D-Link DGS-3700-12, Руководство пользователя

Страница: 64 / 332

DGS-3700-12/DGS-3700-12G Series Layer 2 Gigabit Ethernet User Manual
62
Figure 4- 1 IEEE .Q 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.
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.