xStack® DGS-3620 Series Managed Switch Web UI Reference Guide
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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:
o
Ingress rules – rules relevant to the
classification of received frames belonging
to a VLAN.
o
Forwarding rules between ports – decides
whether to filter or forward the packet.
o
Egress rules – determines if the packet must
be sent tagged or untagged.
Figure 4-1 IEEE 802.1Q Packet Forwarding
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.
Figure 4-2 IEEE 802.1Q Tag
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.
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