Layer switches_ three switches and four switches_ the difference between by fdjerue7eeu


									Layer switches, three switches and four switches, the difference between second floor of the exchange of technology is
relatively mature, the second floor is a data link layer switch devices that can identify
data packets in the MAC address information, according to MAC address forwarding,
and the MAC address recorded with the corresponding port within an address in their
Specific workflow is as follows:
1) When the switch from a port receives a packet, it first reads the header of the
source MAC address, so that it knows the source MAC address of the machine is
connected on which port;
2) go read the header of the purpose of MAC address and the address table to find the
corresponding port;
3) If the table has the objective of MAC addresses with the corresponding port, the
packets directly copied to the port;
4) If the table is not found in the corresponding port put the packet broadcast to all
ports on the machine when the purpose of responding to the source machine, the
switch can also record the purpose of which port the MAC address corresponds to
when the next transmit data no longer need to broadcast to all ports. Continuous cycle
of this process, for the whole network MAC address information can be learned, the
second floor switch is built and maintained its own address table.
The working principle of the switch from the second floor can infer the following
three points:
1) Since the majority of ports on the switch while the data exchange, which requires a
wide exchange of bus bandwidth, if the layer switch with N ports, each
port's bandwidth is M, the switch bus bandwidth than N × M, then This
switch can achieve wire-speed switching;
2) port to connect the machine learning MAC address, write address table, address
table size (generally two kinds of representation: one BEFFER RAM, an entry for the
MAC value), address table size on the capacity of access switches;
3) there is a layer switch is typically designed to process data packets containing the
forwarding ASIC (Application specific Integrated Circuit, ASIC) chip, it can be done
very fast forward speed. As the various manufacturers use different ASIC a direct
impact on product performance.
The above three points is the second evaluation, three main technical advantages and
disadvantages of switch performance parameters, please consider this point when the
equipment selection of more attention.
Three-tier exchange
Following a simple start to look at the three-switch network to the work process.
Using the IP layer switching equipment A ------------------------ ------------------ ------ B
using the IP of the device
Such as A to give B send data, the purpose is known IP, then A would get a network
address with a subnet mask to determine whether the purpose of their own in the same
IP subnet. If the same network segment, but do not know the MAC address required
for forwarding of data, A will send an ARP request, B return to their MAC address, A
MAC with this package and send the packet switch, the switch switching modules
only use the second floor, find MAC address table, will forward the packet to the
appropriate port.
If the destination IP address displayed is not the same segment, then A and B to
achieve the communication, not in the flow cache entry corresponding to MAC
address entries, will be the first to send a normal data packet to a default gateway, the
default network relations generally good in the operating system has been set up,
corresponding to the third layer routing module, we can see that the data is not the
same subnet, the first place in the MAC table is the default gateway MAC address;
then on the three-module receiving this packet, check the routing table to determine
the route to reach B, will construct a new frame header, in which the default gateway
MAC address source MAC address to the host B's MAC address for the
purpose of MAC addresses. Recognition by some trigger mechanism, the
establishment of the host A and B's MAC address and port forwarding
correspondence and records into the flow cache entry table, after the A to B data,
directly referred to the second floor of the exchange of modules completed. This is
commonly referred to as a route many times forward.
These are the three switches, a simple generalization working process, you can see the
exchange of three characteristics:
1) The combination of data from the hardware high-speed forwarding. This is not a
simple superposition of two-story switches and routers, three routing module directly
superimposed on the second floor of the exchange of high-speed backplane bus,
breaking the traditional router interface rate limiting, rate of up to tens of Gbit / s.
Count backplane bandwidth, these are three switches, two important parameters of
2) simple routing software to simplify the routing process. Most of the data
forwarding, in addition to the necessary routing by the routing software, and is
another layer module high-speed forwarding, routing software are treated most
efficient optimization software, not simply follow the router software.
The choice of the second and three switches
Layer switches for small-scale local area network. This would not have much to say,
and in small local area network, broadcast packets little effect on the rapid exchange
function layer switches, multiple access ports and low-modest price for small
networks provide a very comprehensive solution.
Advantage of the router interface type rich Zhichi of three powerful, robust routing
capabilities, suitable for large routing between the networks, it is of advantage to
choose the best routing, load sharing, link backup and route and other network
information exchange, etc. The functional router.
