User Manual

Enterasys X-Pedition User Reference Manual 79
VLAN Overview
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.
Access Ports and Trunk Ports (802.1P and 802.1Q Support)
The ports of an XP can be classified into two types, based on VLAN functionality: access ports
and trunk ports. By default, a port is an access port. An access port can belong to at most one
VLAN of the following types: IP, IPX, or bridged protocols.
Note: The 802.1Q bridge MIB can create an IP-based VLAN with Layer-4 bridging enabled.
Trunk ports (802.1Q) carry traffic belonging to several VLANs and are usually used to connect one
VLAN-aware switch to another. For example, suppose that XP A and B are both configured with
VLANs V1 and V2. A frame arriving at a port on XP A must be sent to XP B, if the frame belongs
to VLAN V1 or to VLAN V2. Thus the ports on XP A and B which connect the two XPs together
must belong to both VLAN V1 and VLAN V2. Also, when these ports receive a frame, they must
be able to determine whether the frame belongs to V1 or to V2. This is accomplished by “tagging”
the frames, i.e., by prepending information to the frame in order to identify the VLAN to which the
frame belongs. In the XP switching routers, trunk ports always transmit and receive tagged frames
only. The format of the tag is specified by the IEEE 802.1Q standard. The only exception to this is
Spanning Tree Protocol frames, which are transmitted as untagged frames.
The XP can automatically determine whether a received frame is an IP frame, an IPX frame or
neither. Based on this, it selects a VLAN for the frame. Frames transmitted out of an access port
contain no special information about the VLAN to which they belong. These frames are classified
as belonging to a particular VLAN based on the protocol of the frame and the VLAN configured on
the receiving port for that protocol. For example, if port 1 belongs to VLAN IPX_VLAN for IPX,
VLAN IP_VLAN for IP and VLAN OTHER_VLAN for any other protocol, then an IP frame
received by port 1 is classified as belonging to VLAN IP_VLAN.