Switching capacity versus Forwarding performance on a switch

[sarahr202]

Hi every one,

 Please consider the following example:

Cisco Catalyst 3850-12S-S 12

https://www.cdw.com/product/Cisco-Catalyst-3850-12S-S-12-Port-Fast-Ethernet-Switch/3378941#PO

• Performance

  • Type : Forwarding performance
  • Value : 68.4 Mpps
  • Type : Stacking bandwidth
  • Value : 480 Gbps
  • Type : Switching capacity
  • Value : 68 Gbps

1)Above what should we go by  Switching capacity or forwarding performance of 68.4 Mpps when deciding if the switch can support /provide desired forwarding rate?

Forwarding performance:

What  packet size is considered in computing this value of 68.4 Mpps?  For example is it Jumbo  frame or 64 packet size ?

For example, If we have to look for a switch that can support  40 GIG forwarding rate, and if we look at 68Mpps only , we can arrive at two different conclusions:

Packet size :64byte

68.4x64 Mbyte/sec or around 35g/sec

Switch can not support our target rate.

Packet size: 9000 byte

68.4x9000 M byte/sec or 615 g/sec

Switch exceeds our target rate.

Appreciated !!

Have a nice weekend!!

[Reza Sharifi · Accepted solution]

Hi Sarah,

It appears that they have used a 1000 byte frame as the baseline to measure the total capacity of this particular switch.  So, 68,000,0000*1000=68,0000,0000,000 which is equal to 68 Gbps.

If you use a 64-byte frame as the baseline with about 26 bytes of overhead for a total of 90 bytes, that number is much lower for a 12 port 1Gig fiber switch full duplex (1Gig in and 1Gig out).

Thanks,

Reza

[Joseph W. Doherty · Accepted solution]

"1)Above what should we go by Switching capacity or forwarding performance of 68.4 Mpps when deciding if the switch can support /provide desired forwarding rate?"

You need to reference both.

"What packet size is considered in computing this value of 68.4 Mpps? For example is it Jumbo frame or 64 packet size ?"

Vendor needs to note which. (NB: Generally, today, vendors quote PPS for minimum sized Ethernet packets - but do keep in mind PPS requirements vary based on packet size and medium.)