Forwarding performance and switching capacity are two standard parameters used to evaluate or measure an Ethernet device’s (switches, routers, firewalls) performance capabilities. This Article describes how to calculate the two parameters and evaluate whether the two parameters of a device meet the requirements.

The Network device has the capacity or the ability to switch/route as many packets as required to achieve wire-rate performance. This metric is called the ‘Packets per Second’ or PPS for short

IP protocol allows for variable payload sizes. The smaller the packet passing on the wire the more packets that need to be switched to achieve wire-rate performance while on contrary, larger packets will require less PPS throughput to achieve wire-rate.

 To calculate how many PPS are needed to achieve wire-speed we need only be concerned with small packet sizes since they will be the most tariff for the switch or Network Device. Naturally, we will assume that no collisions occur on the medium.

The minimum Ethernet frame length is 64 Byte, plus the preamble bytes of 8byte and the interframe gap of 12byte, the total is 84 Byte. That is, the minimum data frame transmitted on the Ethernet is 84byte.

The above calculation however does not take into consideration additional tags that might be available in an Ethernet frame such as VLAN tags or MPLS labels.

Converting Data Speed 100Mbps, 1Gbps, 10Gbps into Bytes per seconds.
8 Bit = 1 Byte
100Mbps = 100 x 10⁶ bit/Sec = 100 x 10⁶ x ⅛  Byte /sec = 12.5 M Byte per Sec
1Gbps = 1 x 10⁹ bit/Sec = 1000 x 10⁶ x ⅛  Byte /sec = 125 M Byte per Sec
10Gbps = 10 x 10⁹ bit/Sec = 10000 x 10⁶ x ⅛  Byte /sec = 1250 M Byte per Sec = 1.25 G Byte per Sec

Forwarding performance (Throughput PPS) and switching capacity (bps) are two common parameters used to measure network devices performance capabilities.

Forwarding Performance – Throughput (pps)

The forwarding performance (or Throughput Rate or packet forwarding rate) of a switch refers to the capability of the switch to forwarding packets, that is, the number of packets that can be forwarded by the switch/router/network devices per second.

The forwarding(or Throughput) rate is expressed in packets per second (pps).

Formula for Calculating Forwarding Rate
Forwarding Rate (Packet Per Second-pps ) = Speed in Byte / Frame Size in Byte

Then we can calculate how many packets per second need to be processed Assuming that all data frames transmitted in the worst case are the smallest 84 byte and if the port is to transmit.

at wire speed: 100Mbps = 12.5M Byte/Sec
Packet Per Second (pps) = (12.5 M Byte /sec) / (84 Byte) = 0.148 M Frame / Sec = 0.1488Mpps

at wire speed: 1Gbps = 125M Byte/Sec
Packet Per Second (pps) = (125 M Byte /sec) / (84 Byte) = 1.48 M Frame / Sec = 1.488Mpps

at wire speed: 10Gbps = 1250M Byte/Sec = 1.25 G Byte Per Sec
Packet Per Second (pps) = (1250 M Byte /sec) / (84 Byte) = 14.88 M Frame / Sec = 14.88Mpps

When the forwarding capabilities are greater than the sum of speeds of all ports, we call the backplane non-blocking

pps Calculation per Port

From the above table we see how many packets per second (pps) an Ethernet device must be able to handle per port in order to achieve wire speed. The above calculation however does not take into consideration additional tags that might be available in an Ethernet frame such as VLAN tags or MPLS labels. Here minimum 84 byte frame Consider.

Here, considering a minimum Frame size of 84 Byte and for a 1Gbps minimum of 1,488,095pps (1.488Mpps) and for a 10Gbps minimum of 1,488,095(14.88Mpps)

Forwarding Rate (pps) = number of 10Gbit/s ports x Forwarding rate of 10Gbps + number of 1 Gbit/s ports x Forwarding Rate of 1Gbps

=  4×14.88Mpps + 24×1.488Mpps
= (59.52 +  35.712)Mpps
= 95.23Mpps

That is, a 24-port 1Gbit/s and 4-port 10Gbit/s Ethernet switch can implement line-rate forwarding only when the packet forwarding rate of the entire device reaches 95.232Mpss.

Switching capacity

Switching capacity or Backplane Bandwidth or Switch fabric capacity or System Throughput (bps)

It 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. The higher the backplane bandwidth of a switch, the stronger the ability to process data, but also the higher the cost of design.

Backplane bandwidth is expressed in bits per second (bps or bit/s)

It is calculated as the sum of the speeds of all ports. The sum of speeds of all ports is counted twice full duplex, one for Transmit(Tx) direction and one for Receive(Rx) direction.

Switching Capacity Backplane Bandwidth (bps) = Number of Ports x port data rate x 2(Full-duplex)

A typical chassis has a physical layout that looks something like this

The backplane bandwidth marks the total data exchange capability of the switch also called the exchange bandwidth.

SPECIFICATIONS
	24G PoE+ 4Sfp+ Switch
I/O ports and slots
	24 RJ-45 autosensing 10/100/1000 ports (IEEE 802.3 Type 10BASE-T, IEEE 802.3u Type 100BASE-TX, IEEE 802.3ab Type 1000BASE-T); Duplex: 10BASE-T/100BASE-TX: half or full; 1000BASE-T: full only
	4 SFP+ 1/10GbE ports; PHY-less.

Solution:

Switching Capacity Backplane Bandwidth (bps) = Number of Ports * port data rate * 2(Full-duplex)

= (24x1Gbps + 4x10Gbps)x2 = 128Gbps

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 1Gbps and 4-port 10Gbps switch must have a forwarding performance of 95.23Mpps and a switching capacity of 128Gbps.

If the two parameters cannot reach this value, the performance of the switch does not meet the requirement.