This question is going to make my head explode!

Any help with this question would be great. I was given the answer in class but was not given the why behind it. I will leave the answer out so I do not sway anyone's thinking

Router1# show ip route


Gateway of last resort is 0.0.0.0 to network 0.0.0.0

           172.16.0.0/20 is subnetted, 1 subnets
S               172.16.0.0 [1/0] via 192.168.0.2
           192.168.0.0/30 is subnetted, 2 subnets
C               192.168.0.0 is directly connected, Serial0/0
C               192.168.0.8 is directly connected, Serial0/1
S*       0.0.0.0/0 is directly connected, Serial0/2

The network administrator issues the command [b]no ip classless[/b] on Router1.  What forwarding action will take place on a packet that is received by Router1 and is destined for host 192.168.0.26?

A. The packet will be dropped

B. The packet will be forwarded to the gateway of last resort.

C. The packet will match the 192.168.0.0 network and be forwarded out Serial 0/0.

D. The packet will most closely match the 192.168.0.8 subnet and be forwarded out Serial 0/1.

A big thanks to anyone who can help me wrap my head around this one!

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127.0.0.1

Hell cacharo,

I'm a Cisco Networking Academy Student, and this same question appeared on my Final Exam for the CCNA Exploration 2: Routing Protocols and Concepts course.

This is actually covered in Chapter 8 of the curriculum too.

I put A. The packet will be dropped as the answer. The "no ip classless" command disables classless routing. Which means that you can only send packets to networks with classful routing only. Classful is when you have fixed Class A /8, Class B /16, or Class C /24 subnets. Classless is when you have variable-length subnets like /28,/29,/15,/20, etc.

For example:

If no ip classless was entered, and the router output showed:
192.168.0.0/24 is subnetted, 2 subnets

Then the packet would be forwarded, because a /24 is a classful subnet

If ip classless was entered, and the router output showed:
172.16.0.0/29 is subnetted, 4 subnets

Then the packet would be forwarded, because ip classless is enabled. Both classful and classless routing would be enabled.

Note:
DO NOT CONFUSE THIS WITH CLASSFUL OR CLASSLESS ROUTING PROTOCOLS! This just affects the behavior of the routing, not the routing protocols. So even if the no ip classless command was entered, classless routing protocols (RIP v2, OSPF, EIGRP, etc.) would still work on the router, as long as they are using a classful addressing scheme.[/b]

cacharo

Thanks for the help. I actually have my final today and this was one of the questions on my review guide. The instructor gave A as the answer but with no explanation. My guess to why was the routing table would be rewritten and have 2 paths to the 192.168.0.0 and 8.0 networks changed to 2 /24 networks and would be dropped because it wouldn't know which way to send it out.

EdTheLad

127.0.0.1 wrote:

Hell cacharo,

I put A. The packet will be dropped as the answer. The "no ip classless" command disables classless routing. Which means that you can only send packets to networks with classful routing only. Classful is when you have fixed Class A /8, Class B /16, or Class C /24 subnets. Classless is when you have variable-length subnets like /28,/29,/15,/20, etc.

For example:

If no ip classless was entered, and the router output showed:
192.168.0.0/24 is subnetted, 2 subnets

Then the packet would be forwarded, because a /24 is a classful subnet

If ip classless was entered, and the router output showed:
172.16.0.0/29 is subnetted, 4 subnets

Then the packet would be forwarded, because ip classless is enabled. Both classful and classless routing would be enabled.

Note:
DO NOT CONFUSE THIS WITH CLASSFUL OR CLASSLESS ROUTING PROTOCOLS! This just affects the behavior of the routing, not the routing protocols. So even if the no ip classless command was entered, classless routing protocols (RIP v2, OSPF, EIGRP, etc.) would still work on the router, as long as they are using a classful addressing scheme.[/b]

Your description is wrong and doesnt make any sense.

After entering the command "no ip classless" a classful entry 192.168.0.0/24 will appear as you mention above.The routing table will also contain the configured subnets under 192.168.0.0/30 which are 192.168.0.0 & 192.168.0.8 as shown in the display.

When a packet arrives with destination address 192.168.0.26 due to a classful lookup the first 24 bits will be checked, they match the major subnet currently in the table 192.168.0.0/24, now the subnets within this classful network are checked for a match.Since 192.168.0.26 does not fall within either of the two subnets 192.168.0.0/30 or 192.168.0.8/30 the packet will be discarded.

If the two subnets 192.168.0.0/30 & 192.168.0.8/30 never existed in the routing table the classful network 192.168.0.0/24 would not be present.If in this case the packet destined to 192.168.0.26 arrived the routing table would be checked, on inspection its noted the major network 192.168.0.0/24 doesnt exist,since the major network is unknown the default route would be used.

marlon23

Simple answer : "(no) ip classless" command changes how your router uses the default route. With Classless routing it will use default route if he hasn't specific route for particular destination under major network section in routing table, when Classless routing is disabled it wouldn't use default route.

That is it, just remember this, and you'll be fine

oRis1024

marlon23 wrote:

Simple answer : "(no) ip classless" command changes how your router uses the default route. With Classless routing it will use default route if he hasn't specific route for particular destination under major network section in routing table, when Classless routing is disabled it wouldn't use default route.

That is it, just remember this, and you'll be fine

I agree with this 100%. It's that simple. In my notes, on my study sheet i have this(maybe it'll help):

PACKET TO 172.16.4.1

no ip classless:

172.16.0.0/24 is subnetted       <- MATCH
    172.16.1.0 > somewhere     <- NO MATCH
    172.16.2.0 > somewhere     <- NO MATCH
    172.16.3.0 > somewhere     <- NO MATCH - DROP! (even if there are default routes)
0.0.0.0 > somewhere

------------------------------------------------------------------------------------------
ip classless:

172.16.0.0/24 is subnetted       <- MATCH
    172.16.1.0 > somewhere     <- NO MATCH
    172.16.2.0 > somewhere     <- NO MATCH
    172.16.3.0 > somewhere     <- NO MATCH (look further)
0.0.0.0 > somewhere              <- MATCH!

And about instructors in Cisco Networking Academies - i suffer from the same thing. They will teach you subnetting to death, for everything else they'll pull up the answer key and tell you the right option.. I don't complain though