CTC Union EOC-20, User Manual, Page: 10 / 30

2.1 Specification
1. The unit supports up to 3 active VLANs with shared VLAN learning bridge out of
4096 possible VLANs specified in IEEE 802.1Q.
2. Each port always belongs to a default VLAN with its port VID (PVID) as an untagged
member. Also, a port can belong to multiple VLANs and be tagged members of these
VLANs.
3. A port must not be a tagged member of its default VLAN.
4. If a non-tagged or null-VID tagged packet is received, it will be assigned with the
default PVID of the ingress port.
5. If the packet is tagged with non-null VID, the VID in the tag will be used.
6. The look up process starts with VLAN look up to determine whether the VID is valid.
If the VID is not valid, the packet will be dropped and its address will not be learned.
If the VID is valid, the VID, destination address, and source address lookups are
performed.
7. The VID and destination address lookup determines the forwarding ports. If it fails,
the packet will be broadcasted to all members of the VLAN, except the ingress port.
8. Frames are sent out tagged or untagged depend on if the egress port is a tagged or
untagged member of the VLAN that frames belong to.
9. If VID and source address look up fails, the source address will be learned.
2.2 Frame Specification
An untagged frame or a priority-tagged frame does not carry any identification of the VLAN to
which it belongs. Such frames are classified as belonging to a particular VLAN based on
parameters associated with the receiving port. Also, priority tagged frames, which, by definition,
carry no VLAN identification information, are treated the same as untagged frames.
A VLAN-tagged frame carries an explicit identification of the VLAN to which it belongs; i.e., it
carries a tag header that carries a non-null VID. This results in a minimum tagged frame length of 68
octets. Such a frame is classified as belonging to a particular VLAN based on the value of the VID
that is included in the tag header. The presence of the tag header carrying a non-null VID means
that some other device, either the originator of the frame or a VLAN-aware bridge, has mapped this
frame into a VLAN and has inserted the appropriate VID.
The following figure shows the difference between a untagged frame and VLAN tagged frame,
where
the Tag Protocol Identifier (TPID) is of 0x8100 and it identifies the frame as a tagged frame. The Tag
Control Information (TCI) consists of the following elements:
1) User priority allows the tagged frame to carry user priority information across bridged
LANs in which individual LAN segments may be unable to signal priority information
(e.g., 802.3/Ethernet segments).
2) The Canonical Format Indicator (CFI) is used to signal the presence or absence of a
Routing Information Field (RIF) field, and, in combination with the Non-canonical
Format Indicator (NCFI) carried in the RIF, to signal the bit order of address
information carried in the encapsulated frame.
3) The VID uniquely identifies the VLAN to which the frame belongs.