1. Network Engineering
Title of the experiment: Use of Null 0 interface for loop avoidance
1) Make the connections of routers as per the following diagram.
R3
S0 10.1.5.2
S1 10.1.5.1 R2

2. R1
S0 S1 E0
10.1.4.2 10.1.4.1
192.168.1.1/24
��.2Consider that router R3 is an ISP backbone router, R1 is an ISP router and router
R2 is a customer premises router. Assume that a block of 192.168.0.0 is assigned by
the service provider and some addresses of 192.168.0.0/24 block is used by router R2.
��.3Configure static ip route between router R1 and R3 and default route from router
R2 to R1as shown above in the diagram.
4) Also configure E0 interface of router R2 as shown above.
5) Try to ping unused IP address such as 192.168.20.1 from router R3. It will be
observed on R3 that it receives message from R1’s S0 interface that “icmp timeout
occurs” because of the loop occurred between R1 and R2.
��.6Rectify the above situation by assigning static route of 192.168.0.0 to Null 0 on R2.
7) Once step 6 is done, the packet destined to 192.168.20.1 will be diverted to Null 0
and discarded later.

3. 8) Hence loop back can be avoided.
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Router configurations -
Router R1 –
L0 = 10.1.1.1 255.255.255.255
S0 = 10.1.4.2/30
S1 = 10.1.5.1/30
ip route 192.168.0.0 255.255.0.0 10.1.4.1
Router R2 –
L0 = 10.2.2.2 255.255.255.255
E0 = 192.168.1.1 255.255.255.0
S1 = 10.1.4.1/30

4. ip route 0.0.0.0 0.0.0.0 10.1.4.2
Router R3 –
L0 = 10.3.3.3 255.255.255.255
S0 = 10.1.5.2/30
ip route 10.1.4.0 10.1.5.1
ip route 192.168.0.0 10.1.5.1