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Regardless of how the default route shows up—whether it’s a gateway of last resort, a route to 0.0.0.0, or a route to some other network with an * beside it in the routing table—it is used according to the rules of classless or classful routing, depending on which is enabled on that router.
Classless Routing
Even if there is a default route in the routing table, it might not be used. Again consider the network shown in Figure 7-5, with the same default route. As shown in Example 7-13, on R3, a ping of 10.1.1.1 works when the default route is used. However, a ping of 168.13.200.1, which would seemingly need to use the default route as well, fails.
Example 7-13 Classful Routing Causes One Ping to Fail
R3#show ip route
Codes: C - connected, S - static, I - IGRP, R - RIP, M - mobile, B - BGP
D - EIGRP, EX - EIGRP external, O - OSPF, IA - OSPF inter area
N1 - OSPF NSSA external type 1, N2 - OSPF NSSA external type 2
E1 - OSPF external type 1, E2 - OSPF external type 2, E - EGP
i - IS-IS, L1 - IS-IS level-1, L2 - IS-IS level-2, ia - IS-IS inter area * - candidate default, U - per-user static route, o - ODR
P - periodic downloaded static route
Gateway of last resort is 168.13.100.1 to network 0.0.0.0
168.13.0.0/24 is subnetted, 4 subnets
R168.13.1.0 [120/1] via 168.13.100.1, 00:00:13, Serial0.1 C 168.13.3.0 is directly connected, Ethernet0
R168.13.2.0 [120/1] via 168.13.100.2, 00:00:06, Serial0.1 C 168.13.100.0 is directly connected, Serial0.1
R3#ping 10.1.1.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 10.1.1.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 84/89/114 ms
R3#
R3#ping 168.13.200.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 168.13.200.1, timeout is 2 seconds:
...
Success rate is 0 percent (0/5)
So the ping 10.1.1.1 command on R3 works, just as it should. However, the ping 168.13.200.1 command does not work. Why?
244 Chapter 7: Advanced Routing Protocol Topics
The classful logic of Example 7-13 works like this:
1.I need to send a packet to 168.13.200.1.
2.I match Class B network 168.13.0.0 because I am using classful routing; there is a match.
3.I do not match a specific subnet of network 168.13.0.0 that contains 168.13.200.1.
4.Because I matched network 168.13.0.0, I cannot use the default route in this case, so I discard the packet.
The classless logic of Example 7-14 works like this:
1.I need to send a packet to 168.13.200.1.
2.I do not match a specific route that contains 168.13.200.1.
3.I use the default route only if there is not a more specific match. Because there is no match, I use the default route.