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The tagging feature allows VLANs to span multiple 802.1Q-compliant switches through a single
physical connection and allows Spanning Tree to be enabled on all ports and work normally.
The main characteristics of IEEE 802.1Q are as follows:
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Assigns packets to VLANs by filtering
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Assumes the presence of a single global spanning tree
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Uses an explicit tagging scheme with one-level tagging
802.1Q Packet Forwarding Decisions
Packet forwarding decisions are made based upon the following three types of rules:
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Ingress rules - rules relevant to the classification of received frames belonging to a VLAN.
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Forwarding rules between ports - decides filter or forward the packet
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Egress rules - determines if the packet must be sent tagged or untagged.
Packet Forwarding in 802.1Q VLANs
The diagram below illustrates packet-forwarding decisions with 802.1Q VLANs.
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.
Switch Management
DES-7000/DES-7100 Layer 2 Switch User's Guide
Figure 5-7. Packet Forwarding with VLANs
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