Sizing a switch

[nsmwella]

Hi,

With regarding sizing the switch what is the impact of below points.

01) Switching capacity

02) Forwarding performance

03) Throughput

As per my research,most of the datasheets of switches are mentioning switch capacity and the forwarding performance not the throughput. 

So my concern is how we size the switch (apart from other points like power redundancy,stacking features etc ) using above points ?

Thanks

[marce1000]

  - You can also have a look at https://www.cisco.com/c/en/us/products/switches/switch-selector.html#~select-switch
     It starts from another viewpoint indeed , but presumably models offered when making a selection
    will incorporate performance needs ,

  M.

[Joseph W. Doherty]

Your points 1 (bandwidth) and 2 (PPS) are what determine point 3.

Point 3 is often the only metric often chosen for sizing a switch, but other points of consideration exist which can be crucial for selecting the "right" switch.

Unfortunately (?) most engineers don't understand many of those considerations, but fortunately, in many cases you don't need to.

I'm not going to try to explain those other considerations, but they explain some of the huge price deltas on different switch models or even line cards that "appear" identical.

If you're trying to learn the distinctions, understand there's a lot to learn, and these forums might be able to answer specific questions, but I don't think they will be able to well educate you on this subject.

[nsmwella]

Hi @Joseph W. Doherty 

Much appreciate your opinion on this subject.

But although the throughput is often the only metric often chosen for sizing a switch in the data sheets I could only see the switching capacity and the forwarding rate information. So how can we calculate the throughput using the given values? As per my understanding although the bandwidth much higher if the throughput is more less that will totally effect to the device performance even if it is a switch,router or firewall 

Thanks

[Joseph W. Doherty]

"So how can we calculate the throughput using the given values?"

Ah, that I might be able to explain.  I have an appointment I need leave for, in a few minutes, but I'll address that in a few hours from now.

[Joseph W. Doherty]

Throughput (bandwidth usage) is PPS times the size of packets.

On switches, they normally provide the "fabric" bandwidth and an aggregate overall PPS rate.

If you had a switch with 48 full duplex gig ports, if would need a "fabric" bandwidth of 96 Gbps (i.e. twice the the sum of the port bandwidths).

For its PPS rating, to drive all ports at max, its PPS depends on the packet sizes.  PPS rate, for the same bandwidth, goes up as packet size decreases.  When you're given a PPS spec, it's very, very important you're also give the packet sizes.  Historically the PPS spec was for 1500 byte packet (for gig, about 83 Kpps needed) but in most cases, average packets sizes are smaller.  To avoid performance issues with smaller packets, sometimes the worse case, 64 byte Ethernet packets (for gig, about 1.5 Mpps needed [notable increase]) spec is provided.  I.e. the latter more-or-less guarantees sufficient PPS for any larger packets, except when you use additional logic when processing (ACLs, QoS, etc.) a packet beyond just forwarding it.  This often reduces the PPS rate, so much, that vendors often provide IMIX performance specs using difference services.  Generally, switches aren't impacted by contents of packets, so if the 64 byte packet size PPS is provided, that's good enough.  (For 48 gig ports, you would need, about, 72 Mpps, i.e. 1.488 Mpps, for each gig port.)

BTW, also, years ago, many Enterprise switches weren't all ports, concurrently, wire-speed capable.  Truthfully, business usage seldom needed that level of performance, but the "market" came to demand it, and so it was met.

A 48 gig switch, with a fabric of 96 Gbps, allowing for 64 byte packets, would have a PPS need of 72 Mpps.  This would be considered a "wire-speed" switch.  Basically, you can run all 48 ports at full speed, w/o issues (as long as you don't oversubscribe any path, like two hosts sending to one host, concurrently - the latter is where other switch factors come into play).

Today, many switches, Enterprise class, are capable of wire-speed between all their edge ports.  Again, how well a switch deals with congestion, or what else it supports (like cut through), is a whole different matter, and varies greatly.

You recent reply mentions routers and FWs.  Although they too may have "throughput" ratings, they are often much, much more impacted by packet sizes, and what processing logic is being applied to transit packets.

I've attached an older Cisco white paper discussing throughput for various routers for different traffic mixes and processing logic.  What's really important is taking notice at the huge throughput differences.  Take notice of the maximum throughput performance at the beginning of the document and the final bandwidth usage recommendations at the end of the document!

Again, switches usually wouldn't have such performance differences for the tests in that document, but they have their own "gotchas".

@marce1000 recommended Cisco's switch selector.  It will, most likely, suggest a switch that will meet your performance demands.  Often, though, it accomplishes that by oversizing the recommended device rather than recommending one that would be "just right".  Although, as the equipment seller, Cisco has a conflict of interest, i.e. they benefit from selling a oversized device, it takes a deep understanding of both switch hardware and your real needs, to "right size" a recommendation.  Further, they don't want to recommend something that's actually undersized, especially easy to walk into that as network bandwidth demand continues to grow.

