Document No. 10-300077, Issue 2 12-21
Configuring IP Routing
Figure 12-7. Null Interface Example
In the preceding figure, Router 1 has a default route that points to the
Border Router for addresses that are not known within the enterprise.
If a null interface is not configured:
1. Router 3 forwards packets that have a destination of 10.10.3.32 to Router
1.
2. Because VLAN 103 is down, Router 1 sends the packets to the Border
Router.
3. The Border Router then sends the traffic back to Router 1 via the route
10.10.0.0/16.
The loop that occurs is shown as a red dotted line in Figure 12-7.
To prevent such a loop, you can create a static route from 10.10.0.0/16 to
the null interface. Once this static route to the null interface is created,
Router 1 drops all packets whose destination address is unreachable and
within the 10.10.0.0/16 network.
When packets are sent to the null interface, the router sends an
Internet
Control Message Protocol (ICMP) destination unreachable message to
the source of the packet (Router 3 in Figure 12-7).
