INS_CWGE24MS2_REV–
10/05/16 PAGE 97
INSTALLATION AND OPERATION MANUAL
CWGE24MS2
TECH SUPPORT: 1.888.678.9427
other. A frame with VID (VLAN Identifier) of null (0) is called a priority frame, meaning that only the
priority level is significant and the default VID of the ingress port is given as the VID of the frame.
Of the 4096 possible VIDs, a VID of 0 is used to identify priority frames and value 4095 (FFF) is
reserved, so the maximum possible VLAN configurations are 4,094.
TPID
User Priority
CFI
VLAN ID
2 bytes
3 bits
1 bit
12 bits
» Forwarding Tagged and Untagged Frames
Each port on the Switch is capable of passing tagged or untagged frames. To forward a frame
from an 802.1Q VLAN-aware switch to an 802.1Q VLAN-unaware switch, the Switch first decides
where to forward the frame and then strips off the VLAN tag. To forward a frame from an
802.1Q VLAN-unaware switch to an 802.1Q VLAN-aware switch, the Switch first decides where
to forward the frame, and then inserts a VLAN tag reflecting the ingress port’s default VID. The
default PVID is VLAN 1 for all ports, but this can be changed.
A broadcast frame (or a multicast frame for a multicast group that is known by the system)
is duplicated only on ports that are members of the VID (except the ingress port itself), thus
confining the broadcast to a specific domain.
» 802.1Q Port base VLAN
With port-based VLAN membership, the port is assigned to a specific VLAN independent
of the user or system attached to the port. This means all users attached to the port should
be members of the same VLAN. The network administrator typically performs the VLAN
assignment. The port configuration is static and cannot be automatically changed to another
VLAN without manual reconfiguration.
As with other VLAN approaches, the packets forwarded using this method do not leak into other
VLAN domains on the network. After a port has been assigned to a VLAN, the port cannot send
to or receive from devices in another VLAN without the intervention of a Layer 3 device.
The device that is attached to the port likely has no understanding that a VLAN exists. The device
simply knows that it is a member of a subnet and that the device should be able to talk to all
other members of the subnet by simply sending information to the cable segment. The switch
is responsible for identifying that the information came from a specific VLAN and for ensuring
that the information gets to all other members of the VLAN. The switch is further responsible for
ensuring that ports in a different VLAN do not receive the information.
This approach is quite simple, fast, and easy to manage in that there are no complex lookup tables
required for VLAN segmentation. If port-to-VLAN association is done with an application-specific
integrated circuit (ASIC), the performance is very good. An ASIC allows the port-to-VLAN mapping
to be done at the hardware level.
Default Settings
The default PVID is 1 for all ports.
The default Acceptable Frame is All for all ports.
All ports join in the VLAN 1.
