3Com Switch 7750 Configuration Guide Guide

84 CHAPTER 9: VLAN OVERVIEW
A VLAN can span across multiple switches, or even routers. This enables hosts in a
VLAN to be dispersed in a looser way. That is, hosts in a VLAN can belong to
different physical network segment.
Compared with the traditional Ethernet, VLAN enjoys the following advantages.
■ Broadcasts are confined to VLANs. This decreases bandwidth utilization and
improves network performance.
■ Network security is improved. VLANs cannot communicate with each other
directly. That is, a host in a VLAN cannot access resources in another VLAN
directly, unless routers or Layer 3 switches are used.
■ Network configuration workload for the host is reduced. VLAN can be used to
group specific hosts. When the physical position of a host changes within the
range of the VLAN, you need not change its network configuration.
VLAN Principles VLAN tags in the packets are necessary for the switch to identify packets of
different VLANs. The switch works at Layer 2 (Layer 3 switches are not discussed in
this chapter) and it can identify the data link layer encapsulation of the packet
only, so you can add the VLAN tag field into only the data link layer encapsulation
if necessary.
In 1999, IEEE issues the IEEE 802.1Q protocol to standardize VLAN
implementation, defining the structure of VLAN-tagged packets.
In traditional Ethernet data frames, the type field of the upper layer protocol is
encapsulated after the destination MAC address and source MAC address, as
shown in
Figure 22
Figure 22 Encapsulation format of traditional Ethernet frames
In Figure 22 DA refers to the destination MAC address, SA refers to the source
MAC address, and Type refers to the protocol type of the packet. IEEE 802.1Q
protocol defines that a 4-byte VLAN tag is encapsulated after the destination MAC
address and source MAC address to show the information about VLAN.
Figure 23 Format of VLAN tag
As shown in Figure 23, 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. By
default, it is 0x8100 in the Switch 7750 Family.
■ Priority is a 3-bit field, referring to 802.1p priority. Refer to section "QoS" for
details.
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