Throughput & Switching Capacity relation

Routing & Switching Switching
1

Hi Rene,

I need some clarity on:

  1. What exactly throughput and capacity means of any switch (assume it has 24 1Gi and 2 10Gi ports) .
  2. I have referred this link: https://supportforums.cisco.com/t5/lan-switching-and-routing/throughput-mpps-switching-capacity-gbps-relation/td-p/2176262

Here I unable to understand the below text :

Your particular switch has 24 gig ports and two 10g ports, so for maximum performance, it needs 44 * 2 (for duplex) = 88 Gbps for its fabric.

Minimum size Ethernet requires 1.488 Mpps per Gbps, so 44 * 1.488 = 65.472 Mpps.

My question is :

Question 1: how the maximum performance it needs 44 * 2 (for duplex) = 88 Gbps for its fabric I am thinking it should be 44 Gbps for both half/full duplex --can you help me with this ?

Question 2: Minimum size Ethernet requires 1.488 Mpps per Gbps, so 44 * 1.488 = 65.472 Mpps". I did not understand this formula, what is 1.488 and mpps mean. Can you please explain this ?

I know minimum ethernet header should be 64 bytes .

Thank you !!
Regards,
Sameer

2

Hi Sameer,
I can attempt to answer your question, to the best of my knowledge we have throughput of the device which has dependent on the core I/O bandwidth of the ASIC.
It doesn’t necessarily mean that if a switch has x number of ports of type 1Gig and the throughput will be 2*xGig.
Forwarding capacity refers to the lower packet sizes (generally 128 byte) uni directional throughput.

you can refer the data sheet of the following, the device has 64*100Gig ports and the ASIC also capable of supporting 6.4T, Bidirectional throughput is 12.8Tbps, forwarding capacity will be 4.2Bpps.

I need to understand why Forwarding capacity convention is Bpps and where as Bidirectional throughput is Tbps

Regards
Vamsi

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3

Hi Sameer,

This sounds correct to me. Keep in mind that when you use full duplex, it means you can send and receive traffic at the same time. For example, with a gigabit interface, you can receive up to 1 Gbit of traffic and send 1 Gbit of traffic at the same time.

PPS stands for packets per second. Here’s how they came up with this number:

A gigabit interface sends 1000000000 bits/second. In bytes, that’s 1000000000 / 8 = 125000000 bytes/second.

For these calculations, they use a minimum Ethernet frame length of 64 bytes, an 8-byte frame header, and an 12-byte frame gap.

64 + 8 + 12 = 84

125000000 / 84 = 1488095

In other words, the switch needs to be able to forward 1488095 packets per second (1.488 million packets per second) to hit the gigabit speed.

Does this help?

Rene

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