'no ip classless' not working?

What IOS are you using? I'm sure I heard that classful support has been dropped from recent IOS, but I can't find a reference to it at the moment.

| I am playing with some routers in a lab and just to say that I've seen | it break things I set 'no ip classless'. However, I am not getting the | result I expected and I was hoping someone could tell me what I'm | missing. | | My lab topology is set up like so: | | RouterA---10.32.20.0/24---RouterB---10.32.10.0/24--- | RouterC---10.32.32.0/24---RouterD---10.35.35.0/24--->

| | RouterB is set with 'no ip classless' and its routing table is below: | | RouterB#show ip route | Gateway of last resort is 10.32.10.254 to network 0.0.0.0 | | 172.16.0.0/24 is subnetted, 1 subnets | C 172.16.30.0 is directly connected, FastEthernet0/1.30 | 10.0.0.0/8 is variably subnetted, 3 subnets, 2 masks | C 10.32.10.0/24 is directly connected, FastEthernet0/0.10 | C 10.32.20.0/24 is directly connected, FastEthernet0/1.20 | S 10.100.0.0/14 [1/0] via 10.32.10.254 | S* 0.0.0.0/0 [1/0] via 10.32.10.254 | | If I understand how classful routing works, I should not be able to | ping 10.35.35.1 from RouterA; the reason being that RouterB is | configured for classful routing and has routes to other networks in | 10.0.0.0/8 (but not 10.35.35.0/24). Therefore, RouterB discards the | packet. | | However, when I try to ping 10.35.35.1, it works fine: | | RouterA#ping 10.35.35.1 | | Type escape sequence to abort. | Sending 5, 100-byte ICMP Echos to 10.35.35.1, timeout is 2 seconds: | !!!!! | Success rate is 100 percent (5/5), round-trip min/avg/max = 1/3/4 ms | RouterA#traceroute 10.35.35.1 | | Type escape sequence to abort. | Tracing the route to 10.35.35.1 | | 1 10.32.20.253 0 msec 0 msec 0 msec | 2 10.32.10.254 4 msec 4 msec 0 msec | 3 10.35.35.1 4 msec 8 msec 0 msec

Is it possible that a router on 10.32.20.X is doing proxy ARP?

With no ip classless in the configuration you have enabled classful routing.. You will notice that if any part of the destination IP addresses classful network is in the routing table, the router will not use the default route to forward the packet. I would suggest trying this with dynamic routing protocols (RIPv2, EIGRP, etc..) to test behavior also.

The routers are all running 12.3 or 12.4 (details below). Regarding Proxy ARP, I don't think it is configured, I certainly didn't configure it explicitly. The A-C configs are below. Thanks for the suggestions.

RouterA is a 1720 running 12.3(23) RouterB is a 1841 running IP Base 12.3(8)T5 RouterC is a 3640 running IP Plus 12.4(16) RouterD is actually a Pix501 running 6.3.5

=3D=3DRouterA config=3D=3D version 12.3 service timestamps debug uptime service timestamps log uptime no service password-encryption ! hostname c1700 ! boot-start-marker boot-end-marker ! no logging buffered enable secret 5 $1$aGun$pr7FjqTJLRWJr7IK7ZybZ/ ! memory-size iomem 25 no aaa new-model ip subnet-zero ! no ip domain lookup ! ip cef ! interface FastEthernet0 ip address 10.32.20.20 255.255.255.0 speed auto ! interface Serial0 no ip address shutdown ! ip classless ip default-network 10.0.0.0 ip route 0.0.0.0 0.0.0.0 172.16.30.253 ip route 0.0.0.0 0.0.0.0 10.32.20.253 ip route 10.32.10.0 255.255.255.0 10.32.20.253 ip route 10.32.10.0 255.255.255.0 172.16.30.253 no ip http server ! line con 0 exec-timeout 0 0 logging synchronous line aux 0 line vty 0 4 exec-timeout 0 0 password cisco logging synchronous login

