Intel BLADE SERVER IXM5414E - Manuale (Pagina 54)

Switch Management and Operating Concepts 31
Port VLAN ID
Packets that are tagged (are carrying the 802.1Q VID information) can be transmitted from one
802.1Q compliant network device to another with the VLAN information intact. This enables
802.1Q VLANs to span network devices (and indeed, the entire network, if all network devices are
802.1Q compliant).
Not all network devices are 802.1Q compliant. These devices are referred to as tag-unaware. 802.1Q
devices are referred to as tag-aware.
Before the adoption of 802.1Q VLANs, port-based and MAC-based VLANs were in common use.
These VLANs relied upon a port VLAN ID (PVID) to forward packets. A packet received on a
given port would be assigned that port PVID and then be forwarded to the port that corresponded to
the packet destination address (found in the switch forwarding table). If the PVID of the port that
receives the packet is different from the PVID of the port that is to transmit the packet, the switch
module will drop the packet.
A switch port can have only one PVID but can have as many VIDs as the switch module has
memory in its VLAN table to store them.
Tagging and untagging
Every port on an 802.1Q compliant switch can be configured to admit or discard packets that are
received without a tag. Untagged packets that are admitted will be tagged with the port’s PVID.
Every port on an 802.1Q compliant switch can also be configured to transmit packets with or
without tags. Ports with tagging enabled will leave the 802.1Q tag received with the packet or
inserted by the ingress port unchanged. Ports with untagging enabled will strip the 802.1Q tag from
all packets that it transmits. Untagging is used to send packets from an 802.1Q-compliant network
device to a noncompliant one.
Ingress filtering and egress rules
A port on a switch where packets are flowing into the switch and VLAN decisions must be made is
referred to as an ingress port. If ingress filtering is enabled for a port, the switch will examine the
VLAN information in the packet header (if present) to decide whether to forward the packet. If
ingress filtering is disabled, packets will not be dropped based on their VLAN classification.
If ingress filtering is enabled and the packet is tagged with VLAN information, the ingress port will
determine whether the ingress port itself is a member of the tagged VLAN. If it is not, the packet
Dest. Src. Length/EType Data
Dest. Src. EType Tag Length/EType Data
Priority VLAN ID VLAN ID
Adding an IEEE 802.1Q Tag Adding an IEEE 802.1Q Tag
Old
CRC Old
CRC
New
CRC New
CRC
Orginal Ethernet
Packet Orginal Ethernet
Packet
New Tagged
Packet New Tagged
Packet