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"Switching Bandwidth" and "Forwarding Rate"

All, I have tried to make good sense of the terms "Switching Bandwidth" and "Forwarding Rate" in the Cisco documentation.  Honestly, I dont seem to be able to grasp a good understanding of what these are talking about.  So to me, its difficult to just go on the 'numbers' when comparing switches.  So Im asking you guys for a "laymans" definition of these two and what they mean.  Any help would be appreciated.

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Participant

"Switching Bandwidth" and "Forwarding Rate"

Also, are the terms "Switching bandwidth" and the switches backplane the same thing? 

VIP Expert

Re: "Switching Bandwidth" and "Forwarding Rate"

Disclaimer

The  Author of this posting offers the information contained within this  posting without consideration and with the reader's understanding that  there's no implied or expressed suitability or fitness for any purpose.  Information provided is for informational purposes only and should not  be construed as rendering professional advice of any kind. Usage of this  posting's information is solely at reader's own risk.

Liability Disclaimer

In  no event shall Author be liable for any damages whatsoever (including,  without limitation, damages for loss of use, data or profit) arising out  of the use or inability to use the posting's information even if Author  has been advised of the possibility of such damage.

Posting

jjoseph01 wrote:

Also, are the terms "Switching bandwidth" and the switches backplane the same thing? 

They are often used interchangeably, although both have shades of meaning that overlap and they have shades of meaning that don't too.

As to a "laymens" explanation, think of many traffic lanes (for network links) leading up to (river) bridge (for network switch [or multi-port bridge]).  How many vehicles (packets) you can move across the bridge depends on how fast each vehicle can pass through the toll process (a device's packets per second, PPS, rate) and how many lanes (bandwidth) the bridge has.  (If you're wondering about "speed" in the sense of MPH, for signal transfer across medium, there's only one "speed" for that medium.)

For Ethernet switches, full "speed" requires 1,488 Kpps, for minimum (64 byte) packts, or 81.3 Kpps, for maximum (1500) packets, and 2 gig of internal bandwidth per duplex gig port.

Participant

"Switching Bandwidth" and "Forwarding Rate"

So, Im sure I dont completely  understand.  dosztal, when you say its the actual throughput, do you  mean that is the throughput on the backplane?  I take it from  JosephDoherty that the switching bandwidth is at least partially the  switche's backplane. 

JosephDoherty, I take it that the "bridge"  in your road example (which I like very much by the way) is the switches  backplane.  This example makes sense to me.  So, my question is when  Cisco refers to the switching bandwidth, are THEY referring to the  backplane of the switch?  I see for a 3750-X that the switching  bandwidth is 160 Gbps.  That is on a 24 port switch, which seems well  above the, what I think would be 96G needed for full capacity for all  ports?  Which I would deem as a good thing.

So, do I understand "switching bandwidth", as you guys see it?

Also,  for the "forwarding rate", I think that makes sense.  Its NOT talking  about the backplane, but the quickness of how many packets it can switch  in a second?

Thanks and sorry for the delay in responding.

VIP Expert

Re: "Switching Bandwidth" and "Forwarding Rate"

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

Yes, the 3560/3750 -X switches are noted as ALL having 160 Gbps switching fabric.  This isn't uncommon in a series, as there's just one internal hardware resource all models use; i.e. one common "bridge" design regardless of the number traffic lanes leading to it.

Also yes, 160 Gbps is more than the 24 port versions need to support all their gig ports, but not by as much as you might think at first glance.  Don't forget these models support optional uplink modules, which supports up to two 10g ports.  Lastly, in the 3750-X, they have stack ring ports, two 16 Gbps.  So we have 24 + 20 + 32 = 76 then we double for duplex giving 152 Gbps.  The internal fabic has the capacity to support all the ports at full capacity for the 24 port versions, but not the 48 ports versions.  (Note: I'm not 100% sure Cisco intended the stack ports should be "counted" against the overall switch fabric; they should be.  It's possible there's additional fabric bandwidth designed just for them.)

