3Com 4007, Руководство по внедрению, Страница: 242 / 684

242 C
HAPTER
14: V
IRTUAL
LAN
S
(VLAN
S
)
You can manage VLAN features in either of these ways:
■
From the
bridge vlan
menu of the Administration Console. (See the
Command Reference Guide .) You can use the Administration Console
after you log in to the system and connect to a module slot.
■
From the Bridge VLAN folder of the Web Management software. (See
the
Switch 4007 Getting Started Guide .)
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
.
For the Switch 4007, a VLAN interface exists entirely within a
single switching module or switch fabric module; you control the
configuration of the VLAN interfaces. A VLAN and a VLAN interface are
analogous to an IP subnetwork and an IP interface on a router.
Need for VLANs If a bridge port in a LAN switching device receives a frame with a
broadcast, multicast, or unknown destination address, it forwards the
data to all bridge ports in the VLAN that are 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 switching device and meet
the demands of growing networks, vendors have responded by:
■
Using customized packet filtering and IP multicast controls such as
Internet Group Management Protocol (IGMP) snooping to provide
further controls on which packets are forwarded through the bridge.
These controls require more complex configuration by the
administrator.
■
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, increase
administration overhead, and raise operating costs.