External routes are managed by other protocols, for example, the External Gateway Protocol (EGP). NCPROUTE needs to know not to interfere with these routes and not to delete them.
An external entry exists in the NCPROUTE routing table as a place holder to prevent a route from being overwritten by a competing RIP route. External routes are not propagated. NCPROUTE does not manage an external route. Therefore, NCPROUTE knows only that there is an existing route to the host or network and that is the route known by NCP.
External routes should be used when the local machine is running a non-RIP routing protocol that dynamically changes the TCP⁄IP routing tables. The remote machine does not need to run any routing protocol, because the only concerns are how to route traffic from the local machine to the remote machine, and how to prevent multiple routing protocols from interfering with each other.