R
EVISED
2018-09-21
F
EATURES
AND
S
PECIFICATIONS
D
RAWING
N
O
. LP0991-G
O
VERVIEW
OF
A
DVANCED
F
EATURES
N-Tron
®
Series NT24K
®
Software Manual
1-3
1.2.4
Quality
of
Service
(QoS)
QoS refers to resource reservation control mechanisms. QoS is the ability to provide different priorities to different
applications, users, or data flows. QoS guarantees are important if the network capacity is insufficient, especially
for real-time streaming multimedia applications such as Voice over IP, high resolution images, online games and
IP-TV, since these often require fixed bit rate and are delay sensitive, and in networks where the capacity is a
limited resource, for example in cellular data communication. In the absence of network congestion, QoS
mechanisms are not required.
The QoS of an egressing frame can be set based on the three QoS mechanisms listed below:
1. IEEE 802.1p (Tagged QOS)
2. DSCP (differentiated services code points) (RFC 2474)
3. Port (Default Priority)
1.2.5
Virtual
Local
Area
Network
(VLAN)
The switch provides support for setting up tagged VLANs. A port may belong to any number of VLANs. The VLAN
membership of an end device is determined by the VLAN(s) defined for the port to which the device is connected.
If a device should move from one port to another, it loses its current VLAN membership and inherits that of its new
port.
VLANs facilitate easy administration of logical groups of devices that can communicate as if they were physically
on the same LAN. Traffic between VLANs is restricted unless the ports are explicitly configured as overlapping
VLANs. Switches forward unicast, multicast, and broadcast traffic only on LAN segments that serve the VLAN to
which the traffic belongs.
A default Virtual LAN (VID=1) exists so if a port is not a member of any other VLAN, it will default back to VLAN 1.
This allows the switch to operate as a 'normal' switch when it is used in a network. A port may be automatically
removed from the default VLAN when it is reconfigured to belong to another VLAN. By default, the switch will
automatically remove a port from the default VLAN when it is reconfigured to another VLAN. See the VLAN
configuration section in Chapter 2 for additional information.
If switch ports are configured to transmit and receive untagged frames, then their connected devices are able to
communicate throughout the LAN. Using Tagged VLANs, the switch has the ability to take non-tagged packets in
some ports, add a VLAN tag to the packet and send it out to tagged ports on the switch. VLANs can also be
configured to accept tagged packets in tagged ports, strip the tags off the packets, and then send the packets back
out to other untagged ports. This allows a network administrator to set up the switch to support devices on the
network that do not support VLAN tagged packets. The administrator can also set up the ports to discard any
packets that are tagged or to discard any packets that are untagged, based on a hybrid VLAN of both tagged and
untagged ports and by using the VLAN Ingress Filter on the switch.
For each switch port there is one and only one PVID (port VLAN ID) setting. If an incoming frame is untagged and
untagged frames are being accepted, then that frame will inherit the tag of the PVID value for that port. Subsequent
switch routing and treatment will be in accordance with that VLAN switch map. By configuring PVIDs properly and
configuring for all frames to exit untagged, the switch can achieve a 'port VLAN' configuration in which all frames in
and out can be untagged, thus not requiring external devices to be VLAN cognizant.
To understand how a VLAN configuration will perform, first look at the port on which the frame enters the switch,
then the VLAN ID (VID) (if the frame is tagged) or the PVID (if the frame is untagged). The VLAN defined by the
VID or PVID defines a VLAN group with a membership of ports. This membership determines whether a port is
included or excluded as to frame egress from the switch.
The NT24k switch also has the ability to allow overlapping VLANs. Overlapping VLANs give the user the ability to
have one or more ports share two or more VLAN groups. For more information and examples on how this could be
implemented, refer to the 'VLAN Configuration Examples' in this document, and/or our website's technical
documents.
Note:
RSTP is only supported on one VLAN.
