Information Technology Reference
In-Depth Information
Task 2: Enable Classful Routing Behavior on the Routers
Use the
no ip classless
command to configure the route lookup process to use classful
route lookups.
Step 1.
R1(config)#
no ip classless
R2(config)#
no ip classless
R3(config)#
no ip classless
Examine the routing table on the R2 router.
Step 2.
R2#
show ip route
<output omitted>
Gateway of last resort is 0.0.0.0 to network 0.0.0.0
172.16.0.0/24 is subnetted, 4 subnets
R 172.16.1.0 [120/1] via 172.16.2.1, 00:00:00, Serial0/0/0
C 172.16.2.0 is directly connected, Serial0/0/0
C 172.16.3.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Serial0/0/1
S* 0.0.0.0/0 is directly connected, Serial0/0/1
Ping from R2 to PC3 and observe the results.
Step 3.
R2#
ping 172.16.4.10
Type escape sequence to abort.
Sending 5, 100-byte ICMP Echos to 172.16.4.10, timeout is 2 seconds:
.....
Success rate is 0 percent (0/5 )
The ping is unsuccessful because the router is using classful routing behavior.
The route lookup process on the R2 router searches the routing table and finds that the
first 16 bits of the destination address are a match with the parent route 172.16.0.0/16.
Because the destination address matches the parent route, the child routes are checked.
What are the child routes of the 172.16.0.0/16 parent network?
172.16.1.0/24
172.16.2.0/24
172.16.3.0/24
How many bits in the destination address must match for a packet to be forwarded using
one of the child routes?
24
Does the destination address of the ping packets match any of the child routes of
172.16.0.0/16?
no
Because the
no ip classless
command has been used to configure the R2 router to use
classful routing behavior, when a level 1 match is found, the router will not search beyond
the child routes for a lesser match. Even though there is a default static route configured, it
will not be used, and the packet will be dropped.
