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Configuring OSI Services

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303535-A Rev 00

End systems generate Hello packets that contain the end system’s subnetwork

address, and specify which NSAPs the end system services. When a router

receives an end system Hello packet, it extracts the configuration information

from the packet (matching the subnetwork address with the corresponding

NSAPs) and stores it in its routing information base. Routers generate Hello

packets that contain the router’s own subnetwork address. When an end system

receives a router Hello packet, the end system extracts the router’s subnetwork

address and stores it in its own routing information base.

Two types of timers control how often Hello packets are exchanged: a

configuration timer and a holding timer. The configuration timer, which is

maintained by each individual system, determines how often a system reports its

availability or any change in its configuration to the other systems attached to the

same subnetwork. The holding timer, which is a value set by the originating

system, is contained in the holding time field of a Hello packet. It specifies how

long a receiving system should retain the configuration information before it is

flushed from the routing information base.

Route Redirecting

The ISO 9542 route redirection function allows routers to inform end systems of

the most desirable route to a particular destination either

• Through a different router, or

• Directly to an end system on the same subnetwork

After the router forwards a data packet to the next hop toward the destination end

system, the router checks to see whether a more direct route exists. The router

determines whether the next hop is

• The destination system, and whether it is attached to the same subnetwork as

the originating system (Figure 1-12

, Example 1)

• Another router that is connected to the same subnetwork as the originating

end system (Figure 1-12

, Example 2)

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Avaya Configuring OSI Services Manual do Utilizador Página 42