Three of the most important function of the switch is to speed up the data within a
large local area network fast forward, adding routing service is also for this purpose.
If a large network in accordance with the department, divided into geographical
factors such as a small local area network, which will lead to a large number of
Internet exchanges, a simple switch can not be achieved using the second floor of
Internet visits; as simple to use a router, because a limited number of interfaces and
routed slow to limit the network speed and network size, using the fast forward with
routing switch becomes the preferred three-tier.
In general, network traffic, including large, fast forward to respond to network
requests, such as all the three switches to do the job, will cause an excessive burden
on three-switch, response rate affected the gateway route exchanges completed by the
router to give full play to the advantages of different devices, be a good networking
strategy, of course, that client's pockets are drums, or the next best thing, so
that three switches are and the Internet interconnection.
Four exchange
A simple exchange of the fourth layer is defined: it is a feature, it decided to transfer
not only based on MAC address (the second level bridge) or source / destination IP
address (the third layer routing), and based on TCP / UDP ( fourth level) application
port number. Switching function is like the fourth level virtual IP, point to the physical
server. It transfers a wide range of business subject to the agreement, there are HTTP,
FTP, NFS, Telnet or other agreement. These business based on the physical server,
require complex load balancing algorithm.
In the IP world, business type from the TCP or UDP port address of the terminal to
determine, in the fourth floor of the exchange zone by the source and the terminal IP
address, TCP and UDP port joint decision. Exchange in the fourth layer for each
server group for setting up the search using virtual IP address (VIP), each server
supports a specific application. In the domain name server (DNS) for each application
server to store address is VIP, but not the real server address. Application when a user
application, a group of VIP with the target server connection requests (such as a TCP
SYN packet) to the server switch. Server switches in the group select the best server,
the terminal address in the VIP IP replaced with the actual server, the server will pass
the connection request. In this way, the same interval all packets by the server
switches to map between the user and the same server for transmission.
The principle of exchange of the fourth layer
The fourth layer of OSI model is the transport layer. Transport layer is responsible for
end to end communication, that is the source and target systems in the network
communication between the co-ordination. In the IP protocol stack is TCP (a transport
protocol) and UDP (user datagram protocol) protocol layer is located.
In the fourth layer, TCP and UDP header contains the port number (port number), they
can be uniquely identified what each packet contains the application protocol (eg
HTTP, FTP, etc.). Endpoint system uses this information to distinguish between
packets of data, particularly the port number to a receiver computer system before it
can determine the IP packet type, and gave it to the appropriate high-level software. IP
address port number and device combination often called "jack
(socket)". 1 and the port number 255 is reserved, they called the
"well known" ports, that is, in all host TCP / IP protocol stack
implementation, the port number is the same. In addition to
"known" ports, the standard UNIX distribution service port in
the 256-1024 range, custom application normally assigned port numbers above 1024.
The most recent list of assigned port numbers can RFC1700 "Assigned
Numbers" found.
TCP / UDP port number to provide the additional information can be exploited for
network switches, which is the basis for the exchange of the fourth floor. Function
with the fourth layer switch connected to the server could play the "virtual
IP" (VIP) front-end role. Each server and support a single or universal
application of the server group are configured a VIP address. The VIP address is sent
out and register the domain name system. In issuing a service request, the fourth layer
switches start by determining TCP, to identify the beginning of a conversation. Then it
uses complex algorithms to determine the best server to handle the request. Once that
decision, the switch will be a session with a specific IP address linked with the real IP
address of the server instead of server VIP address.
Each fourth level switch is to save a match with the selected source IP address of the
server and the source TCP port connection table associated. Then the fourth layer
switch connected to this server forwards the request. All subsequent packets between
the client and the server re-mapping and forward, until the session switch found so far.
The fourth layer using the exchange situation, access can be connected with the real
server to meet the users rules, such as the so each server has an equal number of
access or capacity under the server to allocate different transport streams.
