The difference between two, three, four-tier switches

Two-layer switching technology is a mature development, two-layer switch is a data link layer device, can identify the MAC address information in the packet, according to the MAC address forwarding, and the MAC address and the corresponding port record in their own internal address table. The specific work flow is as follows:

(1) When the switch receives a packet from a port, it first reads the source MAC address in the header, so it knows the source MAC address machine is connected to which port;
(2) to read the destination MAC address in the header and find the corresponding port in the Address table;
(3) If there is a port corresponding to the MAC address in the table, copy the packet directly onto the port;
(4) If the table can not find the appropriate port to the packet broadcast to all ports, when the target machine to respond to the source machine, the switch could learn a destination MAC address and which port corresponding, the next time the data transfer will no longer need to broadcast all the ports.

The continuous cycle of this process, for the whole network of MAC address information can be learned, the second-tier switch is to create and maintain its own address table.

The following three points can be inferred from the working principle of the two-layer switch:

(1) because the switch to the majority of the port data exchange, this requires a very wide exchange bus bandwidth, if the two-tier switch has n ports, each port bandwidth is M, the switch bus bandwidth over NXM, then this switch can achieve wire speed exchange;
(2) Learning port connected to the MAC address of the machine, write Address table, Address table size (generally two representations: one for Beffer RAM, one for the Mac table entry value), Address table size affects the access capacity of the switch;
(3) Another is that a two-tier switch typically contains an ASIC (application specific integrated circuit) chip specifically designed to handle packet forwarding, so the forwarding speed can be very fast. Because each manufacturer uses the ASIC different, the direct influence product performance.

The above three points are also the main technical parameters to evaluate the performance of the two or three-layer switch, please pay attention to the comparison when considering equipment selection.

(ii) Routing technology

Routers work in the third layer of the OSI model---Network-level operations, similar to the two-tier exchange, but the router works on the third tier, which determines that routing and switching use different control information when passing the package, and that the functionality is implemented in a different way. It works by having a table inside the router, what this table indicates is that if you want to go somewhere, the next step should go there, and if you can find the packet from the routing table, go there and forward the link layer information, and if you don't know where the next step is going, discard the package. It then returns a message to the source address.

Routing technology is essentially two functions: determining the optimal Routing and forwarding of packets. The routing table writes a variety of information, the routing algorithm calculates the best path to the destination, and then sends the packet by a relatively straightforward forwarding mechanism. The next router that accepts the data continues forwarding in the same way, and so on until the packet arrives at the destination router.

There are also two different ways to maintain the routing table. One is the updating of routing information, which is to publish some or all of the routing information, routers acquire the topology of the whole network by learning routing information from each other, which is called the distance vector routing protocol, and the other is that routers broadcast their link state information and master the routing information of the whole network through mutual learning. , and then compute the best forwarding path, which is called the link state routing protocol.

Because routers need to do a lot of path computing work, the general processor's ability to work directly determines the quality of its performance. Of course, this judgment is still for middle and low-end routers, because high-end routers often use distributed processing system system design.

(iii) Three-tier switching technology

In recent years, the promotion of three-tier technology ears can be cocoon, shouting three of technology everywhere, some people say that this is a very new technology, but also some people say, three-tier exchange, is not the routers and two-tier switch stack, there is no new gadgets, the fact is indeed so. Let's start with a simple network to see how the three-tier switch works.

The network is relatively simple

Devices using IP a------------------------layer three switch------------------------devices using IP b

For example, a to send data to B, known destination IP, then a subnet mask to obtain the network address, to determine whether the destination IP and its own in the same network segment.

If you are in the same network segment, but do not know to forward the data required MAC address, a to send an ARP request, B return its MAC address, a with this MAC package packet sent to the switch, switch to use two-tier switching module, look for the MAC address table, the packet forwarding to the appropriate port.

If the destination IP address does not display the same network segment, then a to implement and B communication, in the stream cache entry, there is no corresponding MAC address entry, the first normal packet sent to a default gateway, this default gateway is generally in the operating system has been set up, corresponding to the third tier routing module, So for data that is not the same subnet, the MAC address of the default gateway is first placed in the Mac table, then the three-tier module receives the packet, queries the routing table to determine the route to B, and constructs a new frame header with the MAC address of the default gateway as the source MAC address. The MAC address is the destination of the MAC address of Host B. The corresponding relation between the MAC address of host A and B and the port of the transmitter is established by certain trigger mechanism.
, and record the Entry table of the stream cache, then the data from A to B will be directly handed over to the two-layer switching module. This is often referred to as a route forwarding multiple times.

The above is a simple summary of the working process of the three-layer switch, you can see the characteristics of the three-layer exchange:

High-speed forwarding of data is realized by hardware combination.

This is not a simple two-layer switch and router overlay, the three-tier routing module is directly superimposed on the two-layer switched high-speed backplane bus, breaking the traditional router interface rate limit, the rate can reach dozens of gbit/s. To calculate the backplane bandwidth, these are two important parameters of the three-layer switch performance.

Simple routing software simplifies the routing process.

Most of the data forwarding, in addition to the necessary routing options to the Routing software processing, are two-tier module high-speed forwarding, routing software is mostly processed by efficient optimization software, not simply copy the software in the router.

Conclusion

A two-tier switch is used for small local area networks. This is needless to say, in small LAN, broadcast packet impact is not small, the two-tier switch fast switching function, multiple access ports and low modest price for small network users to provide a very complete solution.

The advantage of the router is that it is rich in interface type, powerful in three layers, powerful in routing, and suitable for routing between large networks, which has the advantage of choosing the best route, load sharing, link backup and the exchange of routing information between other networks and so on.

