Information Technology Reference
In-Depth Information
Task 3: Enable Classless Routing Behavior on the Routers
Use the
ip classless
command to reenable classless routing.
Step 1.
R1(config)#
ip classless
R2(config)#
ip classless
R3(config)#
ip classless
Examine the routing table on R2.
Step 2.
Notice that the routing table is the still the same, even though the router configuration has
been changed to use classless routing behavior.
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
Repeat the ping from R2 to PC3 and observe 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 100 percent, round-trip min/avg/max = 28/28/28 ms
The ping is successful this time because the router is using classless routing behavior.
The destination address of the packet is a match with the level 1 parent route
172.16.0.0/16, but there is not a match with any of the child routes of this parent route.
Because classless routing behavior is configured, the router will now continue to search
the routing table for a route where there might be fewer bits that match, but the route is
still a match. The mask of a default route is /0, which means that no bits need to match. In
classless routing behavior, if no other route matches, the default route will always match.
S* 0.0.0.0/0 is directly connected, Serial0/0/1
Because there is a default route configured on the R2 router, this route is used to forward
the packets to PC3.
Examine the routing table on R3 to determine how the traffic generated by the
ping
com-
mand is returned to R2.
Step 4.
R3#
show ip route
<output omitted>
Gateway of last resort is not set
