Refer to the exhibit. In this network, R1 has been configured to advertise a summary route, 192.168.0.0/22, to R2. R2 has been configured to advertise a summary route, 192.168.0.0/21, to R1. Both routers have been configured to remove the discard route (the route to null created when a summary route is configured) by setting the administrative distance of the discard route to 255.
What will happen if R1 receives a packet destined to 192.168.3.1?

A.
The packet will loop between R1 and R2.

B.
It is not possible to set the administrative distance on a summary to 255.

C.
The packet will be forwarded to R2, where it will be routed to null0.

D.
The packet will be dropped by R1, since there is no route to 192.168.3.1.

Explanation:
Here are the configurations and CLI output to prove the answer is A. This is done on IOS 12.4T train of code.
R1

interface Loopback0
ip address 192.168.1.1 255.255.255.0
!
interface FastEthernet0/0
ip address 10.1.2.1 255.255.255.0
ip summary-address eigrp 1 192.168.0.0 255.255.252.0 255
!
router eigrp 1
network 10.0.0.0
network 192.168.1.0
no auto-summary
R2

interface Loopback0
ip address 192.168.4.2 255.255.255.0
!
interface FastEthernet0/0
ip address 10.1.2.2 255.255.255.0
ip summary-address eigrp 1 192.168.0.0 255.255.248.0 255
!
router eigrp 1
network 10.0.0.0
network 192.168.4.0
no auto-summary
Show Output

Look at R1’s routing tablenotice it does NOT have the summary route to null0 for 192.168.0.0/22 because we have disabled it. It does have a summary route to 192.168.0.0/21 from R2 it is receiving
R1#show ip route | b Gateway
Gateway of last resort is not set
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
C 192.168.1.0/24 is directly connected, Loopback0
D 192.168.0.0/21 [90/156160] via 10.1.2.2, 00:03:45, FastEthernet0/0
Now look at R2’s routing table. Notice it does NOT have a summary route to null0 for the 192.168.0.0/21 because we disabled it. It does however have a route for 192.168.0.0/22 received from R1.
R2#sh ip route | b Gateway
Gateway of last resort is not set
C 192.168.4.0/24 is directly connected, Loopback0
10.0.0.0/24 is subnetted, 1 subnets
C 10.1.2.0 is directly connected, FastEthernet0/0
D 192.168.0.0/22 [90/156160] via 10.1.2.1, 00:05:36, FastEthernet0/0
Now, what will happen? As I said When R1 pings 192.168.3.1 it will look in its routing table, and it will see the EIGRP route 192.168.0.0/21 from R2 so it will send the packet to R2 since 192.168.3.1 is part of that summary. R2 will get the packet and look in its routing table. It will find an EIGRP route for 192.168.0.0/22 from R1 and route the packet back to R1 since 192.168.3.1 is within that summary range. This continues in a loop until the IP TTL expires.
Traceroute from R1. Notice it goes to R2 then right back to R1Hope this helps!

R1#trace 192.168.3.1
Type escape sequence to abort.
Tracing the route to 192.168.3.1
1 10.1.2.2 0 msec 0 msec 4 msec
2 10.1.2.1 0 msec 0 msec 4 msec
3 * * *