How Switch Bandwidth are consumed?

[kharyptho84]

Hello All,

1-I have a question that always confuse me , if we have 24G Port switch , each port is configured as a full duplex at speed of 1000Mbps , will the laptop connected to that port will actually send by this speed ? how the switch handles all of this traffic and send it through the 1G UP link without drops ?

2-What does the Switch throughput means ? for example , Switch X Throughput is 400GB , does it mean that the switch can handle 400GB both inbound and outbound at the same time out of all it's available ports?

[balaji.bandi]

it all depends on model (architecture of the switch)  they mention clearly switch does  blocking or non-blocking.

non-blocking means the backplane can process traffic line rate tx/rx(in your case 1GB port) on each port including uplinks

[Seb Rupik]

Hi there,

1 Gbps is the maximum bandwidth of the switchport in one direction, the actual throughput will be less due to several factors such as protocols, latency and congestion. Your first question relates to the subject of over subscription. This has two parts, oversubscription of the switch fabric (which you ask about in your second question) and oversubscription of the uplink ports. Oversubscription of either can lead to congestion on the switch and can be mitigated by using QoS.

Switch throughput is the measure of the switch fabric ability to forward traffic. Keep in mind a switchport is typically operating at full duplex, so a 1GB switchport could consume up to 2 Gbps of that switch throughput.

cheers,

Seb.

[Joseph W. Doherty]

#1 The bits on the wire, are physically sent/received at 1 Gbps, for a gig port.  For a full duplex port, bits on the wire can be both sent and received at the full rate, concurrently.

The effective data transmission rate is lower for reasons noted by Seb.  Further, a host like your laptop, if data is being sent from a disk drive or an application, either might not send at the full possible rate for reasons such as host is "busy" doing multiple things or the hardware (like a disk drive) cannot support a continuous full speed transmission rate.

As to how the switch handles 24 gig ports sending to a single gig uplink port, the port will queue traffic that it cannot immediately send.  Unfortunately, there's only a finite amount of queue resources, and when exceeded, traffic is discarded.  Fortunately, not all other ports usually send a long continuous full data stream.  As, and when, they don't, the uplink is able to dequeue excess traffic, and transmit it, before it's discarded.

#2 A bandwidth number often, but not always, is the switch's fabric bandwidth capacity.  Ideally, the fabric's bandwidth capacity can handle all the switch's port, at full rate, concurrently.

If the switch had just (i.e. no uplink ports too), 24 gig ports, if all were running at full duplex, the switch's maximum needed bandwidth capacity would be (according to Cisco) would be 48 Gbps.  This because Cisco counts all duplex ports as twice their port bandwidth, reflecting ports' in and out bandwidths.

Hand-in-hand with fabric capacity, the switch needs a packets per second (PPS) capacity to forward traffic transiting the switch.  The PPS rate, though is only counted "once" as one port's in is another port's out.  So for 24 gig ports you need a PPS rate for 24 Gbps.

PPS rate, though, varies, for the same bandwidth consumption, based on frame size.  The PPS rate increases as frame size decreases.  (NB: you need [about] 1.488 Mpps per Gbps for minimum size [64b] Ethernet frames/packets.)

In "ye-olde-days", switch fabrics, and PPS rates, could not support all their ports at line-rate/wire-speed, concurrently.  (NB: not generally really needed then, or now.)  Many modern Enterprise switches can support all their ports, concurrently, at full rate.

There's other factors that can determine what one switch makes it "better" than another, especially in certain use cases.  (Also why one 24 gig port switch costs more, possible a lot more, than another 24 gig port switch.)