Black Hole Routing Manual
350
Switch(config)#ip route 192.168.0.0/21 null0 50
Example 2: IPv6 Black Hole Routing function.
Fig 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
41.5
Black Hole Routing Troubleshooting
When configuring the Black Hole Routing function, the configuration may not work
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