Layer 2 Ethernet Technology—Vecloud

OSI's communication model divides network communication into seven relatively independent functional levels-application layer, presentation layer, session layer, transport layer, network layer, data link layer, and physical layer.
Things in the physical layer are responsible for transmitting the bit stream, using high and low pulse circuits to represent 0 and 1.
The data link layer (OOL) has two responsibilities: sending and receiving. At the same time, it also provides an effective end-to-end transmission connection (PPP or something) of data. The data link layer is responsible for encapsulating commands, data, etc. into frames (frames). Son, en (⊙v⊙)), the frame is the structure of the link layer, it must contain enough information (who am I, who I want to send to) to ensure that the data can be safely and correctly transmitted to the destination through the second layer.
So, from this point of view, the most basic needs of each data frame are:
1. As a courier, I need to know who delivered the goods and who received the goods.
2. What I am sending, tell others to load it in the car. What is it?
3. Verify, or if someone sends an iPhone, it turns into a stone.
At the same time, as a courier, the workflow is like this: all the big guys above, what they want to do, I just watch The customer-the package given to me by the network layer, and then I took it all the time to carry it, and threw it to the lower physical layer-tall guys and chunky, walking bumpy. . . . .
If you want to take a detailed look at how many things you have to carry as a courier. . . . Let
’s first take a look at Ethernet, the largest express company:
Although Ethernet is the largest express company, there are also several small branch companies
EthernetⅡ, Netware, 802.3 SAP, 802.3 LLC
SNAP, as a branch company, we implement the express delivery standard It should not be changed. The general process is like this.
So for our express company, I don't care about what the customer carries. I only need to add the express number at the beginning and the end, and verify it.
At the same time, we have requirements for the size of the package, and the requirements of the foreigner head office are like this
Ethernet Frame
Here we discuss both legal and illegal Ethernet frames.The receiver will drop
all illegal frames.
The vast majority of all local (LAN) data traffic in the world is Ethernet
frames !!? There is no single unit of data that is more the Common. (So) the anyone in the any
of the type of Technical Field, Would Benefit by Understanding at The Contents of THESE
Frames, and How They are Transmitted and Received.?
Min Frame Size = Header + CRC + DataMin = 64-
Max Frame Size = HEADER_CRC + DataMax = 1518
simple In other words, the head office requires:
1. The package must be legal, otherwise I don’t want it.
2. The package cannot be too small or too large. The size must be between 64 and 1518. If it is less than 64, I cannot pass it. If it is greater than 1518, I have to consider whether to send two packages, if the client does not allow (DF is set, fragmentation is not allowed), then there is no way, it can only be discarded
Now that we are talking about the size of the package, the user can only rely on the size of the package. The user data is large, and we have to pass it. It is the user’s decision not to send multiple packages. To understand the business, we must first look at our express delivery. How is the order written? As mentioned earlier, the express order must include:
who delivers the goods, who receives the goods, the content of the delivery, and verification. Each express company has different ways of writing the express order number. Overview:
As can be seen from the above, in Ethernet, we use MAC address as the customer’s delivery and receiving address
. Let’s start with EthernetⅡ. First,
it’s obvious that the functions of encapsulating each field in detail are obvious, here is a thing. , The length of each field is added 6+6+2+4=18bytes, so the data part of the customer to send, the minimum is 46, and the maximum is 1500. The
type field indicates the content carried and the content that can be carried It is roughly as follows (Well...who remembers tm...), just remember a few key things, such as IP, ARP, RARP, MPLS, etc., here you can also find a problem -----ARP This protocol is not a three-layer protocol. I asked in an interview before, which layer does ARP work on, and promise and three-layer protocols tell you that type=0x0800. The ARP protocol family is listed separately. Look at MPLS
××× As a label party, it works between the second and third layers, so, we can say that ARP works on the second layer.
Netware Express: The
package is like this.
Compared with EthernetⅡ, there are several changes.
1. Change the type field to length
2. Change the type field to DSAP and SSAP
3. The load can load less 6+6 +2+1+1+4=20 The
802 group companies are almost the same.
Look at 802.3SAP.
Are you looking at 802.3 LLC?
OUI is sold to manufacturers. 24 bits and 22 bits can be sold. The last 2 bits represent unicast or multicast.
Also, the 802.3 header uses lenth to distinguish it from Ethernet Type 2 frames. Length is valid. If the value is 0-1536 (ie 0x600),
the Ethernet type 2 frame must avoid this value field. If it is greater than 0x600, it is an Ethernet type 2 frame. If it is less than 0x600, it is 802.3
because the 802.3 header has no type field, so Supplemented by 802.2 (also called LLC) ISO divides the 2 layers into LLC layer + MAC layer, different protocols are divided into different MAC layers, but a common LLC layer, like
this
is actually written so much (╮(╯_╰) ╭You are embarrassed to say more...), mainly remember the following points
1. The data frame encapsulation transmitted on the Ethernet must be legal, otherwise it will be discarded
2. The data frame size must be between 64-1518, which is less than this. Transmission, larger than this
, the subtle difference between 3, several kinds of encapsulation, the switch uses the lenth length to determine the format, greater than 1536 is the Ethernet type 2 frame, less than 1536 is the 802.3 frame
4. ARP is not a three-layer protocol
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