D-Link DES-3326SRM Product Manual - Page 144

IEEE 802.1Q VLANs, 802.1Q VLAN Packet Forwarding, 802.1Q VLAN Tags, Tagging, Ingress port

Page 144 highlights

D-Link DES-3326S Layer 3 Switch VLAN can be equated to a broadcast domain, because broadcast packets are forwarded only to members of the VLAN on which the broadcast was initiated. IEEE 802.1Q VLANs 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 Switch where packets are flowing into the Switch and VLAN decisions must be made. Egress port - A port on a Switch where packets are flowing out of the Switch, either to another Switch or to an end station, and tagging decisions must be made. IEEE 802.1Q (tagged) VLANs are implemented on the DES-3326S 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. 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 or 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 contained in the packet originally is retained. 134

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D-Link DES-3326S Layer 3 Switch
VLAN can be equated to a broadcast domain, because broadcast packets are forwarded only to members of the VLAN on
which the broadcast was initiated.
IEEE 802.1Q VLANs
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 Switch where packets are flowing into the Switch and VLAN decisions must be made.
Egress port
- A port on a Switch where packets are flowing out of the Switch, either to another Switch or to an end station,
and tagging decisions must be made.
IEEE 802.1Q (tagged) VLANs are implemented on the DES-3326S 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.
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 or 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 contained in the
packet originally is retained.
134