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One of the drawbacks of MAC
address–based VLAN solutions is the require-
ment that all users must initially be configured
to be in at least one VLAN. After that initial
manual configuration, automatic tracking of
users is possible, depending on the specific
vendor solution. However, the disadvantage of
having to initially configure VLANs becomes
clear in very large networks where thousands of
users must each be explicitly assigned to a par-
ticular VLAN. Some vendors have mitigated
the onerous task of initially configuring MAC-
based VLANs by using tools that create
VLANs based on the current state of the
network—that is, a MAC address–based
VLAN is created for each subnet.
MAC address–based VLANs that are
implemented in shared media environments
will run into serious performance degradation
as members of different VLANs coexist on a
single switch port. In addition, the primary
method of communicating VLAN membership
information between switches in a MAC
address–defined VLAN also runs into per-
formance degradation with larger-scale imple-
mentations. This is explained in “Communi-
cating VLAN Membership Information,” later
in this paper.
Another, but minor, drawback to VLANs
based only on MAC-layer addresses emerges
in environments that use significant numbers
of notebook PCs with some docking stations.
The problem is that the docking station and
integrated network adapter (with its hard-wired
MAC-layer address) usually remain on the
desktop, while the notebook travels with the
user. When the user moves to a new desk and
docking station, the MAC-layer address
changes, making VLAN membership
impossible to track. In such an environment,
VLAN membership must be updated con-
stantly as users move around and use different
docking stations. While this problem may not
be particularly common, it does illustrate some
of the limitations of MAC address–based
VLANs.
Layer 3–Based VLANs
VLANs based on layer 3 information take into
account protocol type (if multiple protocols
are supported) or network-layer address (for
example, subnet address for TCP/IP networks)
in determining VLAN membership. Although
these VLANs are based on layer 3 infor-
mation, this does not constitute a “routing”
function and should not be confused with
network-layer routing.
Even though a switch inspects a packet’s
IP address to determine VLAN membership,
no route calculation is undertaken, RIP or
OSPF protocols are not employed, and frames
traversing the switch are usually bridged
according to implementation of the Spanning
Tree Algorithm. Therefore, from the point of
view of a switch employing layer 3–based
VLANs, connectivity within any given VLAN
is still seen as a flat, bridged topology.
Having made the distinction between
VLANs based on layer 3 information and
routing, it should be noted that some vendors
are incorporating varying amounts of layer 3
intelligence into their switches, enabling
functions normally associated with routing.
Furthermore, “layer 3 aware” or “multi-layer”
switches often have the packet-forwarding
function of routing built into ASIC chip sets,
greatly improving performance over CPU-
based routers. Nevertheless, a key point
remains: no matter where it is located in a
VLAN solution, routing is necessary to
provide connectivity between distinct VLANs.
There are several advantages to defining
VLANs at layer 3. First, it enables partitioning
by protocol type. This may be an attractive
option for network managers who are ded-
icated to a service- or application-based VLAN
strategy. Second, users can physically move
their workstations without having to recon-
figure each workstation’s network address—a
benefit primarily for TCP/IP users. Third,
defining VLANs at layer 3 can eliminate the
need for frame tagging in order to commu-
nicate VLAN membership between switches,
reducing transport overhead.
One of the disadvantages of defining
VLANs at layer 3 (vs. MAC- or port-based
VLANs) can be performance. Inspecting
layer 3 addresses in packets is more time con-
suming than looking at MAC addresses in
frames. For this reason, switches that use
4
Acronyms and
Abbreviations
AAL5
ATM Adaptation Layer Type 5
ASIC
Application-specific integrated
circuit
ATM
Asynchronous Transfer Mode
DHCP
Dynamic Host Configuration
Protocol
ELAN
Emulated LAN
FDDI
Fiber Distributed Data Interface
IPX
Internet Packet Exchange
LANE
LAN Emulation
LEC
LAN Emulation client
LES
LAN Emulation server