Interlogix NS2503-24P/2C User Manual - Manual (Page 66)

NS2503-24P/2C User Manual
66
VLAN configuration
VLAN Overview
A Virtual Local Area Network (VLAN) is a network topology configured according to a logical scheme rather than the physical
layout. VLAN can be used to combine any collection of LAN segments into an autonomous user group that appears as a single
LAN. VLAN also logically segment the network into different broadcast domains so that packets are forwarded only between
ports within the VLAN. Typically, a VLAN corresponds to a particular subnet, although not necessarily.
VLAN can enhance performance by conserving bandwidth, and improve security by limiting traffic to specific domains.
A VLAN is a collection of end nodes grouped by logic instead of physical location. End nodes that frequently communicate with
each other are assigned to the same VLAN, regardless of where they are physically on the network. Logically, a VLAN can be
equated to a broadcast domain, because broadcast packets are forwarded to only members of the VLAN on which the
broadcast was initiated.
1. No matter what basis is used to uniquely identify end nodes and assign these nodes VLAN
membership, packets cannot cross VLAN without a network device performing a routing
function between the VLAN.
2. The Managed Switch supports IEEE 802.1Q VLAN. The port untagging function can be used
to remove the 802.1 tag from packet headers to maintain compatibility with devices that are
tag-unaware.
The Managed Switch supports IEEE 802.1Q (tagged-based) and Port-Base VLAN setting in web management page. In the
default configuration, VLAN support is “802.1Q”.
 Port-based VLAN
Port-based VLAN limit traffic that flows into and out of switch ports. Thus, all devices connected to a port are members of the
VLAN(s) the port belongs to, whether there is a single computer directly connected to a switch, or an entire department.
On port-based VLAN, NICs do not need to be able to identify 802.1Q tags in packet headers. NIC send and receive normal
Ethernet packets. If the packet's destination lies on the same segment, communications take place using normal Ethernet
protocols. Even though this is always the case, when the destination for a packet lies on another switch port, VLAN
considerations come into play to decide if the packet is dropped by the Managed Switch or delivered.
 IEEE 802.1Q VLANs
IEEE 802.1Q (tagged) VLAN are implemented on the Managed Switch. 802.1Q VLAN require tagging, which enables them to
span the entire network (assuming all switches on the network are IEEE 802.1Q-compliant).
VLAN allows a network to be segmented in order to reduce the size of broadcast domains. All packets entering a VLAN will only
be forwarded to the stations (over IEEE 802.1Q enabled switches) that are members of that VLAN, and this includes broadcast,
multicast and unicast packets from unknown sources.
VLAN can also provide a level of security to your network. IEEE 802.1Q VLAN will only deliver packets between stations that
are members of the VLAN. Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q
VLAN allows VLAN to work with legacy switches that don't recognize VLAN tags in packet headers. The tagging feature allows
VLAN to span multiple 802.1Q-compliant switches through a single physical connection and allows Spanning Tree to be
enabled on all ports and work normally.
Some relevant terms:
- Tagging - The act of putting 802.1Q VLAN information into the header of a packet.
- Untagging - The act of stripping 802.1Q VLAN information out of the packet header.
 802.1Q VLAN Tags
The figure below shows the 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address. Their
presence is indicated by a value of 0x8100 in the Ether Type field. When a packet's Ether Type field is equal to 0x8100, the
packet carries the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and consists of 3 bits of user priority,
1 bit of Canonical Format Identifier (CFI - used for encapsulating Token Ring packets so they can be carried across Ethernet
backbones), and 12 bits of VLAN ID (VID). The 3 bits of user priority are used by 802.1p. The VID is the VLAN identifier and is
used by the 802.1Q standard. Because the VID is 12 bits long, 4094 unique VLAN can be identified.
The tag is inserted into the packet header making the entire packet longer by 4 octets. All of the information originally contained
in the packet is retained.