轉自 http://hi.baidu.com/pdaliu/blog/item/6b2e9045183aa53c87947387.html
ISL Frame
The ISL frame consists of three primary fields: the encapsulation frame (original frame), which is encapsulated by the ISL header, and the FCS at the end.(ISL幀由三部分組成:原始幀、ISL頭和FCS)
| ISL Header | Encapsulation Frame | FCS |
This example shows the further expansion of the ISL header. The expansion includes the field acronyms and the number of bits for each field:
| No. of bits | 40 | 4 | 48 | 16 | 24 | ||
| Frame field | DA | TYPE | USER | SA | LEN | AAAA03(SNAP) | HSA |
| 15 | 1 | 8 to 196,600 bits (1 to 24,575 bytes) | 32 | |||
| VLAN | BPDU | INDEX | RES | ENCAP FRAME |
Field Descriptions
This section provides detailed descriptions of the ISL frame fields.
DA—Destination Address
The DA field of the ISL packet is a 40-bit destination address. This address is a multicast address and is set at "0x01-00-0C-00-00" or "0x03-00-0c-00-00". The first 40 bits of the DA field signal the receiver that the packet is in ISL format.(ISL包的DA字段是一個40位的目的位址。這個位址是一個多點傳播位址并且被設定成"0x01-00-0C-00-00"或"0x03-00-0C-00-00"。DA字段的40位位址告訴接收者，這個包是ISL格式。)
TYPE—Frame Type
The TYPE field consists of a 4-bit code. The TYPE field indicates the type of frame that is encapsulated and can be used in the future to indicate alternative encapsulations. This table provides definitions of different TYPE codes:（TYPE字段由4比特位組成。TYPE它段指出被封裝幀的類型和有哪些類型可以用于該字段，這個表提供了不同TYPE代碼的定義：）
| TYPE Code | Meaning |
|---|---|
| 0000 | Ethernet |
| Token Ring | |
| 0010 | FDDI |
| 0011 | ATM |
USER—User Defined Bits (TYPE Extension)(使用者定義位，TYPE的擴充)
The USER field consists of a 4-bit code. The USER bits are used to extend the meaning of the TYPE field. The default USER field value is "0000". For Ethernet frames, the USER field bits "0" and "1" indicate the priority of the packet as it passes through the switch. Whenever traffic can be handled in a manner that allows it to be forwarded more quickly, the packets with this bit set should take advantage of the quick path. It is not required that such paths be provided.(USER字段由4比特位組成。USER位用于把TYPE字段的内容詳細化。預設情況下USER字段值是“0000”。例如以太網幀，USER字段的比特位“0”和“1”用于訓示包經過交換機的優先級。包的這個位組的設定決定了是否資料能被一種更快轉發的方式處理。這個字段不像路徑一樣必需被設定的。)
| USER Code | |
|---|---|
| XX00 | Normal Priority |
| XX01 | Priority 1 |
| XX10 | Priority 2 |
| XX11 | Highest Priority |
SA—Source Address
The SA field is the source address field of the ISL packet. The field should be set to the "802.3" MAC address of the switch port that transmits the frame. It is a 48-bit value. The receiving device may ignore the SA field of the frame.（SA字段是ISL包的原位址字段。這個字段應該被設定成發送資料的交換機傳送幀的端口的MAC位址。這是一個48位的值。接收裝置可以忽略幀的SA字段。）
LEN—Length
The LEN field stores the actual packet size of the original packet as a 16-bit value. The LEN field represents the length of the packet in bytes, with the exclusion of the DA, TYPE, USER, SA, LEN, and FCS fields. The total length of the excluded fields is 18 bytes, so the LEN field represents the total length minus 18 bytes.（LEN字段存儲初始包的實際大小用16位表示。LEN字段以位元組的方式描述了包的長度，這個長度包含了排除在外的DA，TYPE，USER，SA，LEN和FCS字段。排除字段的總長度是18個位元組，是以LEN字段所描述的總長度要減去18個位元組。）
AAAA03 (SNAP)—Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC)(子網通路協定和邏輯鍊路控制)
The AAAA03 SNAP field is a 24-bit constant value of "0xAAAA03". （AAAA03 SNAP字段是一個24位不變的常量值“0xAAAA03”）
HSA—High Bits of Source Address(原位址的高位)
