers and shared by other ones of the plurality of routers, wherein each of the plurality of routers that share the physical interface of the first one of the routers store configuration information specifying the shared physical interface installed within the first one of the routers of the multi-router system and a packet header that is required by the shared physical interface, wherein each of the routers that share the physical interface installed within the first one of the routers comprises a packet forwarding engine having respective forwarding information that maps packet keying information to next hops identifying network interfaces, wherein the respective forwarding information for each of the packet forwarding engines specifies a logical network interface assigned to the shared physical interface so that the shared physical interface appears to be local to each of the packet forwarding engines of the plurality of routers and is selectable in response to a route lookup, wherein each of the routers that share the physical interface of the first one of the routers includes a tunnel interface card to form network tunnels, wherein each of the routers share the physical interface by performing route lookups to select as an egress interface the respective logical network interface associated with the shared physical interface of the first one of the routers, constructing based on the configuration information fully-formed packets that have the packet header required by the shared physical interface, encapsulating the fully-formed packets within outbound tunnel packets and forwarding the outbound tunnel packets to the respective tunnel interface card of the router, wherein the tunnel interface cards of the routers receive the outbound tunnel packets and loop the outbound tunnel packets back to the respective routers as inbound tunneled packets received from a network via a tunnel for forwarding to the first one of the routers within which the shared physical interface is installed, and wherein the first one of the routers within which the shared physical interface is installed outputs, on the shared physical interface, the fully-formed packets received from the other ones of the plurality of routers without modification.