Avaya P332G-ML manual Avaya P330 Layer 3 Features, What is Routing?

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.

Layer 3 features are relevant to P332G-ML operating in router mode. You must purchase a Layer 3 preconfigured P332G-ML module or a Routing License Key Certificate for the P332G-ML in order to operate the P332G-ML in router mode.

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

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