提供VLAN識別和服務質量(QoS)級別的IEEE標準。在乙太網一個幀當中加入4個位元組，將一幀的最大長度從1518位元組提高到1522位元組；這4個位元組包含2個位元組的標籤協定標識（TPID）和2個位元組的標籤控制信息（TCI）；其中，TPID用於標識幀的類型，其值為0x8100時表示802.1Q的幀，設備可以根據這個位元組判斷是否接受它；TCI包括用戶優先權（User Priority，3bit）、規範格式指示器（CFI，1bit）和VLAN ID（12bit）。4個位元組中的3比特提供8種優先等級，12比特用於最多識別4096種虛擬乙太網。在第二層區域網路交換，尤其是乙太網中，802.1q成為IEEE虛擬乙太網實施的技術規範。與802.1P相類似，通信業務優先權的確定是通過在幀標題中額外加入一個4位元組數據來完成的。在幀標題中額外加入的大多數數據段僅限於虛擬乙太網操作。幀標題中還包括一個數據段，提供802.1P優先權設定方案中給出的相同的3比特優先標記。除了一般的數據通信業務，802.1Q還通過乙太網交換支持話音和視頻傳輸。簡而言之，802.1Q技術規範為虛擬乙太網幀標籤提供一個32位標題。每一個802.1Q標籤位於乙太網幀源地址欄位和介質訪問控制（MAC）客戶類型/長度欄位之間。802.1Q的一個重要特點是能夠通過一個高速鏈路共享多重子網。
基本介紹
- 中文名：DOT1Q
- 通過：在幀標題中額外加入一個4位元組
- 提供：VLAN
- 適用：所有交換機與路由設備
英文內容,區分 ISL,
英文內容
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.
區分 ISL
dot1q就是802.1q，是vlan的一種封裝方式。dot就是點的意思，就簡寫為dot1q了.
DOT1Q和 ISL的區別：DOT1Q是各類產品的VLAN通用協定模式，Dot1q是一種普遍使用的標準，適用所有交換機與路由設備。DOT1Q支持4096個vlan,而ISL最多支持1024個vlan。ISL是CISCO設備的專用協定，適用於Cisco設備。 ISL(Interior Switching Link)交換機間協定用於實現CISCO交換機間的VLAN中繼。它是一個信息包標記協定，在支持ISL接口上傳送的幀由一個標準乙太網幀及相關的VLAN信息組成。
R2(config)#interface fastethernet 0/0.1
R2(config-subif)#encapsulation dot1q 100 /封裝802.1q協定，並把該連線埠劃分到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
