Knowledge of the switch layers

The two layer switch

The two layer exchange technology development is relatively mature, buy SL4D the two layer switchbelongs to the data link layer device, can be MAC address information to identify data packets in the forwarding, according to the MAC address, the MAC address and corresponding portrecords in their inside a table.

The specific work as follows:

1) when the switch receives a data packet from a port, it reads the source MAC address in Baotou, so that it knows the source MAC address of the machine on which port is connected;

2) to read in Baotou the destination MAC address, and find the corresponding port on the table;

3) if there is corresponding to the destination MAC address port table, the data packet is copied directly to the port of;

Table 4) not found in the corresponding port of the broadcast packet to all ports on thedestination machine, when the source machine to respond, the switch can also record thedestination MAC address and ports corresponding, in the next data will no longer need to all ports for broadcasting. Keep repeating this process, the MAC address information of the whole network can learn, the two layer switch is the way to establish and maintain its own addresstable.

From the work principle of the two layer switch can infer the following three points:

1) because the switch for most port data and exchange, which requires a bus bandwidth is wide,if the two layer switch with N ports, each port bandwidth is M, the switch bus bandwidth is more than N * M, the switch can achieve wire speed switching;

2) learning port connections to the MAC address of the machine, the write address table,address table size (generally two forms: one is BEFFER RAM, a MAC table numerical), address table access capacity switch size effect;

3) there is a layer two switches generally contain specialized for processing ASIC data packet forwarding (Application specific Integrated Circuit, application specific integrated circuit) chip, so the forwarding speed can be done very quickly. Because of various manufacturers use different ASIC, directly affect the performance of the product.

The above three main technical parameters and evaluation of two or three layer switchperformance, this point please consider equipment selection of attention.

The three layer switch

The first to have a look the working process of three layer switch through a simple network.

Devices using IP A------------------------ three layer switch ------------------------ devices using IP B

For example, A to send data to B, known to IP, then A will use the subnet mask made network address, to judge whether the purpose of IP and myself in the same network segment. If in the same network segment, but do not know the data forwarding MAC address, A will send a ARP request, B returns the MAC address, A using the MAC package data packets and send to the switch, the switch uses two layer switching module, MAC address lookup table, forward the packet to the appropriate port.

If the destination IP address display is not the same segment, then A and B to realize thecommunication, in the stream cache entry without a corresponding MAC address entry, will bethe first normal data packets to a default gateway, the gateway in the operating system has been set up, the third layer routing module, so it is visible for is not in the same subnet data, firstin the MAC table is on the default gateway MAC address; and then by the three layer modulereceives the data packet, routing table lookup to determine arrived in B routing, will construct a new frame header, the default gateway MAC address the source MAC address, with host B MAC address as the destination MAC address. The trigger mechanism by certain identification,establish the corresponding relationship between A and B host MAC address and port forwarding, and recorded into the stream cache entry table, after the A to B data, directly by the two layer switch module. This time is usually said multiple forwarding routing.

The above is the three layer switch process simple summary, it can conclude that the threelayer exchange:

1) by the combination of hardware forwarding data. This is not simple superposition of the twolayer switch and router, switch layer three routing module directly superimposed on the two layerof high speed backplane bus, breaks through the traditional router interface rate limit, speed up to tens of Gbit/s. Count the backplane bandwidth, these are two important parameters of the three layer switch performance.

2) routing software to simplify the process of simple routing. Most of the data forwarding, in addition to the necessary routing the routing software processing, are two layer module of high-speed transmission, routing software are the most efficient optimization software processing,and not simply copy router software.

The two layer and the three layer switch selection

The two layer switch for LAN small. This is not to say, in a small local area network, broadcast packets have little effect, provides solutions to perfect the rapid exchange of the two layer switchfunction, multiple access ports and low - price for small network users.

The router has the advantages of various interfaces, three layer function support of the powerful, routing ability strong, suitable for large inter network routing, its advantage is to select the best route, load sharing, link backup and other network routing information exchange, etc.the function of router.

The three layer switch is the most important function is to accelerate the large-scale local area network internal data fast forwarding, the routing function is to serve this purpose. 

Generally speaking, in the network data flow, fast forwarding the response in the network, such as all the layer three switch to do the work, will cause the layer three switch overburdened,response speed is affected, the inter network routing by the router to complete, give full play to the advantages of different devices, it is a good networking strategy, of course, the premise isthe customer's purse is a drum, or draw back asks next, let the three layer switch and also forinternet.

The four layer switch

A simple definition of the fourth layer exchange is: it is a function, it not only determines thetransmission based on MAC address (the second layer bridge) or the source / destination IP address (the third layer routing), and on the basis of TCP/UDP (fourth) application of the port number. The fourth layer switching function is like a virtual IP, pointing to the physical server. Itfollows the protocol transmission business variety, HTTP, FTP, NFS, Telnet or other protocols.These operations in a physical server basis, the need for complex load balancing algorithm.

In the IP world, business type is determined by the terminal TCP or UDP port address,application of interval in the fourth layer exchange is determined by the source terminal and theterminal IP address TCP and UDP port, common. In the fourth layer exchange for each searchserver group set up for the use of virtual IP address (VIP), each server to support someapplication. The domain name server (DNS) for each application server address is stored in theVIP, not the actual server address. When a user application, a target server group VIP connection request (such as a TCP SYN packet to the server switch). The server switch selectsthe best server in the group, the terminal address in VIP with the actual server IP to replace, and the buy CXL connection request to the server. In this way, the same interval of all packets are mapped bythe server switch, transmission in the user and the same server