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Enterasys Xpedition User Reference Manual 37

VLAN Overview

The XP can also be used purely as a router, i.e., each physical port of the XP is a separate

routing interface. Packets received at any interface are routed and not bridged. In this

case, no VLAN configuration is required. Note that VLANs are still created implicitly by

the XP as a result of creating L3 interfaces for IP and/or IPX. However, these implicit

VLANs do not need to be created or configured manually. The implicit VLANs created by

the XP are subnet-based VLANs.

Most commonly, an XP is used as a combined switch and router. For example, it may be

connected to two subnets S1 and S2. Ports 1-8 belong to S1 and ports 9-16 belong to S2.

The required behavior of the XP is that intra-subnet frames be bridged and inter-subnet

packets be routed. In other words, traffic between two workstations that belong to the

same subnet should be bridged, and traffic between two workstations that belong to

different subnets should be routed.

The XP switching routers use VLANs to achieve this behavior. This means that a L3

subnet (i.e., an IP or IPX subnet) is mapped to a VLAN. A given subnet maps to exactly

one and only one VLAN. With this definition, the terms VLAN and subnet are almost

interchangeable.

To configure an XP as a combined switch and router, the administrator must create

VLANs whenever multiple ports of the XP are to belong to a particular VLAN/subnet.

Then the VLAN must be bound to an L3 (IP/IPX) interface so that the XP knows which

VLAN maps to which IP/IPX subnet.

Ports, VLANs, and L3 Interfaces

The term port refers to a physical connector on the XP, such as an ethernet port. Each port

must belong to at least one VLAN. When the XP is unconfigured, each port belongs to a

VLAN called the “default VLAN.” By creating VLANs and adding ports to the created

VLANs, the ports are moved from the default VLAN to the newly created VLANs.

Unlike traditional routers, the XP has the concept of logical interfaces rather than physical

interfaces. An L3 interface is a logical entity created by the administrator. It can contain

more than one physical port. When an L3 interface contains exactly one physical port, it is

equivalent to an interface on a traditional router. When an L3 interface contains several

ports, it is equivalent to an interface of a traditional router which is connected to a layer-2

device such as a switch or bridge.

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Cabletron Systems SSIM-R8-02 Spécifications Page 57