Classification of switches and main reference factors

Today, let’s talk about the most used switch in our weak current network engineering.

1. Classification of switches

Network composition mode: access layer switch, aggregation layer switch and core layer switch

OSI model: Layer 2 switches, Layer 3 switches, Layer 4 switches, etc., all the way to Layer 7 switches.

The manageability of the switch: the difference between the manageable switch and the unmanageable switch lies in the support for network management protocols such as SNMP and RMON.

Second, those factors are mainly referred to when choosing a switch

a. Backplane bandwidth, L2/3 switching throughput rate.

b. VLAN type and quantity.

c. The number and type of switch ports.

d. Protocols and methods that support network management. The switch is required to provide more convenient and centralized management.

e, Qos, 802.1q priority control, 802.1X, 802.3X support.

f. Stacking support.

g. The switch buffer and port buffer, main memory, forwarding delay and other parameters.

h. Wire-speed forwarding, routing table size, access control list size, support for routing protocols, support for multicast protocols, packet filtering methods, machine expansion capabilities, etc. are all parameters worth considering, and should be investigated according to actual conditions .

The above are the factors that need to be referred to when choosing a switch. Usually, we can judge by the following factors when choosing a switch.

1. Backplane bandwidth

The backplane bandwidth of a switch is the maximum amount of data that can be handled between the switch interface processor or interface card and the data bus. The backplane bandwidth indicates the total data exchange capacity of the switch, in Gbps, also called exchange bandwidth. Therefore, only modular switches (with expandable slots and the flexibility to change the number of ports) have this concept. Fixed port switches do not have this concept, and the backplane capacity and switching capacity of fixed port switches are equal. The backplane bandwidth determines the upper limit of the connection bandwidth between each board (including the board that has not been installed in the expandable slot) and the switching engine. Due to the different architectures of modular switches, the backplane bandwidth cannot fully represent the true performance of the switch. The concept of backplane bandwidth does not exist in fixed port switches.

2. Switching capacity and forwarding capacity

Since the switching engine is the core of the packet forwarding of the modular switch, this indicator can truly reflect the performance of the switch. For fixed port switches, the switching engine and the network interface template are integrated, so the forwarding performance parameter provided by the manufacturer is the forwarding performance of the switching engine, which is the key to determining the performance of the switch. For devices that support Layer 3 switching, manufacturers will provide Layer 2 forwarding rate and Layer 3 forwarding rate respectively. Generally, Layer 2 capability uses bps and Layer 3 capability uses pps. Modular switches with different architectures are used. These two parameters The meaning is different. However, for general LAN users, they only need to care about these two indicators, which are the key indicators that determine the performance of the system. For large-scale campus network and metropolitan area network users, it is meaningful to discuss the architecture of the switch and the third-layer optimization algorithm.

3. Backplane bandwidth calculation

Backplane bandwidth is the maximum amount of data that can be handled between the switch interface processor or interface card and the data bus.

Calculation formula: Number of ports × corresponding port rate × 2 (full duplex mode)

24 ports 100M + 2 ports Gigabit

24*2*100+2*2*1000=8.8Gbps

4. Calculation method of wire-speed packet forwarding rate

(1) Backplane bandwidth (switching capacity)

Check the total bandwidth that all ports on the switch can provide. The calculation formula is the number of ports*corresponding port rate*2 (full-duplex mode). If the total bandwidth ≤ the nominal backplane bandwidth, then the backplane bandwidth is line-rate.

(2), packet forwarding wire speed

Packet forwarding rate=number of gigabit ports×1.488Mpps+number of 100M ports*0.1488Mpps+number of other types of ports*corresponding calculation method.

5. The origin of the parameter 1.488Mpps

So, how did you get 1.488Mpps?

The measurement standard of packet forwarding wire speed is based on the number of 64-byte data packets (minimum packets) sent per unit time as the calculation basis. For Gigabit Ethernet, the calculation method is as follows: 1,000,000,000bps/8bit/(64+8+12)byte=1,488,095pps Note: When the Ethernet frame is 64byte, the 8byte frame header and Fixed overhead of 12byte frame gap.

Therefore, a wire-speed Gigabit Ethernet port has a packet forwarding rate of 1.488Mpps when forwarding 64byte packets.

The wire-speed port packet forwarding rate of Fast Ethernet is exactly one-tenth of that of Gigabit Ethernet, which is 148.8kpps. For 10 Gigabit Ethernet, the packet forwarding rate of a wire-speed port is 14.88Mpps.

For Gigabit Ethernet, the packet forwarding rate of a wire-speed port is 1.488Mpps.

For Fast Ethernet, the packet forwarding rate of a wire-speed port is 0.1488Mpps.

For Ethernet, the packet forwarding rate of a wire-speed port is 0.01488Mpps.

6. Packet forwarding rate

The packet forwarding rate indicates the ability of the switch to forward data packets. It refers to how many data packets the switch can forward per second (Mpps), that is, the number of data packets that the switch can forward at the same time. The packet forwarding rate reflects the switching capability of the switch in units of data packets. Calculation method = number of gigabit ports × 1.488Mpps + number of 100M ports × 0.1488Mpps + number of other types of ports

24 100M ports, 2 Gigabit ports

2*1.488+24*0.1488≈6.6Mbps

7. Wire-speed exchange

What wire-speed exchange:

Wire-speed switching refers to the ability to achieve data exchange without bottlenecks in accordance with the data transmission speed on the network communication line. Its realization is based on the ASIC chip, which completes the protocol analysis and data packet forwarding through dedicated hardware, rather than through software based on the CPU of the switch. The realization of wire-speed switching also relies on distributed processing technology, and the data streams of multiple ports of the switch can be processed simultaneously. Therefore, the LAN switch can be regarded as a parallel processing device using CPU, RISC and ASIC.

Three, the difference between MB, Bps, bps, pps

The former is a bit, the latter is a byte

1byte=8bit

1MB=8Mb=8M

1MB=1024byte

Mbps where p is equivalent to "/", and can also be written as Mb/s (Megabits per second Mbps)

Generally speaking, the broadband of 100M can be expressed as 100Mbps (100Mb/s) or 12.5MB/s (12.5MBps)

12.5MB/s is the data we see through software download.

1. bps

The unit of broadband rate is expressed in bps (or b/s);

bps means bits per second, that is, how many bits of information are transmitted per second. In fact, 1M bandwidth means 1Mbps (It is megabits per second Mbps not megabytes per second MBps)

The line unit is bps, which means bit (bit)/second (second), note that it is a lowercase letter b; the rate unit displayed when users download on the Internet is often Byte (byte)/s (second), note that it is a capital letter B. The relationship between bytes and bits is 1Byte=8Bits

The theoretical rate of 2M (ie 2Mb/s) broadband is: 256KB/s (ie 2048Kb/s), and the actual rate is about 103-200kB/s

2. pps

The packet forwarding rate indicates the ability of the switch to forward data packets. The unit is generally pps (packets per second)

The packet forwarding rate refers to how many million data packets (Mpps) the switch can forward per second, that is, the number of data packets that the switch can forward at the same time.

Calculation method

The wire-speed packet forwarding rate of a Gigabit port is 1.4881Mpps, and the wire-speed packet forwarding rate of a 100M port is 0.14881Mpps

The specific data packet will be added with a 64-byte data packet in front of each packet during the transmission process, originally only 512 bits, but in the transmission process actually =512+64+96=672bit, Gigabit port line Fast packet forwarding rate=1000Mbps/672=1.488095Mpps

Multiply the pps value in the device parameter by 672=bps

6.6Mpps*672=4435.2Mbps