Black Hole Routing Troubleshooting - Planet XGS3-42000R User Manual

Example 2: IP v6 Black Hole Routing function.
2004:1:2:3:1::/80
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 net work address is 2004:1: 2:3::/64. Commonly, this configuration will work well. However, if
one of t he Layer 3 interfaces in S witch 2 goes down, for example, the int erface belonged to 2004:1:2:3: 1/80.
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.
ipv6 rout e 2004:1:2:3::/64 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)#ipv6 route 2004:1:2:3::/64 null0 50

41.5 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 distanc e. Attention should be paid to the
following items:

IP v6 should be enabled before IP v6 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 net work 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.
For problems that cannot be fixed through above methods, please issue the command show ip route distance
2004:1:2:3::1/64
SWITCH2
Figure 41-4-2 IP v6 Black Hole Routing Configuration Example
SWITCH1
2004:1:2:3::2/64
.........
2004:1:2:3:7::/80
41-3