3Com CoreBuilder 3500, Implementation Manual, Page: 156 / 592

156 C
HAPTER
9: V
IRTUAL
LAN
S
VLAN Overview
A virtual LAN (VLAN) is a logical grouping that allows end users to
communicate as if they were physically connected to a single LAN,
independent of the physical configuration of the network. A VLAN is
generally considered equivalent to a Layer 2 broadcast domain or a
Layer 3 network.
Your system’s point of attachment to a given VLAN is called a
VLAN
interface . A VLAN interface exists entirely within a single switch; you
control the configuration of the VLAN interfaces on the switch. A VLAN
and a VLAN interface are analogous to an IP subnet and an IP interface
on a router.
Need for VLANs If a bridge port in a LAN switch receives a frame with a broadcast,
multicast, or unknown destination address, it forwards the data to all
bridge ports in the VLAN that is associated with the frame, except the
port on which it was received. This process is referred to as bridge
flooding . As networks grow and the amount and types of traffic increase,
bridge flooding may create unnecessary traffic problems that can clog the
LAN.
To help control the flow of traffic through a switch and meet the
demands of growing networks, vendors have responded by:
■
Using customized packet filtering to further control which packets are
forwarded through the bridge. These filters can be complex to
configure.
■
Using more and more routers as broadcast firewalls to divide the
network into broadcast domains. As the number of legacy routers
increase, latency begins to degrade network performance,
administration overhead increases, and operating costs rise.
■
Using the Spanning Tree algorithm in switches to control the flow of
traffic among LANs (for redundant links). These mechanisms work
best only in certain types of LAN topologies.