We had a very interesting time in our labs this week. One of the topics covered this week in our CCNA2 (CIT1122) Routing Class was Classful and Classless Routing behavior. We have been studying classfull and classless routing but, this week, focused on classful and classless routing behavior.
Unfortunately, or interestingly, the lab did not work as expected, using RIPv1, a classful protocol. According to what we read in our text and online curriculum, we should be able to use the "no ip classless" command with RIPv1. As a result of using that command, if we ping to a subnet that doesn't exist in the RIPv1 routing table, the packet destined to that subnet should be dropped. Because we were using the, "no ip classless" command, the router, as described in our studies, should not have been able to reach the default route. The ping should have failed.
I contacted Rick Graziani, author of our text and professor at Cabrillo Community College, and he is trying to get an answer for us.
The question we are asking is, does the "no ip classless" command still work in current IOS images for RIPv1? As stated above, in the lab this week, we found that it didn't matter if we disabled the "ip classless" command with the "no ip classless" command - RIPv1 still used the classless behavior and found the default route.
Below is a example with the routing table looking like this (I don't remember the exact networks but found this one on the Internet):
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 3 subnets
R 172.16.1.0 [120/1] via 172.16.2.2, 00:00:12, Serial0/1/0
C 172.16.2.0 is directly connected, Serial0/1/0
C 172.16.3.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Serial0/1/1
S* 0.0.0.0/0 is directly connected, Serial0/1/1
Regardless if "no ip classless" or "ip classless" was in use, the ping to
the network 172.16.4.0 was successful (but with classfull behaviour shouldn't):
Router#ping 172.16.4.1
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.4.1, timeout is 2 seconds:
!!!!!
Success rate is 100 percent (5/5), round-trip min/avg/max = 12/14/16 ms
As soon as we here back from Rick, I will update you with his findings. We now have him curious!!
Joanne
hello there,
ReplyDeletei know this post was well over 2 years ago, but i also have the same issue.
using the same discontiguous network as an example above, i found that the ping packet still goes through 172.16.4.1 even with the "no ip classless" command turned on. as stated, this shouldn't happen since the router was instructed to employ a classful routing behavior. the default route is not supposed to be used when classful routing behavior is in effect.
did you get answers from Rick Graziani? what did he tell you? does it have something to do with the IOS version? thanks!
Loumarven
ReplyDeleteI am really sorry I am just seeing your comment as I never have an opportunity to get back to this blog since adding Coordinator duties to my teaching.
I don't remember hearing back from Rick but I do know the reason for the issue.
In order for the lab to work, you must disable the 'ip cep' command from your configuration. This command in current versions of the ios will be enabled by default. Bottom line is that it bypasses the route processor.