Whether this is a "good thing", depends on your needs.  Most actual production usage doesn't run all ports at full capacity most of the time.  If your actual bandidth demand is much lower than the capacity of the device, you often are paying extra for something you don't use nor may ever use.

Regarding "forwarding rate", it's the count of how many packets you can examine, per second, as they transit your switch.  Again, my "toll gate" analogy.

For 24 gig and 2 10g, worst case (minimum size packets - if vehicles were motorcycles) Ethernet requires 24 + 20 = 44 * 1.488 = 65.472 Mpps.  Cisco notes Mpps for their 24 port model, with dual 10g uplinks, as 65.5 Mpps.  So, switch appears to support wire rate.  (Note: appears - they didn't account for the stack ring ports.)

Once again, if want a bridge to allow vehicles to cross it without delay, it must have enough lanes to support all the lanes leading up to it (bandwidth) and a tolling system (forwarding rate, PPS) to deal with the number of vehicles (packets) crossing it as they arrive.  Many times actual switches (or real bridges) can't sustain full capacity but that's fine when there's less loading (like crossing a bridge at 2 AM vs. 5 PM).

Participant

"Switching Bandwidth" and "Forwarding Rate"

Forwarding rate is the amount of packets the device can switch. Switching bandwidth is the actual throughput. Here's an example to make it easier to understand:

If the forwarding rate is 1000 pps (packets/s), and you're sending 1 kB packets, then the switching bandwidth is ~8 Mbps (1000 x 1 KB x 8). When sending 100 kB packets, the throughput is 800 Mbps.

Low-end GbE switches may have 1 Gbps/port switching bandwidth only if you send large packets (1500 kB). If you're sending small packets (like voice traffic), the bandwidth will be much slower because of the poor forwarding rate.

Participant

"Switching Bandwidth" and "Forwarding Rate"

So, Im sure I dont completely understand.  dosztal, when you say its the actual throughput, do you mean that is the throughput on the backplane?  I take it from JosephDoherty that the switching bandwidth is at least partially the switche's backplane. 

JosephDoherty, I take it that the "bridge" in your road example (which I like very much by the way) is the switches backplane.  This example makes sense to me.  So, my question is when Cisco refers to the switching bandwidth, are THEY referring to the backplane of the switch?  I see for a 3750-X that the switching bandwidth is 160 Gbps.  That is on a 24 port switch, which seems well above the, what I think would be 96G needed for full capacity for all ports?  Which I would deem as a good thing.

So, do I understand "switching bandwidth", as you guys see it?

Also, for the "forwarding rate", I think that makes sense.  Its NOT talking about the backplane, but the quickness of how many packets it can switch in a second?

Thanks and sorry for the delay in responding.

Participant

Re: "Switching Bandwidth" and "Forwarding Rate"

So, my question is when Cisco refers to the switching bandwidth, are THEY referring to the backplane of the switch?

So, do I understand "switching bandwidth", as you guys see it?

Yes, the datasheet refers to the backplane.

I see for a 3750-X that the switching bandwidth is 160 Gbps.  That is on a 24 port switch, which seems well above the, what I think would be 96G needed for full capacity for all ports?  Which I would deem as a good thing. 

The 3750-X also has a StackWise port (64 Gbps), that's why its switching bandwidth is 160 Gbps.

Also, for the "forwarding rate", I think that makes sense.  Its NOT talking about the backplane, but the quickness of how many packets it can switch in a second?

Let's look at another example, using the previously mentioned 24 port 3750-X. It's bandwidth is 160 Gbps, forwarding rate is 65.5 Mpps. It means that the smallest packet size can be ~320 B (160 Gbps / 65.5 Mpps) to utilize the maximum available bandwidth.

If the forwarding rate is only 20 Mpps, the smallest packet size would be ~1 KB (160 Gbps / 20 Mpps). If you send smaller packets, you cannot utilize the 160 Gbps bandwidth. With 512 B packets, the bandwidth would be 80 Gbps, even if the switching bandwidth in 160 Gbps.

