about the logical LAN which the host (or message) belongs to. Figure below shows an
Ethernet frame and its 802.1Q tag position
Figure 45: 802.1Q Ethernet frame
The information at 802.1Q tag is divided in 4 fields:
TPID (Tag Protocol Identifier): 16-bit length, this field presents VLAN protocol
and will be equal to 0x8100;
PCP (Priority Code Point): 3-bit length, this field presents the priority of the
packet at the network;
CFI (Canonical Format Unit): 1-bit length, this field is always set to 0 in
Ethernet communication;
VID (VLAN Identifier): 12-bit length, this field shows explicitly VLAN number
identifier which the frame belongs to. It is also called VLAN tag.
VLAN tag usage can have many concepts in its background. Actually, there are many VLAN tag types, used in different
environments, and with specific standards related. As example, C-tag, S-tag and I-tag can be cited. Description given in this
manual applies to Customer tag (C-tag), the most common VLAN usage in power system communication. As Reason Switches
main application is be a path to interconnect IED equipment in power systems communication, information contained in this
manual should be enough to use the equipment. If application of switch requires the usage of specific VLAN for trunk links, where
S-tag is used, service VLAN tag (S-tag) is also supported by Reason Switches.
VID number is the number that explicitly defines which VLAN the incoming packet
belongs to. There are, in theory, 2
12
(4,096) possibilities of numbers, as this is a 12-bit
field. Besides, there are two VID numbers that are reserved:
VID = 0xFFF is reserved and must not be used;
VID = 0x000 means Priority-tagged frame. This frame is not associated with
any VLAN identification, but explicitly shows the priority of the packet.
When using VLAN mechanism, be aware that there is no 0xFFF VLAN usage.
By default, in Reason switches, Priority-tagged frames (VID = 0x000) will be mapped to native VLAN ID (VID = 0x001) and will
have its priority information removed at the outgoing frame.
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