3Com Switch 7757, Руководство по настройке

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Port-Based VLAN 89
In Figure 26 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 27 Format of VLAN tag
As 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. By
default, it is 0x8100 in 3Com series Ethernet switches.
■
Priority is a 3-bit field, referring to 802.1p priority. Refer to “QoS
Configuration” on page 657 for details.
■
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.
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.
Link Types of Ethernet
Ports
An Ethernet port on a Switch 7750 can be of the following three link types.
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Access. An access port can belong to only one VLAN. It is used to provide
network access for terminal users.
■
Trunk: A trunk port can belong to more than one VLAN. It can receive/send
packets from/to multiple VLANs, and is generally used to connect another
switch.
■
Hybrid: A hybrid port can belong to more than one VLAN. It can receive/send
packets from/to multiple VLANs, and can be used to connect either a switch or
a user PC.
n
A hybrid port allows the packets of multiple VLANs to be sent without tags, but a
trunk port only allows the packets of the default VLAN to be sent without tags.
TPID DA&SA Type Priority CFI VLAN ID
VLAN Tag