The HSA field is a 24-bit value. This field represents the upper 3 bytes (the manufacturer ID portion) of the SA field. The field must contain the value "0x00-00-0C".（HSA字段是一個24位的值。這個字段描述了SA字段的高3位[高3位是廠商ID]。這個字段必須包含值“0x00-00-0C”。）
VLAN—Destination Virtual LAN ID
The VLAN field is the VLAN ID of the packet. It is a 15-bit value that is used to distinguish frames on different VLANs. This field is often referred to as the "color" of the frame.（VLAN字段是資料包的VLAN ID。它是一個15位的值，用于差別不同VALN的資料幀。）
BPDU—Bridge Protocol Data Unit (BPDU) and Cisco Discovery Protocol (CDP) Indicator
The bit in the BPDU field is set for all BPDU packets that are encapsulated by the ISL frame. The BPDUs are used by the spanning tree algorithm in order to determine information about the topology of the network. This bit is also set for CDP and VLAN Trunk Protocol (VTP) frames that are encapsulated.(BPDU字段隻有一位，所有被ISL幀封裝的BPDU包設定這個字段。BPDU字段被生成樹算法決定網絡拓撲的資訊。這一位也被CDP和VLAN幹線協定[ＶＴＰ，因為VTP用于交換機和交換機之間的通信，是以我把Trunk翻譯成“幹線”。小弟英語也不怎麼樣，感覺這段應該是這個意思]封裝的幀設定。）
INDX—Index(索引)
The INDX field indicates the port index of the source of the packet as it exits the switch. This field is used for diagnostic purposes only, and may be set to any value by other devices. It is a 16-bit value and is ignored in received packets.(INDX字段指出資料包離開源交換機的端口索引。這個字段僅是用于診斷的目地，并且可以被其它裝置把它設成任務值。它是一個16位的值并且在收到的包裡忽略。)
RES—Reserved for Token Ring and FDDI（用于令牌環和FDDI網絡接收）
The RES field is a 16-bit value. This field is used when Token Ring or FDDI packets are encapsulated with an ISL frame. In the case of Token Ring frames, the Access Control (AC) and Frame Control (FC) fields are placed here. In the case of FDDI, the FC field is placed in the Least Significant Byte (LSB) of this field. For example, an FC of "0x12" has a RES field of "0x0012". For Ethernet packets, the RES field should be set to all zeros.(RES字段是一個16位的值。這個字段用于被ISL幀封裝的令牌環和FDDI包。在令牌環情況下，通路控制[AC]和幀控制[FC]被放在這裡。在FDDI的情況下，FC字段被放置在這個字段的最低有效位元組[LSB]中。例如：FC值是0x12在RES字段中是0x0012。在以太網中這個字段應該全被設定成零。
ENCAP FRAME—Encapsulated Frame（封裝幀）
The ENCAP FRAME field is the encapsulated data packet, which includes its own cyclic redundancy check (CRC) value, completely unmodified. The internal frame must have a CRC value that is valid after the ISL encapsulation fields are removed. The length of this field can be from 1 to 24,575 bytes in order to accommodate Ethernet, Token Ring, and FDDI frames. A receiving switch may strip off the ISL encapsulation fields and use this ENCAP FRAME field as the frame is received (associating the appropriate VLAN and other values with the received frame as indicated for switching purposes).（ENCAP FRAME字段是被封裝的資料包，這個字段中包含了完整沒有被修改過的被封裝資料的循環有冗校驗[CRC]值。在ISL封裝被移除後内部幀[被封裝在ISL内的原始資料幀]必須有一個有效的CRC值。這個字段的長度可以是從1到24575位元組為了容納以太網、令牌環和FDDI幀。接收資料的交換機剝去ISL封裝的字段并且使用ENCAP FRAME字段做為被接收的幀[被接收的幀與适合的一些VLAN聯接，做為交換資料的目的地。]）
FCS—Frame Check Sequence(幀校驗隊列)
The FCS field consists of 4 bytes. This sequence contains a 32-bit CRC value, which is created by the sending MAC and is recalculated by the receiving MAC in order to check for damaged frames. The FCS is generated over the DA, SA, Length/Type, and Data fields. When an ISL header is attached, a new FCS is calculated over the entire ISL packet and added to the end of the frame.(FCS字段由4個位元組組成。這個隊列包含32位CRC[循環有冗校驗]值，為了檢查幀是否已經損壞，CRC被發送方的MAC建立并且由接收方的MAC重新計算。FCS被生成其中包含了DA，SA，Length/Type和資料字段。當ISL頭資訊被加到原始幀的開頭時，一個新的FCS根據全部的ISL包被計算出來并且被添加到原始幀的末尾)
