Question

1. A network engineer is troubleshooting a small LAN network with one border router, GW01 that connects to the Internet Service Provider’s (ISP) network. GW01 uses its Serial 0/2/1 interface to connect to the ISP’s router. Everyone on the LAN network lost connectivity to the Internet. Upon troubleshooting the issue, the network engineer notices the following message after typing show ip route. Gateway of last resort required but not set. What do you suggest the network engineer do to rectify this issue? Assume the problem is only on the organization’s LAN side of the network. Provide as much information as possible including specific commands and all required parameters. (10 points)

Answer 1

The Gateway of Last Resort or Default gateway is a route used by the router when no other known route exists to transmit the IP packet. Known routes are present in the routing table. So if ,any route not known by the routing table is forwarded to the default route. Each router which receives this packet will treat the packet the same way, if the route is known, packet will be forwarded to the known route.

There should be a default route into the routing table. The default gateway is what we restored last. So what we used to resort last are the unknown subnets. All the known subnets are included into the routing table but the unknown subnets or destination are forward to the default route 0.0.0.0/0
You should have something like:
ip route 0.0.0.0 0.0.0.0 <Next Hop IP or local interface>
or learn a default route 0.0.0.0/0 via a dynamic routing protocol, also you can try for testing with.
ip route 0.0.0.0 0.0.0.0 null0

Because you need a valid entry.