4000 Router Series Layer 2 - LAN Switching Configuration Guide

Configuring VLANs

The VLAN feature is available on only routers with Layer 2 Ethernet switching interface modules installed.

For more information about Layer 2 Ethernet switching interface modules, see HP MSR Router Series

Interface Module Guide.

Overview

Ethernet is a family of shared-media LAN technologies based on the CSMA/CD mechanism. An Ethernet

LAN is both a collision domain and a broadcast domain. As the medium is shared, collisions and

broadcasts are common in an Ethernet LAN. Typically, bridges and Layer 2 switches can reduce

collisions in an Ethernet LAN, but to confine broadcasts, a Layer 2 switch must use the Virtual Local Area

Network (VLAN) technology.

VLANs enable a Layer 2 switch to break a LAN down into smaller broadcast domains, as shown

in Figure 17.

Figure 17 A VLAN diagram

A VLAN is logically divided on an organizational basis rather than on a physical basis. For example, you

can assign all workstations and servers used by a particular workgroup to the same VLAN, regardless of

their physical locations. Hosts in the same VLAN can directly communicate with one another. You need

a router or a Layer 3 switch for hosts in different VLANs to communicate with one another.

All these VLAN features reduce bandwidth waste, improve LAN security, and enable flexible virtual

group creation.

VLAN frame encapsulation

To identify Ethernet frames from different VLANs, IEEE 802.1Q inserts a four-byte VLAN tag between the

destination and source MAC address (DA & SA) field and the upper layer protocol type (Type) field, as

shown in Figure 18.

VLAN 2

VLAN 5

Switch BSwitch A

Router