转自 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帧由三部分组成:原始帧、ISL头和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

4

48

16

24

24

Frame field

DA

TYPE

USER

SA

LEN

AAAA03(SNAP)

HSA

No. of bits

15

1

16

16

8 to 196,600 bits (1 to 24,575 bytes)

32

Frame field

VLAN

BPDU

INDEX

RES

ENCAP FRAME

FCS

 

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字段是一个40位的目的地址。这个地址是一个组播地址并且被设置成"0x01-00-0C-00-00"或"0x03-00-0C-00-00"。DA字段的40位地址告诉接收者，这个包是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比特位组成。TYPE它段指出被封装帧的类型和有哪些类型可以用于该字段，这个表提供了不同TYPE代码的定义：）

TYPE Code

Meaning

0000

Ethernet

0001

Token Ring

0010

FDDI

0011

ATM

 

USER—User Defined Bits (TYPE Extension)(用户定义位，TYPE的扩展)

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比特位组成。USER位用于把TYPE字段的内容详细化。默认情况下USER字段值是“0000”。例如以太网帧，USER字段的比特位“0”和“1”用于指示包经过交换机的优先级。包的这个位组的设置决定了是否数据能被一种更快转发的方式处理。这个字段不像路径一样必需被设置的。)

USER Code

Meaning

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包的原地址字段。这个字段应该被设置成发送数据的交换机传送帧的端口的MAC地址。这是一个48位的值。接收设备可以忽略帧的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字段存储初始包的实际大小用16位表示。LEN字段以字节的方式描述了包的长度，这个长度包含了排除在外的DA，TYPE，USER，SA，LEN和FCS字段。排除字段的总长度是18个字节，所以LEN字段所描述的总长度要减去18个字节。）

AAAA03 (SNAP)—Subnetwork Access Protocol (SNAP) and Logical Link Control (LLC)(子网访问协议和逻辑链路控制)

The AAAA03 SNAP field is a 24-bit constant value of "0xAAAA03". （AAAA03 SNAP字段是一个24位不变的常量值“0xAAAA03”）

HSA—High Bits of Source Address(原地址的高位)

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字段是一个24位的值。这个字段描述了SA字段的高3位[高3位是厂商ID]。这个字段必须包含值“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字段是数据包的VLAN ID。它是一个15位的值，用于区别不同VALN的数据帧。）

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字段只有一位，所有被ISL帧封装的BPDU包设置这个字段。BPDU字段被生成树算法决定网络拓扑的信息。这一位也被CDP和VLAN干线协议[ＶＴＰ，因为VTP用于交换机和交换机之间的通信，所以我把Trunk翻译成“干线”。小弟英语也不怎么样，感觉这段应该是这个意思]封装的帧设置。）

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字段指出数据包离开源交换机的端口索引。这个字段仅是用于诊断的目地，并且可以被其它设备把它设成任务值。它是一个16位的值并且在收到的包里忽略。)

RES—Reserved for Token Ring and FDDI（用于令牌环和FDDI网络接收）

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字段是一个16位的值。这个字段用于被ISL帧封装的令牌环和FDDI包。在令牌环情况下，访问控制[AC]和帧控制[FC]被放在这里。在FDDI的情况下，FC字段被放置在这个字段的最低有效字节[LSB]中。例如：FC值是0x12在RES字段中是0x0012。在以太网中这个字段应该全被设置成零。

ENCAP FRAME—Encapsulated Frame（封装帧）

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字段是被封装的数据包，这个字段中包含了完整没有被修改过的被封装数据的循环有冗校验[CRC]值。在ISL封装被移除后内部帧[被封装在ISL内的原始数据帧]必须有一个有效的CRC值。这个字段的长度可以是从1到24575字节为了容纳以太网、令牌环和FDDI帧。接收数据的交换机剥去ISL封装的字段并且使用ENCAP FRAME字段做为被接收的帧[被接收的帧与适合的一些VLAN联接，做为交换数据的目的地。]）

