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Appendix A:
LANs vs. VLANs
A LAN is a broadcast domain composed of hubs, switches, or bridges that are physically
wired to each other and to multiple nodes and hosts. Typically, hosts within one LAN can
communicate directly with each other, but inter-LAN communication requires one or more
routers depending on the complexity of the network. Use of routers increases the
possibility of network traffic delays and gaps in security.
VLANs allow you to divide a network into logical subnets without modifying the
underlying physical structure of the network, thus minimizing the latency and security
problems associated with additional routers or gateways.
You define VLAN members by assigning an identical VLAN ID to each node in the group.
The nodes do not have to be physically connected to the same switch or hub.
For example, as shown in Figure A-1, VLAN ID 1 contains three nodes from Switch A
and one node from Switch B. Similarly, VLAN ID 2 contains nodes that are not all
connected to the same switch.
By creating VLANs in your network, you can enable a single switch or hub to support
more subnets than the number of available physical ports on the switch or hub would
otherwise allow.
Tagging Formats
A VLAN port can forward traffic entering from a physical LAN or from another node in
the VLAN depending on the tagging format a packet supports. A VLAN port can receive
and forward frames with these tagging formats:
•
untagged - regular Ethernet frame
•
VLAN-tagged - frame containing a four-byte VLAN ID
The Bluesocket BSC VLAN Implementation
On the BSC, each defined VLAN interface is bound to either the protected or managed
physical interface. Each VLAN sharing the same physical interface must have a unique ID
A-2
Switch A
VLAN-1
Figure A-1: Sample VLANs
Router
S
C I S C O Y S T E M S
Po w e r S u p p l y 0
C a t a l y s t
850 0
S E R I E S
Sw i t c h
Pr o c e ss o r
LAN Backbone
Switch B
VLAN-2
Po w e r S u p p l y 1
S D
Switch C
VLAN-3
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