=3D=3DRouterB config=3D=3D version 12.3 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname 1841 ! boot-start-marker boot-end-marker ! enable secret 5 $1$hiFe$mTnPJtBBtct5FO6c8D/Dp0 ! username cisco secret 5 $1$f6gj$PjxDNivBCz/JtXQZbi7w3. mmi polling-interval 60 no mmi auto-configure no mmi pvc mmi snmp-timeout 180 no aaa new-model ip subnet-zero ip cef ! ! ! ! no ip domain lookup no ftp-server write-enable ! ! ! ! interface FastEthernet0/0 no ip address duplex auto speed auto ! interface FastEthernet0/0.10 encapsulation dot1Q 10 ip address 10.32.10.253 255.255.255.0 no cdp enable ! interface FastEthernet0/0.20 ! interface FastEthernet0/1 no ip address duplex auto speed auto ! interface FastEthernet0/1.20 encapsulation dot1Q 20 ip address 10.32.20.253 255.255.255.0 ! interface FastEthernet0/1.30 encapsulation dot1Q 30 native ip address 172.16.30.253 255.255.255.0 ! interface Serial0/0/0 no ip address shutdown ! no ip classless ip route 0.0.0.0 0.0.0.0 10.32.10.254 ip route 10.100.0.0 255.252.0.0 10.32.10.254 no ip http server ! ! ! control-plane ! ! line con 0 exec-timeout 0 0 logging synchronous history size 15 line aux 0 line vty 0 4 exec-timeout 0 0 logging synchronous login local history size 15 line vty 5 15 exec-timeout 0 0 logging synchronous login local history size 15

=3D=3DRouterC config=3D=3D ! version 12.4 service timestamps debug datetime msec service timestamps log datetime msec no service password-encryption ! hostname r2 ! boot-start-marker boot-end-marker ! enable secret 5 $1$Z7iL$d6iyr1T0G5GGJIBeEB5IX1 ! no aaa new-model ! ip cef no ip domain lookup ! username cisco secret 5 $1$xFnX$32qbiWsFGoGCIWrmPURPO. ! interface FastEthernet0/0 no ip address duplex auto speed auto ! interface FastEthernet0/0.1 encapsulation isl 1 ip address 10.32.32.254 255.255.255.0 no ip redirects ! interface FastEthernet0/0.10 encapsulation isl 10 ip address 10.32.10.254 255.255.255.0 no ip redirects ! interface Serial0/0 no ip address shutdown ! interface Serial0/1 no ip address shutdown clock rate 2000000 ! interface Serial0/2 no ip address shutdown ! ip http server ! no ip classless ip route 0.0.0.0 0.0.0.0 10.32.32.1 ip route 10.32.20.0 255.255.255.0 10.32.10.253 ip route 172.16.30.0 255.255.255.0 10.32.10.253 ! control-plane ! line con 0 exec-timeout 0 0 logging synchronous line aux 0 line vty 0 4 exec-timeout 0 0 logging synchronous login local line vty 5 15 exec-timeout 0 0 logging synchronous login local

I'm pretty sure classfull routing is no longer supported as well, either that or its broken. I do know that classless routing is now the default in the newer versions of IOS, I'm not sure what version this changed. If it were broken, I doubt that Cisco would fix it because the entire concept of classfull routing was itself sent to the bit bucket more than 15 years ago. A workaround is to add a static route for 10/8 to null0. This is exactly what "no ip classless" would do anyway.

ip route 10.0.0.0 255.0.0.0 null0

I don't doubt that the functionality was eliminated; it seems prehistoric, and I've been trying to think of a situation in which it would be useful. But if it is indeed depreciated, i wonder why it would even still be a configuration option. Furthermore, I wonder why it would still be included in the CCNA material, other than as a "this is how things were in the bad old days" aside.

You'd hope the CLI would warn you if you tried to configure it, if it is indeed now non-functional. It could of course just be a bug.

Indeed. My CCNA tutor seemed to think that serial interfaces were what made the world go round.