IOS command:IP classless

Posted by Blog - uTIcARdI on February 9, 2019

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:
Topology
Topology
Configuration
  1. Configured IP addresses on the routers
    Note: The subnets of R1’s Loopback1 and R2’s Loopback1 belongs 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 default route.
  • With ‘no ip classless’
    If the route entry of class A exists in the route table, the router will forward the packets by following the entry instead of default route.

IOS command:IP classless


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