HP ProCurve 7000dl Series Basic Management And Configuration Manual page 603

To configure a null route, enter this command from the global configuration
mode context:
Syntax: ip route <A.B.C.D> <subnet mask | /prefix length> null 0 [<administrative
distance>]
You might configure a route through the null interface in order to drop traffic
to network addresses that do not yet exist in your network.
For example, an organization has allocated the address space 192.168.20.0 /24
to a remote site. However, currently the site is only using half of the addresses.
Network management have divided the network into two /25 subnets and left
the second subnet (192.168.20.128 /25) unused. You can prevent the local
router from forwarding traffic across the WAN link that will only dropped by
the remote router. Enter this command:
ProCurve(config)# ip route 192.168.20.128 /25 null 0
You could also use a null route in order to force the router to:
drop traffic to destinations that you have determined to be unauthorized
However, a better way to control traffic is to use an ACL or an ACP. (See
Chapter 5: Applying Access Control to Router Interfaces of the Advanced
Management and Configuration Guide.)
advertise a route not included in its routing table
When a router uses a routing protocol, its routing table must include a
route in order to advertise that route. You could configure a null route if
you wanted the router to advertise a route, but not to forward traffic using
that route. (For more information on this topic, see "Advertising Local
Networks" on page 15-73 in Chapter 15: IP Routing—Configuring RIP,
OSPF, BGP, and PBR of the Advanced Management and Configuration
Guide.)
IP Routing—Configuring Static Routes
Configuring Static Routes
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