You should issue the ip default-gateway 10.10.100.1 command to create a gateway of last resort on the router in this scenario. The gateway of last resort is used when no route can be identified for routing packets. If a route cannot be determined and the gateway of last resort has been defined, packets are routed to the gateway's address. The ip default-gateway command should be used to configure a gateway of last resort when IP routing is disabled on a Cisco router or switch.
You should not issue the ip default-network 10.10.100.0 command to create a gateway of last resort on the router in this scenario, because the ip default-network command can be used only on devices that have IP routing enabled. Because IP routing is disabled on the router in this scenario, you cannot use the ip default-network command to create a gateway of last resort on the router. The ip default-network command is issued to flag routes in the routing table as candidates for the default route. If the ip default-network command is issued with a network address that matches an entry appearing in the routing table, that route becomes the default route. If there is no matching entry in the routing table, the route will not become a default route. You can issue multiple ip default-network commands on a router; the router will use administrative distances (ADs) and metrics to determine the best default route.
You should not issue the ip route command to create a gateway of last resort on the router in this scenario, because the ip route command can be used only on devices that have IP routing enabled. Because IP routing is disabled on the router in this scenario, you cannot use the ip route command to create a gateway of last resort on the router. You can create a static gateway of last resort on a router by issuing the ip route command with a static route followed by an IP address. For example, you could issue the ip route 0.0.0.0 0.0.0.0 10.10.100.1 command to create a static gateway of last resort to the nexthop router at 10.10.100.1.
Alternatively, you could specify a physical interface on the router as the gateway. For example, you could issue the ip route 0.0.0.0 0.0.0.0 s0/0 command to specify the Serial 0/0 interface on the local router as the gateway of last resort? packets with destinations not found in the routing table will be forwarded to the Serial 0/0 interface.