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Avaya P333R-LB User’s Guide 89
Chapter 12
Avaya P330 Layer 3 Features
Introduction
This section describes the Avaya P330 Layer 3 features. It provides the basic
procedures for configuring the P330 for Layer 3 operation.
What is Routing?
Routing enables transfer of a data packet from source to destination using a device
called a router. Routing involves two basic activities:
• determining optimal routing paths
• transmitting information packets through an internetwork
Routers use routing tables to determine the routes to particular network
destinations and, in some cases, metrics associated with those routes. Routers
communicate with one another and maintain their routing tables through the
transmission of a variety of messages.
The Routing Update Message is one such message. Routing Updates generally
consist of all or a portion of a routing table. By analyzing Routing Updates from all
routers, a router can build a detailed picture of network topology.
A Link-State Advertisement is another example of a message sent between routers.
Link-State Advertisements inform other routers of the state of the sender's links.
Link information can also be used to build a complete picture of the network's
topology. Once the network topology is understood, routers can determine optimal
routes to network destinations.
Routers can route only those messages that are transmitted in a routable protocol,
such as IP or IPX. Messages in non-routable protocols, such as NetBIOS and LAT,
cannot be routed, but they can be transferred from LAN to LAN via a bridge.
When a router receives a packet, it examines the packet's destination protocol
address. The router then determines whether it knows how to forward the packet to
the next hop. If the router does not know how to forward the packet, it typically
drops the packet unless a default gateway is defined. If the router knows how to
forward the packet, it changes the packet destination’s physical address to that of
the next hop and transmits the packet.
The next hop may or may not be the ultimate destination host. If not, the next hop is
usually another router, which executes the same switching decision process. As the
packet moves through the internetwork, its physical address changes but its
protocol address remains constant. This process is illustrated in the figure below.