How to choose the appropriate exchange of the fourth layer
1) speed
In order to be effective in the enterprise networks, fourth tier exchange must provide
wire-speed router with a third layer of comparable performance. In other words, the
fourth layer switching all ports must operate at full media speed, even in multiple
Gigabit Ethernet connections is all about. Gigabit Ethernet speed is equal to 1.488
million packets per second maximum speed routing (assuming the worst case, that all
network packets as well as the definition of the minimum size of 64 bytes long).
2) the server capacity balancing algorithm
Hope that the capacity of the balance followed by an interval of size, the fourth
distribution layer switch will apply the algorithm to the server There are many kinds
of testing a simple loop recent connection, detection loop delay or detection server
itself closed-loop feedback. In all forecasts, the closed-loop feedback to provide the
server existing portfolio reflects the most accurate test.
3) Table Capacity
It should be noted that the fourth layer switches need to have distinction between the
exchange and storage capacity of a large number of entries to send. Switches in the
core of an enterprise network, especially when. Many second / three switches tend to
send the table size is proportional to the number of network devices. Switch on the
fourth floor, this number must be multiplied by the different applications used in
network protocols and the number of sessions. Thus sending the size of the table with
the number of endpoint devices and application types of growth and rapid growth.
The fourth layer switch designers to design their products in the form of this growth
to be considered. Large table capacity of the manufacturing support wire-speed traffic
to send high-performance switches, the fourth level is essential.
4) Redundancy
Fourth layer switches support redundant topology of internal functions. Card that has
dual-link fault-tolerant connection, it is possible to build a server from the network
card, link and server switches fully redundant system.
Managed switches can be managed
Managed switches can be managed in several ways: through the RS-232 serial port
(or parallel port) management, through a web browser management and management
through network management software.
1. Through the serial port management
Managed switch with a serial cable for switch management and use. Plug one end of
the first serial port cable serial port on the switch on the back, the other end inserted in
the computer serial port in general. Then connect the switches, and PC. In Windows
98 and Windows 2000 are provided in the "Super Terminal"
program. Open the "super terminal" in setting the connection
parameters, you can interact with the switch through a serial cable, shown in Figure 1.
This approach does not occupy the bandwidth of the switch, so called
"band management" (Out of band).
In this management mode, the switch provides a menu-driven console interface or
command line interface. You can use the "Tab" key or the arrow
keys to move the menu and sub menu, press the Enter key to perform the appropriate
command, or use a dedicated switch manage manage switch command set. Different
brands of switches command set is different, and even the same brand of switch, the
command is also different. Use the menu command in the operation more convenient.
2. Through the Web Management
Managed switches can be Web (Web browser) to manage, but to switch to specify an
IP address. The IP address for the management of switches used in addition to outside,
and no other purpose. By default, the switch no IP address, must specify a serial port
or other IP address, to enable such management.
Management switch using a web browser, the switch is equivalent to a Web server,
but pages are not stored in the hard drive inside, but inside the switch NVRAM,
NVRAM through the program can upgrade inside the Web program. When the
administrator in the browser, enter the IP address of the switch, the switch as a server
as the web is passed to the computer, then you feel like visiting a Web site, as shown
in Figure 2. In this way the bandwidth occupied by the switch, so called
"in-band management" (In band).
If you wish to manage the switch, just click the corresponding Web page features, and
in the text box or drop-down list to change the switch parameters on it. Web
management in this way can be carried out in the local area network, so it can realize
remote management.
3. Through network management software
Managed switches are followed SNMP protocol (Simple Network Management
Protocol), SNMP protocol is a set of international standards for network equipment
management practices. Those who follow the SNMP protocol equipment, can be
managed by network management software. You only need a single workstation to
install a SNMP network management network management software, local area
network can be easily managed network switches, routers, servers and so on. SNMP
network management software through the interface shown in Figure 3, it is also an
in-band management.
The management of network switches can be managed by more than three ways. To
figure out which way it? In the initial switch setting, they often go through band
management; the configured IP address, you can use in-band management of the.
In-band management because the management data is passed through the public use
of the LAN, and can realize remote management, but security is not strong. Band
management through serial communication, data is only used in switches and
management of transfer between machines, so security is strong; However, due to
serial cable length restrictions, remote management can not be achieved. So which
way to look at you with the security and manageability of the request.
4. Main purpose
Used to control other computers, such as schools, teachers have been logged by the
switch control computer students

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