3Com Switch 8800 Advanced Software V5 Configuration Guide

156 CHAPTER 14: VLAN CONFIGURATION
A VLAN is not restricted by physical factors, that is to say, hosts that reside in
different network segments may belong to the same VLAN; a VLAN can be with
the same switch, or span across multiple switches or routers.
VLAN technology has the following advantages:
■ Broadcast traffic is confined to each VLAN, reducing bandwidth utilization and
improving network performance.
■ LAN security is improved. Packets in different VLANs cannot communicate with
each other directly. That is, users in a VLAN cannot interact directly with users
in other VLANs, unless routers or Layer 3 switches are used.
■ A more flexible way to establish virtual working groups. With VLAN
technology, clients can be allocated to different working groups, and users
from the same group do not have to be within the same physical area, making
network construction and maintenance much easier and more flexible.
VLAN Fundamental To enable network devices to identify packets of different VLANs, a field
identifying VLANs is added to packets. As common switches operate on the data
link layer, the field thus needs to be inserted to the data link layer encapsulation.
The format of the packets carrying the fields identifying VLANs is defined in IEEE
802.1Q, which is issued in 1999.
In the header of a traditional Ethernet packet, the field following the destination
MAC address and the source MAC address is protocol type, which indicates the
upper layer protocol type.
Figure 44 illustrates the format of a traditional Ethernet
packet, where DA stands for destination MAC address, SA stands for source MAC
address, and Type stands for upper layer protocol type.
Figure 44 The format of a traditional Ethernet packet
IEEE 802.1Q defines a four-byte VLAN Tag field between the DA&SA field and the
Type field to carry VLAN-related information, as shown in
Figure 45.
Figure 45 The position and the format of the VLAN Tag field
The VLAN Tag field comprises four sub-fields: the TPID field, the Priority field, the
CFI field, and the VLAN ID field.
■ The TPID field, 16 bits in length, indicates that this data frame is VLAN-tagged.
IEEE 802.1Q defines the value of this filed as 0x8100.
■ The Priority field, three bits in length, indicates the priority of a packet. For
information about packet priority, refer to
“Priority Mapping” on page 851.
Type(2)DA&SA(12) DATA
Type
DA&SA(12) DATA
DA&SA DATA
Type(2)DA&SA(12) DATADA&SA(12) DATA
Type
DA&SA(12) DATA
DA&SA
DATA
Type(2)DA&SA(12) DATADA&SA(12) DATA
Type
DA&SA(12) DATA
DA&SA DATA
Type(2)DA&SA(12) DATADA&SA(12) DATA
Type
DA&SA(12) DATA
DA&SA
DATA
TPID Prioity CFI VLAN ID
VLAN Tag
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN ID
VLAN Tag
TPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
VLAN Tag
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Priority CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN ID
VLAN Tag
TPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
VLAN Tag
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
VLAN Tag
TPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
VLAN Tag
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN ID
DA&SA
TypeTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Prioity CFI VLAN IDTPID Priority CFI VLAN ID
DA&SA
Type