DOT1Q

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.

翻譯：

一種提供VLAN標識和服務質量(QoS)級別的IEEE標準。將4個位元組添加到一個乙太網幀中，將最大幀大小從1518位元組增加到1522位元組。三位被使用允許八個優先順序級!s (QoS)和12位被用來識別多達4096個vlan。802.1q是IEEE規範，用於實現第2層LAN交換機中的VLANs(虛擬區域網)，重點是乙太網。與802.1P類似，流量的優先順序是通過幀標頭中額外的4位元組數據來實現的。這個附加到header中的大多數數據欄位都是特定於VLAN操作的。還包括一個欄位，它提供與802.1P的優先順序映射方案中指定的相同的3位優先順序標誌。除了傳統的數據傳輸，802.1Q還支持通過乙太網交換機進行語音和視頻傳輸。簡而言之，802.1Q規範為VLAN幀標記提供了32位標頭。每個802.1Q標記位於源地址欄位和媒體訪問控制(MAC)客戶端類型/長度欄位之間的乙太網幀中。802.1Q的一個重要特性是能夠在高速鏈路上共享多個子網。這一功能不僅減少了物理分隔所需的低速鏈路的數量，而且還允許非對稱流量管理，以便更容易地管理不同的高速鏈路。在ieee802.1 p和802.1Q中，我們看到了一些重要概念的引入，這些概念是為進一步的Qos(服務質量)開發而提出的。這些802.1特性也可以映射到更高的層協議，如IP和ATM。

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通用協議模式，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設備的專用協議，適用於Cisco設備。　ISL(Interior Switching Link)交換機間協議用於實現CISCO交換機間的VLAN中繼。它是一個信息包標記協議，在支持ISL介面上發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)#encapsulationdot1q100 /封裝802.1q協議，並把該埠劃分到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)#encapsulationdot1q200

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)#encapsulationdot1q300

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