Overview; Layer 2 Vs. Layer 3 Vlans - HP procurve 8100fl series Management And Configuration Manual

VLAN Configuration

Overview

6-2

Overview

Virtual LANs (VLANs) are a means of dividing a physical network into several
logical (virtual) LANs. The division can be done on the basis of various criteria,
giving rise to different types of VLANs. For example, the simplest type of VLAN
is the port-based VLAN. Port-based VLANs divide a network into a number of
VLANs by assigning a VLAN to each port of a switching device. Then, any
traffic received on a given port of a switch belongs to the VLAN associated
with that port.
VLANs are primarily used for broadcast containment. A Layer 2 (L2) broadcast
frame is normally transmitted all over a bridged network. By dividing the
network into VLANs, the range of a broadcast is limited. This means the
broadcast frame is transmitted only to the VLAN to which it belongs. This
reduces the broadcast traffic on a network by an appreciable factor.

Layer 2 vs. Layer 3 VLANs

VLANs are an integral part of the 8100fl switch, which can function both as
Layer 2 (L2) switches and as fully-functional Layer 3 (L3) routers. Hence they
can be viewed as a switch and a router in one box. To provide maximum
performance and functionality, the L2 and L3 aspects of the 8100fl switch are
tightly coupled.
The switch can be used purely as an L2 switch. Frames arriving at any port
are bridged and not routed. In this case, setting up VLANs and associating
ports with VLANs is all that is required.
The switch can also be used purely as a router, that is, each physical port of
the switch 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 as a result of creating L3 interfaces
for IP. However, these implicit VLANs do not need to be created or configured
manually. The implicit VLANs created by the switch are subnet-based VLANs.
Most commonly, the 8100fl switch is used as a combined switch and router.
For example, the switch may be connected to two subnets S1 and S2. Ports 1­
5 belong to S1 and ports 6-10 belong to S2. The required behavior of the switch
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.