D-Link DGS-3312SR [99/290] Q vlan packet forwarding

DGS-3312SR Stackable Gigabit Layer 3 Switch

IEEE 802.1Q VLANs

itch where packets are flowing into the Switch and VLAN decisions must be made.

er Switch or to an

end station, and tagging decisions must be made.

IEEE 802.1Q (tagged) VLANs are implemented on the DGS-3312SR Switch. 802.1Q VLANs require tagging, which

enables the VLANs to span an entire network (assuming all Switches on the network are IEEE 802.1Q-compliant).

Any port can be configured as either tagging or untagging. The untagging feature of IEEE 802.1Q VLANs allow VLANs

to work with legacy Switches that don’t recognize VLAN tags in packet headers. The tagging feature allows VLANs to

span multiple 802.1Q VLAN compliant Switches through a single physical connection and allows Spanning Tree to be

enabled on all ports and work normally.

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 filter or forward the packet

• Egress rules – determines if the packet must be sent tagged or untagged.

Some relevant terms:

• Tagging - The act of putting 802.1Q VLAN information into the header of a packet.

• Untagging - The act of stripping 802.1Q VLAN information out of the packet header.

• Ingress port - A port on a Sw

• Egress port - A port on a Switch where packets are flowing out of the Switch, either to anoth

Figure 4- 48. 802.1Q Packet Forwarding

802.1Q VLAN Tags

shows th

indicated

0x8100, the packet carries onsists of

three bits or user priority, one bit of Canonical Format Identifier (CFI – used for encapsulating Token Ring packets so they

can be carried across Ether

e VL ng, 4094

an be ident

The tag is inserted into the ned

in the packet originally is retained.

The figure below

Their presence is

e 802.1Q VLAN tag. There are four additional octets inserted after the source MAC address.

by a value of 0x8100 in the EtherType field. When a packet’s EtherType field is equal to

the IEEE 802.1Q/802.1p tag. The tag is contained in the following two octets and c

net backbones) and twelve bits of VLAN ID (VID). The three bits of user priority are used by

AN identifier and is used by the 802.1Q standard. Because the VID is twelve bits lo

ified.

packet header making the entire packet longer by four octets. All of the information contai

802.1p. The VID is th

unique VLANs c