Planet XGS3-42000R User Manual page 346

192.168.1.0/ 24
As it is shown in the figure, in Switch 2, eight in all interfaces are configured as Layer 3 VLAN interfaces for
access interfaces. The net work addresses are 192.168. 1.0/24 ~ 192.268.7.0/24.
configured on Switch 2 to connect to Switch 1. And a backward default routing is configured on Switch 1 to
Switch 2, whose net work address is 192.168.0.0/21. Commonly, this configuration will work well. However, if
one of the Layer 3 interfaces in Switch 2 goes down, for example, the interface belonged to 192. 168.1. 0/24.
When datagrams arrives at VLAN1 in S witch 2, there will be no routing rules for these datagrams. The switch
then will forward these datagrams according to the default routing, back to Switch 1. When Switch 1 receives
these datagrams, it will forward them back to Switch 2. Thus, loopback exists. To solve this problem, Black
Hole Routing can be introduced on S witch 2.
ip route 192. 168. 0.0/21 null0 50
Then S witch 2 will drop the datagrams from interface V LAN1 that match the Black Hole Routing rule. And
loopback routing is prevented.
Configuration steps are listed as below:
XGS 3-42000R#config
XGS 3-42000R(config)#ip route 192.168.0.0/21 null0 50
192.168.0.1/ 21
SWITCH2
Figure 41-4-1 IP v4 Black Hole Routing Configuration Example
SWITCH1
192.168.0.2/ 21
.........
192.168.7.0/ 24
41-2
A default routing is