The most important function of the three-tier switch is to speed up the rapid forwarding of data within a large local area network, which is also served by the addition of routing functions. If you put a large network according to the department, geographical factors such as a small local area network, which will lead to a large number of Internet exchange visits, the use of two-tier switches can not achieve the Internet exchange; For example, the use of routers, due to limited number of interfaces and slow forwarding speed, will limit the speed of the network and network size, A three-tier switch with fast forwarding with routing capabilities is preferred.

Generally speaking, in the intranet data flow is big, requirements for fast forwarding of the response network, such as all by the three-tier switch to do this work, will result in three-tier switch overload, response speed is affected, the network routing to the router to complete, give full play to the advantages of different equipment, is a good networking strategy, of course, The premise is that the customer's purse is very drum, or the second, so that the three-tier switch also for the Internet interconnection.

A simple definition of layer fourth exchange is that it is a function that determines that the transmission is based not only on the MAC address (the second Layer Bridge) or the source/destination IP address (third-tier routing), but also on the TCP/UDP (layer fourth) application port number. Layer Fourth switching functions are like virtual IP, pointing to the physical server. It transmits a variety of business compliance protocols, with HTTP, FTP, NFS, Telnet, or other protocols. These services require a complex load balancing algorithm based on the physical server. In the IP world, the business type is determined by the terminal TCP or UDP port address, and the application interval in layer fourth switching is determined by the source and terminal IP addresses, TCP, and UDP ports.

A virtual IP address (VIP) is set up for each server group for search in layer fourth exchange, and each group of servers supports some application. Each application server address stored in the Domain name server (DNS) is a VIP, not a real server address.

When a user applies for an application, a VIP connection request (such as a TCP SYN packet) with the target server group is sent to the server switch. The server switch selects the best server in the group, replaces the VIP in the terminal address with the IP of the actual server, and passes the connection request to the server. In this way, all packets in the same interval are mapped by the server switch and transmitted between the user and the same server.

The principle of layer fourth exchange

The fourth layer of the OSI model is the transport layer. The transport layer is responsible for end-to-end communication, which is to coordinate communication between the network source and the target system. In the IP protocol stack, this is the protocol layer where TCP (a transport Protocol) and UDP (User Packet protocol) reside.

In layer fourth, TCP and UDP headers contain port numbers (PortNumber), which can uniquely differentiate which application protocols each packet contains (for example, HTTP, FTP, and so on). The endpoint system uses this information to differentiate the data in the packet, especially the port number, which enables a receiving computer to determine the type of IP packet it receives and hand it over to the appropriate high-level software. The combination of port number and device IP address is often referred to as "socket". The port number between 1 and 255 is reserved, they are called "well-known" ports, that is, in all host TCP/IP protocol stack implementations, these port numbers are the same. In addition to the "familiar" port, standard UNIX services are allocated in 256 to 1024 port ranges, and custom applications typically allocate port numbers above 1024. The most recent list of allocation port numbers can be found on the RFc1700 "Assigned Numbers". The additional information provided by the TCP/UDP port number can be used by the network switch, which is the basis of layer 4th exchange.
　
Examples of "familiar" port numbers:

Apply Protocol port number
FTP 20 (data)
21 (Control)
TELNET 23
SMTP 25
HTTP 80
NNTP 119
NNMP 16
162 (SNMP traps)
The additional information provided by the TCP/UDP port number can be used by the network switch, which is the basis of layer fourth exchange.

A switch with layer fourth functionality can function as a front-end to the "Virtual IP" (VIP) connected to the server.

Each server and server group that supports a single or universal application is configured with a VIP address. This VIP address is sent out and registered on the domain Name System.

When a service request is issued, layer fourth switches to identify the start of a session by determining TCP start. It then uses complex algorithms to determine the best server to process the request. Once this decision is made, the switch links the session to a specific IP address and replaces the VIP address on the server with the server's real IP address.

Each layer fourth switch holds a connection table that matches the selected server's source IP address and the source TCP port. Then layer fourth switches forward the connection request to this server. All subsequent packets are again mapped and forwarded between the client and server until the switch discovers
Session to date.
　
In the case of layer fourth exchange, access can be connected to a real server to meet user-established rules, such as having an equal number of accesses per server or allocating transport flows based on the capacity of different servers.

How to choose the right layer fourth exchange
　　
A, speed
In order to be effective in the enterprise network, layer fourth Exchange must provide performance comparable to the third-tier wire-speed routers. In other words, layer fourth Exchange must operate at full media speed on all ports, even on multiple gigabit Ethernet connections. Gigabit Ethernet speeds are equal to the maximum speed of 488,000 packets per second (assuming the worst-case scenario, that is, all packets are the minimum size for the network definition, and 64 bytes long).

b, server capacity balance algorithm
Depending on the desired capacity balance interval, layer fourth switches will apply a number of algorithms to the server, with a simple detection loop connection, detection loop delay, or closed loop feedback for the server itself. In all predictions, closed-loop feedback provides the most accurate detection of the server's existing business volume.

C, table capacity
It should be noted that switches for layer fourth Exchange require the ability to differentiate and store a large number of send table entries. This is especially true when the switch is at the core of an enterprise network. Many second-and three-tier switches tend to send table sizes in direct proportion to the number of network devices. For layer fourth switches, this number must be multiplied by the number of different application protocols and sessions used in the network. As a result, the size of the sending table grows rapidly as the number of endpoint devices and application types grows. Layer fourth switch designers need to consider this growth of the table when designing their products. Large table capacity is critical to manufacturing high-performance switches that support wire-speed fourth-tier traffic.
　　
D, redundancy
The layer fourth switch has the capability to support redundant topologies internally. In the case of a fault-tolerant connection with a De Chinglu network card, it is possible to establish a fully redundant system from a server to a network adapter, link, and server switch.

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