Port-Based VLAN 89In Figure 26 DA refers to the destination MAC address, SA refers to the sourceMAC address, and Type refers to the protocol type of the packet. IEEE 802.1Qprotocol defines that a 4-byte VLAN tag is encapsulated after the destination MACaddress and source MAC address to show the information about VLAN.Figure 27 Format of VLAN tagAs shown in Figure 27, a VLAN tag contains four fields, including TPID, priority,CFI, and VLAN ID.■ TPID is a 16-bit field, indicating that this data frame is VLAN-tagged. Bydefault, it is 0x8100 in 3Com series Ethernet switches.■ Priority is a 3-bit field, referring to 802.1p priority. Refer to “QoSConfiguration” on page 657 for details.■ CFI is a 1-bit field, indicating whether the MAC address is encapsulated in thestandard format in different transmission media. This field is not described indetail in this chapter.■ VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packetbelongs. It is in the range of 0 to 4,095. Generally, 0 and 4,095 is not used, sothe field is in the range of 1 to 4,094.VLAN ID identifies the VLAN to which a packet belongs. When the switch receivesan un-VLAN-tagged packet, it will encapsulate a VLAN tag with the default VLANID of the inbound port for the packet, and the packet will be assigned to thedefault VLAN of the inbound port for transmission.Port-Based VLAN Port-based VLAN technology introduces the simplest way to classify VLANs. Youcan isolate the hosts and divide them into different virtual workgroups throughassigning the ports on the device connecting to hosts to different VLANs.This way is easy to implement and manage and it is applicable to hosts withrelatively fixed positions.Link Types of EthernetPortsAn Ethernet port on a Switch 7750 can be of the following three link types.■ Access. An access port can belong to only one VLAN. It is used to providenetwork access for terminal users.■ Trunk: A trunk port can belong to more than one VLAN. It can receive/sendpackets from/to multiple VLANs, and is generally used to connect anotherswitch.■ Hybrid: A hybrid port can belong to more than one VLAN. It can receive/sendpackets from/to multiple VLANs, and can be used to connect either a switch ora user PC.n A hybrid port allows the packets of multiple VLANs to be sent without tags, but atrunk port only allows the packets of the default VLAN to be sent without tags.TPIDDA&SA TypePriority CFI VLAN IDVLAN Tag
