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## Switching Forward Rate Calculation Tool in PPS

is there is a tool can calculate the forwarding rate of any switch in PPS based on 64 Byte or any items.

5 REPLIES
Hall of Fame Super Silver

## Re: Switching Forward Rate Calculation Tool in PPS

Hello Shaymaa,

you can use the following formula

L2 frame size [B] = IPsize + 14 (header) + 4 (FCS)

you need to take in account the interframe GAP that is a silence interval between two frames that is equivalent to 20 -20,1 bytes on the line.

so to find the pps for a given rate on the wire

R bps

pps = R /[(ipsize+18+20)*8]

where 8 is the conversion between B bytes and b bits.

you can use the formula in an spreadsheet to build your own tool.

Hope to help

Giuseppe

New Member

## it is not that clear. Could

it is not that clear. Could you explain it in little low level more, please

Hall of Fame Super Silver

## Hello Mcepedar3,

Hello Mcepedar3,

to calculate the wire speed in packet per second or more correctly in frames per second over a Gigabit ethernet link or other type of ethernet technologies we need to take in account some facts:

on the wire we can send ethernet frames, but they need to be distinguishible in other words a receiver has to be able to detect the end of each ethernet frame. This is achieved by the presence of the inter frame gap that as I have written in my previous is a time interval equivalent to the time to send 20 - 20,1 Byte on the wire.

Following this consideration when we want to calculate how many ethernet frames of minimum size 64 bytes we can send on a 1Gbps link we need to perform the following calculation:

R = 1000000000 bps

the L1 effective size of the frame becomes 64+

20,0 Byte

and the conversion factor from bytes to bits is 8.

so the possible line rate is given by:

pps = 1000000000 / ( 84,0 * 8) =

1488095 frames per seconds

I hope this example is more clear.

Hope to help

Giuseppe

Super Bronze

## Re: Switching Forward Rate Calculation Tool in PPS

I haven't seen a tool, but if you're referencing Ethenet, for 64 bytes, the conversion is 1.488095 mpps for 1 gbps to support "wire speed". Don't forget the need to double for duplex, if necessary, and you can scale for 10/100 Ethernet.

PS:

As an example, to support "wire speed" on 48 gig ports for 64 bytes packets: 1.488095 Mpps * 48 ports = 71.42856 Mpps (NB: the Cisco 4948 switch is spec'ed at 72 Mpps, while 3560G-48 is spec'ed at 38.7 Mpps).

BTW, "wire speed" pps requirements drop, much, as packet size increases. For instance, gig with 1518 frames only requires about 81 Kpps.

Many switch vendors often used to document pps for a larger frame size, to make it appear it could support "wire speed". Which it could for the larger frames, although not for smaller. Although misleading, normally switches don't need to contend with every port running "wire speed" with minimum size packets.

PPS:

With switches, don't forget to also check what the fabric bandwidth is. "Wire speed" backplane bandwidth is often spec'ed at 2x the ports' bandwidth. E.g. 48 gig ports would need 96 Gbps (NB: the Cisco 4948 switch is spec'ed at 96 Gbps, while 3560G-48 is spec'ed at 32 Gbps).

New Member