IOS command:IP classless

I wrote down this article in 2009-03-13 when I was looking for my first job. Taking this again today as preparation for CCNP.

Topology:

Configuration

1. Configured IP addresses on the routers
   Note: The subnets of R1’s Loopback1 and R2’s Loopback1 belong to the class A address.

2. Execute command “ip classless” on both R1 and R2 (The command has been the default since IOS 11.3)

3. Configure static routes

   R1(config)#ip route 0.0.0.0 0.0.0.0 192.168.12.2 

   R2(config)#ip route 10.1.1.0 255.255.255.0 192.168.12.1 

4. Ping

   R1#ping 2.2.2.2
   Type escape sequence to abort.
   Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
   !!!!!
   Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/1 ms
   R1#ping 10.2.2.2
   Type escape sequence to abort.
   Sending 5, 100-byte ICMP Echos to 10.2.2.2, timeout is 2 seconds:
   !!!!!
   Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/3 ms

5. no IP classless

   R1(config)#no ip classless

   R2(config)#no ip classless

6. Ping again

   R1#ping 2.2.2.2
   Type escape sequence to abort.
   Sending 5, 100-byte ICMP Echos to 2.2.2.2, timeout is 2 seconds:
   !!!!!
   Success rate is 100 percent (5/5), round-trip min/avg/max = 1/1/4 ms
   R1#ping 10.2.2.2.2
   % Unrecognized host or address, or protocol not running.
   R1#show ip route
   Codes: L - local, C - connected, S - static, 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
      i - IS-IS, su - IS-IS summary, 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, H - NHRP, l - LISP
      a - application route
      + - replicated route, % - next hop override
   
   Gateway of last resort is 192.168.12.2 to network 0.0.0.0
   
   S*    0.0.0.0/0 [1/0] via 192.168.12.2
     10.0.0.0/8 is variably subnetted, 2 subnets, 2 masks
   C        10.1.1.0/24 is directly connected, Loopback0
   L        10.1.1.1/32 is directly connected, Loopback0
     192.168.12.0/24 is variably subnetted, 2 subnets, 2 masks
   C        192.168.12.0/24 is directly connected, Ethernet0/0
   L        192.168.12.1/32 is directly connected, Ethernet0/0

   On Router1:

   • It failed to ping 10.2.2.2 with default route still present
   • 2.2.2.2 still works

Conclusion

With the default route: 0.0.0.0/0, The connectivity from R1 to 10.2.2.0/24 (on R2) depends on the command: ip classless.

• With ‘ip classless’
  The packets will be forwarded by following the default route.
• With ‘no ip classless’
  If the route entry of class A exists in the routing table, the router will forward the packets by following the entry instead of the default route.

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IOS command:IP classless

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