Chapter 29: Security
STANDARD Revision 1.0
C4® CMTS Release 8.3 User Guide
© 2016 ARRIS Enterprises LLC. All Rights Reserved.
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Routing to a Null Interface
A null interface is a pseudo-interface that can neither receive nor forward packets. Any packets routed to a null interface
are dropped. Routing to a null interface is a way to filter traffic that is simpler than using Access Control Lists. It functions
like a global Access Control List (ACL) since a single entry in the routing table applies to all ingress interfaces.
With this feature enabled, the C4/c CMTS system administrator can define a route in the C4/c CMTS routing table to a null
interface. A Classless InterDomain Routing (CIDR) match with this defined route on the destination IP address of an ingress
IP datagram results in the dropping of the packet.
Routing to a null interface can be used to discard certain IP prefixes in order to remove the potential for routing loops. By
default the packets routed to the null interface are discarded silently, but the C4/c CMTS can be configured to return an
Internet Control Message Protocol (ICMP) "destination unreachable" error to the source IP. The cost of the route is zero by
default, but it can be increased by user command.
CLI Commands for Routing to a Null Interface
To enable the sending of ICMP unreachable messages:
configure interface null 0 ip unreachables
The only valid interface number for this command is 0. The result is global.
To add a route to the null interface:
configure ip route [destination ip] [destination mask] null 0
To add an IPv6 route to the null interface:
configure ipv6 route [vrf <VRF>] null <0-0> [admin-distance <0-255>] [metric <INTEGER>]
To display IP routes:
show ip route
Or …
show ipv6 route
