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Advanced Redistribution
Alternatively, we could have used the out argument with the RIP routing
process. The out argument tells the RIP routing process to advertise only
routes that are permitted by the access list. If we had used the out argument,
the syntax would be
router rip
distribute-list 1 out
Using the Null0 Interface
Like the distribute-list feature, the Null0 interface can be used to cre-
atively control routes. Specifically, Null0 is a logical interface, and when
traffic is directed (routed) to this interface, the traffic is discarded. In some
of the literature, you may see Null0 referred to as a "bit bucket" or a "black
hole," because whatever goes in does not come out. Traffic can be directed
into the Null0 interface using a static route. Consider the following example.
In order to conserve IP numbers, some networks use private IP addresses
(e.g.,,, or If we were using pri-
vate address space inside our network, then we would certainly not want to
advertise these networks outside our network, because we might acciden-
tally advertise these routes to someone else who was using private address-
ing. Therefore, we could use our distribute-list command to prevent the
advertisement of private address space. Alternatively, we could create a
static route that directed all traffic destined for this private address space to
be discarded, as shown in Figure 10.10.
F I G U R E 1 0 . 1 0
The bit bucket
ip route null0
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