I am attaching my scenario and my "show ip route" and "show running" for my two routers.
I read about Classful Routing: When a packet's destination only matches a router's default route, and does not match any other routes, only use the default route if this router does not know any routes in the classful network in which the destination IP address reside.
I am trying to configure the scenario so that the ping from PC1 to PC2 fails because the routers are configure with the "no ip classless" and both PCs are in the same classful network.
Any idea why my ping is working? I was expecting this wouldn't work. Please help to understand!!
The link is clear but I still don't understand why my scenario still works.
As indicated in the link:
- This is the essence of classful routing: If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped. ......... MY ROUTER "ROUTER-LAR" HAS A SUBNET /25 OF MAJOR NETWORK 192.168.0.X CONNECTED. IF I PING IP 192.168.0.131 WHOSE SUBNET /25 IS IN THE OTHER SIDE OF THE NETWORK (AND IT IS PART OF THE MAJOR NETWORK 192.168.0.X) THE PING WORKS!!! WHY DOES IT WORK IF "ROUTER-LAR" KNOWS PART OF THE MAJOR NETWORK? I THINK IT SHOULD DROP THE PACKET SINCE IT KNOWS ABOUT THE MAJOR NETWORK..... PLEASE HELP!!! PLEASE SEE MY ATTACHMENTS.
- This is the essence of classful routing: If one part of a major network is known, but the subnet toward which the packet is destined within that major network is unknown, the packet is dropped.
Isn't the major network refering to a network of class A, B or C? In my case both routers have a /25 subnet that belong to class C 192.168.0.0 network. That is why I think the packet should be dropped.
It worked as you said. I only had to disable CEF with the "no ip cef" command and ping doesn't work with "no ip classless" and it works with "ip classless". Now the question is... is there something that gets affected by disabling CEF? I haven't really get to that concept.
I am not all too familiar with CEF and its workings, but I assume disabling it forces every packet to be passed to the control plane for routing, and not being routed/switched in hadware via the FIB ("Forwarding Information Base"). no ip classless does not seem to apply when using the FIB.
For more details I suggest you check out the online resources e.g.
Hi everyone, I would like to thank you in advance for any help you can provide a newcomer like myself!
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