FCS—Frame Check Sequence(帧校验队列)

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个字节组成。这个队列包含32位CRC[循环有冗校验]值，为了检查帧是否已经损坏，CRC被发送方的MAC建立并且由接收方的MAC重新计算。FCS被生成其中包含了DA，SA，Length/Type和数据字段。当ISL头信息被加到原始帧的开头时，一个新的FCS根据全部的ISL包被计算出来并且被添加到原始帧的末尾)

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不会修改被封装的原始帧的FCS。)

Frame Size （帧大小）

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封装帧是30个字节，最小FDDI包的大小是17字节。因此最小的被ISL封装的FDDI帧大小是47字节。最大令牌环包是18000字节。因此最大的ISL包是18000加30个字节的ISL头信息，总大小为18030字节。如果仅封装以太网包，ISL帧大小的范围是从94到1548字节。)

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. （对于使用ISL封装系统的隐含意义是最大总长度是30个字节。因此如何被封装的包是1518字节，那么ISL包就是1548字节长度。另外，如果除了被封装的是以太网包，最大长度可以很大程度的增长。当你评估拓扑是否能支持ISL包大小时必需注意这个长度变化。

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). （其它隐含意义还有ISL包有两个FCS值。第一个FCS值是被原始数据计算出来的。第二个FCS值是在原始包已经被封装到ISL里后被计算出来的。如果原始数据没有包含一个有效的CRC值，这个无效的CRC直到ISL头被剥去、终端设备检查初始数据的FCS值之前都不会被发现。）

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使用了在原始以太网帧的SA和Type/Length字段中间插入一个4字节标志字段的机制。由于帧被修改过了，所以干线设备[trunking device]要重新计算被修改过的帧的FCS值。)

DA

SA

TAG

TYPE/LEN

DATA

FCS

 

This example shows the further expansion of the Tag field. The expansion includes the field acronyms and the number of bits for each field.（下面这个例子进一步的说明了Tag[标志]字段。这个例子包含了字段的缩写字母开头和每一个字段所占位数。）

No. of bits

16

3

1

12

Frame field

TPID

PRIORITY

CFI

VID

 

Field Descriptions （字段说明）

This section provides detailed descriptions of the 802.1Q frame fields.（这一部分提供了详细的802.1Q帧字段的说明）

TPID—Tag Protocol Identifier(Tag协议标识符)

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协议标识是一个１６位的字段。它被设置成0x8100这个值，为了说明这个帧是一个IEEE 802.1Q标志帧）

Priority（优先级）

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).(就像用户所知道的优先级一样。这是一个3位的字段用于指示IEEE 802.1!Q的优先级。这个字段批出了帧在传输过程中的优选次序。这个字段可以被声明成8个级别[从0到7])

CFI—Canonical Format Indicator(规范格式指示符)

The Canonical Format Indicator is a 1-bit field. If the value of this field is 1, the MAC address is in noncanonical format. If the value is 0, the MAC address is in canonical format.（规范格式指示符是是一个1位的字段。如果该值被设置成1，那么MAC地址是非规范格式。如果该值被设置成0，那么MAC地址是规范格式。[什么是Canonical Format，什么又是Noncanonical Format？？？点击查看CFI做用]）

VID—VLAN Identifier（VLAN标识符）

The VLAN Identifier is a 12-bit field. It uniquely identifies the VLAN to which the frame belongs. The field can have a value between 0 and 4095.（VLAN标识符是一个12位的字段。它唯一的指出帧属于哪个VLAN。这个字段的任可以是从0到4095中的一个。）

Frame Size （帧大小）

The 802.1Q tag is 4 bytes. Therefore, the resulting Ethernet frame can be as large as 1522 bytes. The minimum size of the Ethernet frame with 802.1Q tagging is 68 bytes. （802.1Q标志位是4个字节大小。因此，包含IEEE 802.1Q标志字段的以太网的帧最大1522字节，最小68字节。）