VIP Expert

Re: "Switching Bandwidth" and "Forwarding Rate"

Disclaimer

The Author of this posting offers the information contained within this posting without consideration and with the reader's understanding that there's no implied or expressed suitability or fitness for any purpose. Information provided is for informational purposes only and should not be construed as rendering professional advice of any kind. Usage of this posting's information is solely at reader's own risk.

Liability Disclaimer

In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

JosephDoherty, I take it that the "bridge" in your road example (which I like very much by the way) is the switches backplane.  This example makes sense to me.  So, my question is when Cisco refers to the switching bandwidth, are THEY referring to the backplane of the switch?  I see for a 3750-X that the switching bandwidth is 160 Gbps.  That is on a 24 port switch, which seems well above the, what I think would be 96G needed for full capacity for all ports?  Which I would deem as a good thing. 

So, do I understand "switching bandwidth", as you guys see it?

On the 3750-X datasheet, we see:

Table 9. Cisco Catalyst 3750-X and 3560-X Performance Specifications

Performance Numbers for All Switch Models

Switching Fabric

160 Gbps

This, as described, is the bandwidth capacity all these switch series internal switching fabric, i.e. the bandwidth capacity between switch's ports.  BTW, backplane might refer to the switch's internal bandwidth capacity, but it has other meanings too.  I would avoid using it in this context.

For 24 (duplex) gig ports you need 48 Gbps.  However, these switches often support an optional module that can support two 10g (duplex) ports, so then your bandwidth requirement is 88 Gbps ((24 + 20 [dual 10g]) * 2 [duplex]).  (For the 48 gig ports, you would need 136 Gbps.)

In Andras's post, he mentions bandwidth of the ring ports (note: this would be only for the 3750-X).  I'm unsure Cisco "counts" them against the fabric capacity, as Andras supposes, but assuming they do, each port is 16 (duplex) gig.  So a 24 port 3750-X would need (24 + 20 + 32 [dual 16g]) * 2 = 152 Gbps, while the 48 port 3750-X would need (48 + 20 + 32) * 2 = 200 Gbps.

The described 160 Gbps is more than enough for any 3560-X or any 3750-X not stacked.  However, if we do need to count StackWisePlus port bandwidths against the fabric, the 48 port 3750-X has insufficient bandwidth to support full concurrent port utilization.

Again, Cisco isn't clear about whether StackWisePlus capacity should be counted against the provided fabric bandwidth, but if we look at the original 3750 series, its switching fabric bandwidth is described as 32 Gbps.  Considering StackWise bandwidth is also described as 32 Gbps, I suspect this bandwidth was not intended to "count" again the switch's fabric bandwidth.  As the original StackWise is described as placing all switch traffic on the ring, I suspect this bandwidth is in "parallel" to the fabric's bandwidth.

When the total fabric capacity exceeds port capacity, there's no benefit to performance as it will remain unused.

As to why all the 3560-X/3750-X series have the same fabric bandwidth, likely it's a one-size for all design.  Different models probably just "connect" to the same basic fabric chip set.  (Somewhat similar to old 6500 SFMs [switch fabric modules].  I.e. they provided a fabric that supported up to 256 Gbps.)

Regarding "forwarding rate", that's a measure of how many packets per second the switch can process for certain sized packets.  When packet's size isn't described, today it's usually denoted for minimum size Ethernet packets, i.e. each 64 bytes.

Minimum size Ethernet, to run at gig rate, is 1.488 Mpps.  Unlike fabric, you don't need to account for duplex as one port's in is another port's out.

So for 24 gig ports, and for the optional dual 10g ports, we need (24 + 20) * 1.488 = 65.472 Mpps to support full rate.

The spec sheet notes the 24 ports switch's Mpps as 65.5, which would be a rounded number of the 65.472 Mpps just calculated.

Notice how this would be fine for all the ports excluding the StackWisePlus ports.  (Again, why I believe they are not "counted" as part of these bandwidth/Mpps specs.)

Participant

Re:"Switching Bandwidth" and "Forwarding Rate"

Thsnk you. I think I have it now. Actually, thznk you both very much. I appreciate your help!

Sent from Cisco Technical Support Android App

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