Lab Objective
Configure a default static route on an edge router pointing toward an ISP, verify it appears correctly as the gateway of last resort, and confirm it successfully routes traffic to a destination with no more specific matching route.
Lab Purpose
Maintaining an individual static route for every possible internet destination would be completely impractical. A default route, matching any destination not covered by a more specific entry, is the standard solution for edge routers connecting to an ISP or any upstream network.
Lab Topology
R1 (edge router) ---- Serial0/0/1 ---- ISP Router
R1's internal LAN: 192.168.50.0/24
R1's serial address: 203.0.113.1/30
ISP's serial address: 203.0.113.2/30
(ISP simulated as directly reachable for
this lab, representing the path to the
wider internet)Task 1: Configure Basic Addressing
Configure R1's serial interface toward the ISP and its internal LAN interface.
Task 2: Configure the Default Static Route
Configure a default route on R1 pointing to the ISP's serial address.
Task 3: Verify the Gateway of Last Resort
Confirm the routing table identifies this route as the gateway of last resort.
Task 4: Test the Default Route
Ping an address with no specific matching route in R1's table (simulating an internet destination) and confirm the default route is used.
Solution and Verification
R1(config)# interface serial0/0/1
R1(config-if)# ip address 203.0.113.1 255.255.255.252
R1(config-if)# no shutdown
R1(config-if)# exit
R1(config)# interface gigabitethernet0/0
R1(config-if)# ip address 192.168.50.1 255.255.255.0
R1(config-if)# no shutdownR1(config)# ip route 0.0.0.0 0.0.0.0 203.0.113.2
-- 0.0.0.0 0.0.0.0 matches every possible
-- destination, since a mask of all zeros
-- means "any value in every octet is
-- acceptable" -- this is the standard
-- notation for a default routeR1# show ip route
Gateway of last resort is 203.0.113.2 to network 0.0.0.0
S* 0.0.0.0/0 [1/0] via 203.0.113.2
C 192.168.50.0/24 is directly connected, GigabitEthernet0/0
C 203.0.113.0/30 is directly connected, Serial0/0/1R1# ping 8.8.8.8
-- (simulated destination beyond the ISP)
!!!!!
Success rate is 100 percent (5/5)
R1# show ip route 8.8.8.8
% Network not in table
-- Confirms 8.8.8.8 has no specific matching
-- entry, yet the ping succeeded because the
-- default route caught it insteadKey Takeaway
The asterisk (S*) next to a static route in the routing table specifically marks it as the candidate default route, and the Gateway of last resort line at the top of show ip route output is the fastest way to confirm whether a default route is actually configured and active — a router with no gateway of last resort shown will drop any packet with no more specific matching route rather than forwarding it anywhere.