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The main characteristics of IEEE 802.1Q are as follows:
•
Assigns packets to VLANs by filtering.
•
Assumes the presence of a single global
spanning tree.
•
Uses an explicit tagging scheme with one-
level tagging.
•
802.1Q VLAN Packet Forwarding
•
Packet forwarding decisions are made based
upon the following three types of rules:
•
Ingress rules – rules relevant to the
classification of received frames belonging to
a VLAN.
•
Forwarding rules between ports – decides
whether to filter or forward the packet.
•
Egress rules – determines if the packet must
be sent tagged or untagged.
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 EtherType field. When a packet's EtherType 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 VLANs 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.
The EtherType and VLAN ID are inserted after the MAC
source address, but before the original
EtherType/Length or Logical Link Control. Because the
packet is now a bit longer than it was originally, the
Cyclic Redundancy Check (CRC) must be recalculated.
DES-3810 Series Layer 3 Managed Ethernet Switch Web UI Reference Guide
Figure 5-1 Packet Forwarding window
Figure 5-2 802.1Q VLAN Tags window
Figure 5-3 802.1Q VLAN Tags window
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