Also, even when you have the expertise, doing an in depth analysis, to "right size" a recommendation, may cost more, possibly much, much more, than just going ahead and paying the additional cost for a more expensive oversized switch.

[nsmwella]

@Joseph W. Doherty , 

Got your point so far. 

' Today, many switches, Enterprise class, are capable of wire-speed between all their edge ports.  Again, how well a switch deals with congestion, or what else it supports (like cut through), is a whole different matter, and varies greatly.'

Still my concern is this. Bandwidth and the PPS all are theoretical values right ?. Value for one port multiplying by the number of ports . But the actual capacity ( generally called throughput ?) depends on the switch CPU, RAM etc performance. If the customer requirement is ' i need a switch which support this amount of throughput ' how can I address that ? 

'Again, how well a switch deals with congestion, or what else it supports (like cut through), is a whole different matter, and varies greatly.' according to this we can not say the actual performance is the bandwidth and PPS.

I am asking this subject to correction as per my understanding. 

Thanks

[Joseph W. Doherty]

"Bandwidth and and the PPS all are theoretical values right ?."

No, incorrect, they are very real.

"But the actual capacity ( generally called throughput ?) depends on the switch CPU, RAM etc performance."

Not exactly, much depends on "performance" doing what.

"If the customer requirement is ' i need a switch which support this amount of throughput ' how can I address that ?"

Depends on whether you just want to meet the requirements of what the customer asked for, which may, or may not, actually satisfy the customer in deployed usage.  (You do have a great defense, it does meet your specifications.  It's not my fault you failed to mention other needs.)

Or, you try to ferret out actual customer needs.  (Not always appreciated by all customers.)

"according to this we can not say the actual performance is the bandwidth and PPS."

Again, maybe yes, maybe no, depends on performance doing what.

Let me try an automobile analogy.

I want a car that seats 4 and can do 150 MPH.

Given those two performance requirements, both very real, either a particular automobile meets them or not.

So you recommend a specific automobile that does.  All's great, right?

Possibly, possibly not!

When you start to use this automobile, you find it has a very small gas tank, it obtains very poor MPG, seats are very uncomfortable, no leg room in back, its maximum payload is 500 pounds, it has no luggage space, it rides like a truck, it must take curves very slowly, etc.

Now if your only requirement is being able to keep up or even pass other traffic on Germany's Autobahn, and nothing else matters, you selected what you need.

In real life, other than for bragging, many other considerations likely are important for selecting an automobile, but for switches, often the focus in on speed and bandwidth capacity, both easy to measure, and both can be important, but, also in real world, other considerations may be as important if not more so.

For example, many years ago. common performance considerations, for switches, are they blocking and/or subject to head-of-line blocking?  Are you aware of these?  Their potential impact to performance?  (Unlikely, such would be an issue on any current gen Enterprise class switch, but they may still be on very low cost switches.)

With automobiles, we can run into similar performance considerations with tires.  How many attributes do they have?  Tires have speed ratings, wear ratings, traction ratings, load ratings, yet all those don't cover all driving conditions.

With chassis/modulay switches, there are different performance attributes with the chassis, supervisors, line cards, and possibly with service modules.

Again, bandwidth capacity and PPS rates are real, and can have a huge performance impact, but so can many other variables.

If you wade through these forums, you'll find posts about network devices not meeting their expectations, but it has x number of fast ports, what's wrong with it?  Often the answer is it's not the "right" device for how it's being used.

Are you starting to see selecting the optimal switch isn't easy?  So, also again, oversizing is commonly done, in an attempt to avoid performance issues.  Unfortunately, it doesn't always succeed.

Lastly, although oversizing adds to cost, it may actually be less expensive than doing a detailed analysis.

[balaji.bandi]

Many parameters need to be considered, and different models of switches need to be compared to meet the requirements.

Instead of all these hassles, contact a Cisco partner to provide requirements; they have sales tools to project the model you need to buy. Then, verify that the model meets the requirements to place an order.

[sdroy]

Sizing a switch, prioritize switching capacity to ensure the device can handle simultaneous high-bandwidth traffic across all ports, especially in data-intensive environments. Assess forwarding performance (measured in packets per second) to confirm its ability to process packets efficiently, which is crucial for latency-sensitive applications like VoIP or IoT. Since throughput often isn’t directly listed in datasheets, calculate it by considering port speeds and forwarding performance to understand the switch's real-world data handling capability. Finally, balance these metrics with considerations like power redundancy, stacking options, and scalability to select a switch that meets both current and future network demands effectively.