Note: The addition of the new FCS does not alter the original FCS that is contained within the encapsulated frame.(注意：附加的新的FCS不會修改被封裝的原始幀的FCS。)
Frame Size （幀大小）
The ISL frame encapsulation is 30 bytes, and the minimum FDDI packet is 17 bytes. Therefore, the minimum ISL encapsulated packet for FDDI is 47 bytes. The maximum Token Ring packet is 18,000 bytes. Therefore, the maximum ISL packet is 18,000 plus 30 bytes of ISL header, for a total of 18,030 bytes. If only Ethernet packets are encapsulated, the range of ISL frame sizes is from 94 to 1548 bytes.(ISL封裝幀是30個位元組，最小FDDI包的大小是17位元組。是以最小的被ISL封裝的FDDI幀大小是47位元組。最大令牌環包是18000位元組。是以最大的ISL包是18000加30個位元組的ISL頭資訊，總大小為18030位元組。如果僅封裝以太網包，ISL幀大小的範圍是從94到1548位元組。)
The biggest implication for systems that use ISL encapsulation is that the encapsulation is a total of 30 bytes, and fragmentation is not required. Therefore, if the encapsulated packet is 1518 bytes long, the ISL packet is 1548 bytes long for Ethernet. Additionally, if packets other than Ethernet packets are encapsulated, the maximum length can be greatly increased. You must consider this length change when you evaluate whether a topology can support ISL packets size. （對于使用ISL封裝系統的隐含意義是最大總長度是30個位元組。是以如何被封裝的包是1518位元組，那麼ISL包就是1548位元組長度。另外，如果除了被封裝的是以太網包，最大長度可以很大程度的增長。當你評估拓撲是否能支援ISL包大小時必需注意這個長度變化。
Another system implication is that ISL packets contain two FCSs. The first FCS is calculated for the original data. The second FCS is calculated after the packet has been encapsulated in ISL. If the original data does not contain a valid CRC, the invalid CRC is not detected until the ISL header is stripped off and the end device checks the original data FCS. This typically is not a problem for switching hardware, but can be difficult for routers and network interface cards (NICs). （其它隐含意義還有ISL包有兩個FCS值。第一個FCS值是被原始資料計算出來的。第二個FCS值是在原始包已經被封裝到ISL裡後被計算出來的。如果原始資料沒有包含一個有效的CRC值，這個無效的CRC直到ISL頭被剝去、終端裝置檢查初始資料的FCS值之前都不會被發現。）
IEEE 802.1Q Frame
IEEE 802.1Q uses an internal tagging mechanism which inserts a 4-byte tag field in the original Ethernet frame itself between the Source Address and Type/Length fields. Because the frame is altered, the trunking device recomputes the FCS on the modified frame.(IEEE 802.1Q使用了在原始以太網幀的SA和Type/Length字段中間插入一個4位元組标志字段的機制。由于幀被修改過了，是以幹線裝置[trunking device]要重新計算被修改過的幀的FCS值。)
| TAG | TYPE/LEN | DATA |
This example shows the further expansion of the Tag field. The expansion includes the field acronyms and the number of bits for each field.（下面這個例子進一步的說明了Tag[标志]字段。這個例子包含了字段的縮寫字母開頭和每一個字段所占位數。）
| 3 | 12 | |||
| TPID | PRIORITY | CFI | VID |
Field Descriptions （字段說明）
This section provides detailed descriptions of the 802.1Q frame fields.（這一部分提供了詳細的802.1Q幀字段的說明）
TPID—Tag Protocol Identifier(Tag協定辨別符)
The Tag Protocol Identifier is a 16-bit field. It is set to a value of 0x8100 in order to identify the frame as an IEEE 802.1Q-tagged frame.（Tag協定辨別是一個１６位的字段。它被設定成0x8100這個值，為了說明這個幀是一個IEEE 802.1Q标志幀）
Priority（優先級）
Also known as user priority, this 3-bit field refers to the IEEE 802.1p priority. The field indicates the frame priority level which can be used for the prioritization of traffic. The field can represent 8 levels (0 through 7).(就像使用者所知道的優先級一樣。這是一個3位的字段用于訓示IEEE 802.1!Q的優先級。這個字段批出了幀在傳輸過程中的優選次序。這個字段可以被聲明成8個級别[從0到7])
![802.1Q與Cisco ISL介紹[圖]](data:image/gif;base64,R0lGODlhAQABAIAAAP///wAAACwAAAAAAQABAAACAkQBADs=)