Cisco 3650 Switching Capacity- How did they do it?

[een4hc007]

Hi all,

Apologies if this may sound a silly question but I am kinda stumped by the Switching capacity of a 3650 (datasheet description) for a 24 port model with (non-multigigabit models) to be at  92Gbps.

In an ideal world, Switching capacity should be (24 x 1G port + 4x 1G port)*2 = 56 Gbps at full duplex (going by the datasheet of non-multigigabit models).

This is nowhere near to 92Gbps as claimed in the datasheet.

Where am I going wrong here?

Thank you all in advance. Have a lovely day ahead.

Regards,

ee

[Mark Malone]

Hi you wont get line rate on a campus switch like that 1 to 1 speed for each port  , you would need a 5k data centre switch to get 1:1 wire speed like that as an example or a 4948 switch , there are ASICs behind the ports and the full set of ports usually broken down into sets behind the ports and then work of the back plane speed for oversubscription  , so you may end up with a ratio of 3:1 depending on the switch or line card etc

ASIC breakdown how to see it on campus switch

https://ciscointerworking.wordpress.com/2015/06/14/asic-redundancy/

Looking at this doc some of them have wire rate backplane on newer models and some are oversubscribed like 2:1 ratio

https://www.cisco.com/c/en/us/products/collateral/switches/catalyst-3560-e-series-switches/product_data_sheet0900aecd805bac22.html

[Joseph W. Doherty]

Cisco may be listing the actual fabric switching capacity which can be in excess of the ports installed for a particular model. (I believe Cisco often uses the same internal switch fabric for different switch models within the same series regardless of the provided ports. For example, a 24 port and 48 port 3650 may have the same internal fabric switching capacity.)

Of course, in cases where the internal switch capacity exceeds the provided ports, you'll never obtain or utilize the full capacity of the switch fabric.

[sbhadrav@cisco.com]

The calculation is the same to provide for "wire-speed/line-rate" performance, but actual performance numbers either need to be obtained from the vendor.
Switching capacity generally documents the interior bandwidth capacity between ports.  On Cisco switches, to avoid queuing any port to port traffic, for full duplex ports, you need twice the port bandwidth capacity.  So, 92 Gbps, supports 46 Gbps (duplex).

Forwarding rate is the capacity to forward frames.  Minimal size Ethernet requires 1.488 Mpps per gig.  68.4 Mpps (if rated for minimum size - most modern rates are for minimum size) would support about 46 Gbps (notice about the same as the switch's bandwidth capacity).

Stacking bandwidth is bandwidth supported within the whole stack or between a stack pair.  How vendors "count" this varies.  I believe Cisco totals the pair of stack ports, counting their duplex bandwidth.  I.e., I think each stack ports is equal to an Ethernet port of 120 Gbps (duplex)..