Step 1

Prior to the attack, select an address for the intended black hole next-hop. Configure the forwarding plane of each edge router so that packets forwarded to this next-hop are dropped:

Create a black hole VLAN (Virtual LAN) with an IP address that is in the same subnet as the chosen black hole next-hop.
Add an active port to the black hole VLAN (usually an unused port in the switch).
Create a static FDB entry that maps a well-chosen, unused MAC address to the black hole VLAN and the active port added to that VLAN.
Create a static ARP entry that maps the black hole next-hop to the above MAC address.
Create an filter to deny packets that exit the blackhole VLAN.
In the following example configuration, 192.168.2.0/24 is the subnet of the black hole VLAN, “BH_VLAN,” and 192.168.2.66 is the chosen black hole next-hop. The active port 6:9 is added as the egress port for “BH_VLAN.”
```
create vlan BH_VLAN
configure vlan BH_VLAN tag 666
enable loopback-mode vlan BH_VLAN
configure vlan BH_VLAN ipaddress 192.168.2.1 255.255.255.0
enable ipforwarding vlan BH_VLAN
disable igmp snooping vlan BH_VLAN
disable igmp vlan BH_VLAN
create fdb 00:02:03:04:05:06 vlan BH_VLAN port 6:9
configure iparp add 192.168.2.66 vr VR-Default 00:02:03:04:05:06
configure access-list BH_ACL vlan BH_VLAN egress
```
When a packet arrives in the forwarding plane and looks up a route that has the above black hole next-hop as its next-hop, a subsequent ARP and FDB (forwarding database) look-up occurs that forwards the packet to exit the switch using the above black hole VLAN, “BH_VLAN,” and port “6:9.” The packet is dropped due to the deny action in the egress ACL filter.

The following policy file discards any traffic that exits the black hole VLAN, “BH_VLAN.” Note that the match on “source-address 0.0.0.0/0” matches any egress packet ensuring that all packets exiting via the black hole VLAN are dropped:
```
edit policy BH_ACL
entry bh-acl {
if {
	source-address 0.0.0.0/0;
} then {
deny ;
}
}
```

Published March 2020prev | next

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