PLANET XGS3-24042 - Manual (Page 369)

42-3
Example 2: IPv6 Black Hole Routing function.
Figure 41-2 IPv6 Black Hole Routing Configuration Example
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 network addresses are 2004:1:2:3:1/80~2004:1:2:3:7/80. A default routing is
configured on Switch 2 to connect to Switch 1. And a backward default routing is configured on Switch 1 to
Switch 2, whose network address is 2004:1:2:3::/64. 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 2004:1:2:3:1/80.
When datagrams arrives at VLAN1 in Switch 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 Switch 2.
ipv6 route 2004:1:2:3::/64 null0 50
Then Switch 2 will drop the datagrams from interface VLAN1 that match the Black Hole Routing rule. And
loopback routing is prevented.
Configuration steps are listed as below:
Switch#config
Switch(config)#ipv6 route 2004:1:2:3::/64 null0 50
42.5 Black Hole Routing Troubleshooting
When configuring the Black Hole Routing function, the configuration may not work due to some reasons such
as incorrect network address mask, and incorrect management distance. Attention should be paid to the
following items:
 IPv6 should be enabled before IPv6 Black Hole Routing can work.
 It is suggested that the length of the network address mask should be longer than that of normal routing
configuration, in order to prevent the Black Hole Routing from intervening other routing configuration.
 When the network address mask of Black Hole Routing configuration is the same with some other
configuration, it is suggested that the distance of Black Hole Routing is set lower.
2004:1:2:3:1::/80 2004:1:2:3:7::/80
………
SWITCH2
SWITCH1 2004:1:2:3::1/64
2004:1:2:3::2/64