249
routing, but using null0 as the output interface.
Example:
To configure a route to 2001:2:3:4::/64 as a black hole routing.
Switch(config)#ipv6 route 2001::/64 null0
2.5 Black Hole Routing Configuration Exmaple
Example 1: To configure IPv6 black hole routing.
Fig
2-1
IPv4 Black Hole Routing Configuration Example
As it is shown in the figure, in Switch2, eight in all interfaces are configured as Layer 3 VLAN
interfaces for access interfaces. The network addresses are 192.168.1.0/24 ~ 192.268.7.0/24. A
default routing is configured on Switch2 to connect to Switch1. And a backward default routing is
configured on Switch1 to Switch2, whose network address is 192.168.0.0/21. Commonly, this
configuration will work well. However, if one of the Layer 3 interfaces in Switch2 goes down, for
example, the interface belonged to 192.168.1.0/24. When datagrams arrives at VLAN1 in
Switch2, there will be no routing rules for these datagrams. The switch then will forward these
datagrams according to the default routing, back to Switch1. When Switch1 receives these
datagrams, it will forward them back to Switch2. Thus, loopback exists. To solve this problem,
black hole routing can be introduced on Switch2.
ip route 192.168.0.0/21 null0 50
Then Switch2 will drop the datagrams from interface vlan1 that match the black hole routing
rule.
Configuration steps are listed as below: