标题：DOT1Q
英文内容
802.1Q
An IEEE standard for providing VLAN identification and quality of service (QoS) levels. Four bytes are added to an Ethernet frame, increasing the maximum frame size from 1518 to 1522 bytes. Three bits are used to allow eight priority levels (QoS) and 12 bits are used to identify up to 4096 VLANs. 802.1q is the IEEE specification for implementation of VLANs (Virtual Local Area Networks) in Layer 2 LAN switches, with emphasis on Ethernet. Similar to 802.1P, prioritization of traffic is accomplished through an additional four bytes of data in the frame header. Most data fields in this addition to the header are specific to VLAN operation. Also included is a field which provides the same 3-bit priority flag specified in 802.1P's priority-mapping scheme. In addition to conventional data traffic, 802.1Q supports voice and video transmission through Ethernet switches. In short, the 802.1Q specification provides a 32-bit header for VLAN frame tagging. Each 802.1Q tag sits in an Ethernet frame between the source address field and the media access control (MAC) client type/length field. An important feature of 802.1Q is the ability to share multiple subnets across a high-speed link. This capability not only reduced the number of lower speed links needed for physical separation, but it also allowed for asymmetrical traffic management so that different speed links could be managed more easily. With IEEE 802.1p and 802.1Q, we saw the introduction of some important concepts that have been carried forward for further QoS (Quality of Service) development. These 802.1 features also can be mapped into higher layer protocols like IP and ATM.
An IEEE standard for providing VLAN identification and quality of service (QoS) levels. Four bytes are added to an Ethernet frame, increasing the maximum frame size from 1518 to 1522 bytes. Three bits are used to allow eight priority levels (QoS) and 12 bits are used to identify up to 4096 VLANs. 802.1q is the IEEE specification for implementation of VLANs (Virtual Local Area Networks) in Layer 2 LAN switches, with emphasis on Ethernet. Similar to 802.1P, prioritization of traffic is accomplished through an additional four bytes of data in the frame header. Most data fields in this addition to the header are specific to VLAN operation. Also included is a field which provides the same 3-bit priority flag specified in 802.1P's priority-mapping scheme. In addition to conventional data traffic, 802.1Q supports voice and video transmission through Ethernet switches. In short, the 802.1Q specification provides a 32-bit header for VLAN frame tagging. Each 802.1Q tag sits in an Ethernet frame between the source address field and the media access control (MAC) client type/length field. An important feature of 802.1Q is the ability to share multiple subnets across a high-speed link. This capability not only reduced the number of lower speed links needed for physical separation, but it also allowed for asymmetrical traffic management so that different speed links could be managed more easily. With IEEE 802.1p and 802.1Q, we saw the introduction of some important concepts that have been carried forward for further QoS (Quality of Service) development. These 802.1 features also can be mapped into higher layer protocols like IP and ATM.
中文内容
IEEE关于提供VLAN（虚拟局域网）识别和服务质量（QoS）等级的标准。在以太网一个帧当中加入4个字节，将一帧的最大长度从1518字节提高到1522字节；这4个字节包含2个字节的标签协议标识（TPID）和2个字节的标签控制信息（TCI）；其中，TPID用于标识帧的类型，其值为0x8100时表示802.1Q的帧，设备可以根据这个字节判断是否接受它；TCI包括用户优先级（User Priority，3bit）、规范格式指示器（CFI，1bit）和VLAN ID（12bit）。4个字节中的3比特提供8种优先等级，12比特用于最多识别4096种虚拟以太网。在第二层局域网交换，尤其是以太网中，802.1q成为IEEE虚拟以太网实施的技术规范。与802.1P相类似，通信业务优先权的确定是通过在帧标题中额外加入一个4字节数据来完成的。在帧标题中额外加入的大多数数据段仅限于虚拟以太网操作。帧标题中还包括一个数据段，提供802.1P优先权设定方案中给出的相同的3比特优先标记。除了一般的数据通信业务，802.1Q还通过以太网交换支持话音和视频传输。简而言之，802.1Q技术规范为虚拟以太网帧标签提供一个32位标题。每一个802.1Q标签位于以太网帧源地址字段和介质访问控制（MAC）客户类型/长度字段之间。802.1Q的一个重要特点是能够通过一个高速链路共享多重子网。这一功能不仅减少了物理分离所需的低速链路数量，而且还允许进行非对称通信业务管理，这样能够更容易对不同速度的链路进行管理。通过IEEE 802.1p和802.1Q，我们可以看到一些已经为服务质量（QoS）进一步开发所采纳的重要理念。这些802.1特性还可以被引入更高一层的协议，如IP和ATM。
DOT1Q和 ISL的区别
dot1q就是802.1q，是vlan的一种封装方式。dot就是点的意思，就简写为dot1q了.
DOT1Q和 ISL的区别：DOT1Q是各类产品的VLAN通用协议模式，Dot1q是一种普遍使用的标准，适用所有交换机与路由设备。DOT1Q支持4096个vlan,而ISL最多支持1024个vlan。ISL是CISCO设备的专用协议，适用于Cisco设备。　ISL(Interior Switching Link)交换机间协议用于实现CISCO交换机间的VLAN中继。它是一个信息包标记协议，在支持ISL接口上发送的帧由一个标准以太网帧及相关的VLAN信息组成。
R2(config)#interface fastethernet 0/0.1
R2(config-subif)#encapsulation dot1q 100 /封装802.1q协议，并把该端口划分到vlan 100
R2(config-subif)#ip address 202.114.12.1 255.255.255.192
R2(config-subif)#no shutdown
R2(config-subif)#exit
R2(config)#interface fastethernet 0/0.2
R2(config-subif)#encapsulation  dot1q 200
R2(config-subif)#ip address 202.114.12.65 255.255.255.192
R2(config-subif)#no shutdown
R2(config-subif)#exit
R2(config)#interface fastethernet 0/0.3
R2(config-subif)#encapsulation  dot1q 300
R2(config-subif)#ip address 202.114.12.129 255.255.255.192
R2(config-subif)#no shutdown
R2(config-subif)#exit
R2(config)#interface fastether0/0
R2(config-if)#no shutdown
Switch0(config)#interface f0/24
Switch0(config-if)# switchport mode trunk
Switch0 (config-if)#switchport trunk encapsulation dot1q
Switch0(config-if)# switchport trunk allowed all
Switch0(config-if)#exit