3Com Switch 7750 Configuration Guide Guide

Port-Based VLAN 85
■ CFI is a 1-bit field, indicating whether the MAC address is encapsulated in the
standard format in different transmission media. This field is not described in
detail in this chapter.
■ VLAN ID is a 12-bit field, indicating the ID of the VLAN to which this packet
belongs. It is in the range of 0 to 4,095. Generally, 0 and 4,095 is not used, so
the field is in the range of 1 to 4,094.
VLAN ID identifies the VLAN to which a packet belongs. When the switch receives
an un-VLAN-tagged packet, it will encapsulate a VLAN tag with the default VLAN
ID of the inbound port for the packet, and the packet will be assigned to the
default VLAN of the inbound port for transmission. For the details about setting
the default VLAN of a port, refer to section "Port Basic Configuration" in 3Com
Switch 7750 Family Ethernet Switches - Operation Manual.
Port-Based VLAN Port-based VLAN technology introduces the simplest way to classify VLANs. You
can isolate the hosts and divide them into different virtual workgroups through
assigning 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 with
relatively fixed positions.
Protocol-Based VLAN
Introduction to
Protocol-Based VLAN
Protocol-based VLAN is also known as protocol VLAN, which is another way to
classify VLANs besides port-based VLAN. Through the protocol-based VLANs, the
switch can analyze the received un-VLAN-tagged packets on the port and match
the packets with the user-defined protocol template automatically according to
different encapsulation formats and the values of the special fields. If a packet is
matched, the switch will add a corresponding VLAN tag to it automatically. Thus,
the data of the specific protocol is assigned automatically to the corresponding
VLAN for transmission.
This feature is used for binding the ToS provided in the network to VLAN to
facilitate management and maintenance.
Encapsulation Format of
Ethernet Data
This section introduces the common encapsulation formats of Ethernet data for
you to understand well the procedure for the switch to identify the packet
protocols.
Ethernet II and 802.3 encapsulation
In the link layer, there are two main packet encapsulation types: Ethernet II and
802.3, whose encapsulation formats are described in the following figures.
Ethernet II packet:
Figure 24 Ethernet II encapsulation format
802.3 standard packet:
Type(2)DA&SA(12) DATAType(2)DA&SA(12) DATADA&SA(12) DATA