Avaya Configuring IP Services Uživatelský manuál Strana 110

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Configuring IP Services
3-46 114065 Rev. A
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:
Enter the supernet address/mask pair as the destination IP address and address
mask.
To create the black hole, enter the black hole encoding (255.255.255.255) as
the next-hop address and the next-hop mask.
Configuring Router Discovery
Before a host can send IP datagrams beyond its directly attached subnet, the host
must discover the address of at least one operational router on that subnet. Router
Discovery is an extension of the Internet Control Message Protocol (ICMP) that
enables hosts attached to multicast or broadcast networks to discover the IP
addresses of their neighboring routers.
Routers configured with Router Discovery periodically multicast or broadcast a
router advertisement from each of their interfaces, announcing the IP address or
addresses of that interface. Hosts discover the addresses of their neighboring
routers by listening for these advertisements. Hosts will use the router with the
highest preference level as a gateway.
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