Vlan Fundamental - H3C S5500-SI Series Operation Manual

Operation Manual – VLAN
H3C S5500-SI Series Ethernet Switches
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, users in a VLAN can be connected
to the same switch, or span across multiple switches or routers.
VLAN technology has the following advantages:
1) Broadcast traffic is confined to each VLAN, reducing bandwidth utilization and
improving network performance.
2) LAN security is improved. Packets in different VLANs are isolated at Layer 2. That
is, users in a VLAN cannot communicate with users in other VLANs directly,
unless routers or Layer 3 switches are used.
3) A more flexible way to establish virtual workgroups. With VLAN technology, a
virtual workgroup can be created spanning physical network segments. That is,
users from the same workgroup do not have to be within the same physical area,
making network construction and maintenance much easier and more flexible.

1.1.2 VLAN Fundamental

To enable packets being distinguished by the VLANs they belong to, The VLAN tag
fields used to identify VLANs are added to packets. As common switches operate on
the data link layer of the OSI model, they only process data link layer encapsulation
information and the VLAN tag thus needs to be inserted to the data link layer
encapsulation.
The format of the packets carrying the VLAN tag fields is defined in IEEE 802.1Q, which
is issued by IEEE in 1999.
In the header of a traditional Ethernet data frame, the field following the destination
MAC address and the source MAC address is the Type field, which indicates the upper
layer protocol type.
where DA stands for destination MAC address, SA stands for source MAC address,
and Type stands for the upper layer protocol type of the frame.
Figure 1-2 The format of a traditional Ethernet frame
IEEE802.1Q defines a four-byte VLAN Tag between the DA&SA field and the Type field
to carry VLAN-related information, as shown in
Figure 1-3 The position and the format of the VLAN Tag
Figure 1-2 illustrates the format of a traditional Ethernet frame,
1-2
Chapter 1 VLAN Configuration
Figure 1-3.