Forwarding performance and switching capacity

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Forwarding performance and switching capacity are two common parameters used to measure network devices. This post describes how to calculate the two parameters and evaluate whether the two parameters of a device meet the requirements.

1. Forwarding performance

The forwarding performance(packet forwarding rate) of a switch refers to the capability of the switch to forwarding packets, in PPS (Packet per second), that is, the number of packets that can be forwarded by the switch per second.

Because of the Ethernet conflict detection mechanism, the data frame size is limited when the Ethernet transmits data frames. The minimum data frame is 64byte, plus the preamble bytes of 8byte and the interframe gap of 12byte, the total is 84byte. That is, the minimum data frame transmitted on the Ethernet is 84byte.

Take the 100 Mbit/s Ethernet interface as an example. Every eight bits form a byte. Therefore, the Ethernet interface rate of 100 Mbit/s =12.5Mbyte/s, that is, the Ethernet interface can forward 12.5M bytes =12500000byte per second. Assuming that all data frames transmitted in the worst case are the smallest 84byte, the data frame forwarded by the 100 Mbit/s Ethernet port per second is 12500000/84=148809pps (frame/second) =148.8kpps=0.1488Mpps.

Therefore, it can be concluded that the packet forwarding rate of the 100 Mbit/s Ethernet interface is 0.1488Mpps, then the packet forwarding rate of the Gigabit Ethernet interface is 1.488Mpps, and the packet forwarding rate of the 10G Ethernet interface is 14.88Mpps.

For example, if there is a 24-port 10/100Base-TX Ethernet switch, the packet forwarding rate of the switch is 24*0.1488Mpps=3.5712Mpps, plus four GE ports 4*1.488Mpps=5.952Mpps. So the total is 3.5712Mpps+5.952Mpps=9.5232Mpps. That is, a 24-port 100 Mbit/s +4 gigabit Ethernet switch can implement line-rate forwarding only when the packet forwarding rate of the entire device reaches 9.5232Mpss.

2. Switching capacity

The switching capacity (backplane bandwidth) of a switch refers to the maximum amount of data that can be transmitted between a switch interface processor or interface card and a data bus. The switching capacity indicates the total data exchange capability of the switch, in bps.

The switching capacity of the switch = Number of ports * Rate of the port *2 (full-duplex).  

For example, the switching capacity of a 24-port 100M switch is =24*100*2=4.8Gbps.

Switch capacity =Forwarding performance *8* (64+8+12)*2 =Forwarding performance*1344 .

Therefore, you need to evaluate the performance of a switch based on the forwarding performance and switching capacity, but not only the interface rate and the number of interfaces.

A 24-port 100 Mbit/s switch must has a forwarding performance of 3.5712Mpps and a switching capacity of 4.8 Gbps. If the two parameters cannot reach this value, the performance of the switch does not meet the requirement.

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