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Configuring IP Services

2-22

Deﬁning a Static Route

A static route is a manually conﬁgured route that speciﬁes the transmission path a

datagram must follow, based on the datagram’s destination address. A static route

speciﬁes a transmission path to another network. You conﬁgure a static route if

you want to restrict the paths that datagrams follow to paths you speciﬁcally

conﬁgure.

Static routes remain in IP routing tables until you remove them. Note, however,

that if the interface that was used to reach the next hop in the static route becomes

disabled, the static route disappears from the IP routing table.

Deﬁning a Black Hole for a Supernet

A router that advertises an aggregate route by using a supernet address to

represent multiple explicit routes must be able to discard packets that match the

supernet address but that do not match any of the explicit routes.

For example, consider a router that advertises an aggregate route using the

supernet address 192.32.0.0/255.255.248. The supernet address represents eight

speciﬁc networks: 192.32.0.0 through 192.32.7.0. Once the aggregate route has

been propagated, the router receives network trafﬁc for each of these speciﬁc

destinations.

At some point, the router loses connectivity to network 192.32.3.0, one of the

networks in the supernet. The router continues to forward trafﬁc that matches

destinations 0.0 through 2.0 and 4.0 through 7.0. However, the router can no

longer ﬁnd a complete match in the routing table for the disconnected network,

3.0. The router must drop all trafﬁc destined for 192.32.3.0.

To force the router to drop the packet for an unmatched destination, you conﬁgure

a special type of static route for a supernet called a black hole. Speciﬁcally, you

enter the supernet address/mask pair as the Destination IP Address and Address

Mask parameter values on the IP Static Routes window. To create the black hole,

you enter the black hole encoding (255.255.255.255) as the Next Hop Addr and

Next Hop Mask parameter values.

For instructions on using Site Manager to conﬁgure a static route, see “Conﬁguring Static

Routes” on page 2-63.

For instructions on using Site Manager to deﬁne a black hole in an aggregate route, see

“Conﬁguring Static Routes” on page 2-63.

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