Router vs. Layer-3-Switch - Routing Performance of Cisco 3850 Series

[Kevin_W]

Hello,

Maybe you can help me to compare the routing-performance between some Cisco router and layer-3-switches.
I have the following example:

- Cisco 3850 (WS-C3850-24T-S) vs. Cisco 1811 ISR

In the data sheets you can normally see no values for routing-performance in mbbs (mega bits in seconds). Often I can only find "Forwarding Rate in Mpps" (million packets per second). But are these two values comparable? 
The Cisco 3850 (WS-C3850-24T-S) has a forwarding rate about 68,4 mpps and the Cisco 1811 has 70 mpps. Does it mean, that the 1811 has also a higher layer-3-performance?

Maybe you can explain me how to compare those devices generally or with these example models.


Thank you in advance


Best regards

[Mark Malone]

Hi just remember no NAT on 3850 , limited BGP and PBR support for these switches. There very good switch and capable of routing but if you want a router style device capable of all functions and not limited get an ISR especially if its wan facing , 38s would be fine for doing layer 3 in and around your network depending on your design , just my opinion anyway , I have 38s and isrs running

[Kevin_W]

Thanks for your fast answers.

Yes I know that the 1811 router has much more layer 3 features like NAT oder BGP.

In this case I just wanted to compare the performance. The 3850 in this case only needs to do inter-vlan-routing and maybe HSRP. 

But to sum it up:
Is the "forwarding rate" in the data sheets the same as "routing performance"?
E.g. 68,4 million packets per second with 64 byte packets would be round about 33 Gbit/s. Or am I wrong?

Thanks in advance

[Joseph W. Doherty]

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 wha2tsoever (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

Minimum size Ethernet needs (about) 1.488 Mpps per Gbps, so 68.4 Mpps / 1.488 = (about) 46 Gbps.  (NB: that's unidirectional, don't forget duplex.)

BTW, PPS requirements drop as packet size increases.

Also BTW, Jon wrote:

Hardware devices will always have much larger forwarding rates than software devices.

That's almost always true, and should be true, but I did want to add there are some (fortunately extremely unlikely) pathological conditions, where a "slow" ISR can out perform a L3 switch.  If for some reason route forwarding is processed by a switch's CPU, rather than its dedicated hardware, that CPU is often slower than many ISRs CPUs (because it wasn't really intended to be used for packet forwarding).

[Kevin_W]

So to make it clear in a "real situation":

- Client A (192.168.1.10 /24)
- Server B (192.168.2.20 /24)
> Client A wants to transfer data to Server B and the default gateway of both devices is a Cisco 3850. Both devices are connected via one 1000 Mbit/s interfaces to the switch. No other devices are connected to the layer-3-switch.

The maximum available bandwith between the both devices would be theoretical 46/23 Gbps? For sure the devices could only use 1 Gbps, because their interfaces are only 1 Gbps.

Is that right?

Thanks in advance

[Joseph W. Doherty]

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 wha2tsoever (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

For sure the devices could only use 1 Gbps, because their interfaces are only 1 Gbps.

Correct.

The maximum available bandwith between the both devices would be theoretical 46/23 Gbps?

Correct for minimum size packets and if just configured for packet forwarding.  Again, as packet size increases, PPS requirement decrease, so unless all traffic is using minimal size packets, possible maximum Gbps rate might be much higher.  (I can tell you what the PPS rate is needed for different packet sizes, but without testing the platform, or without vendor documenting performance for different packet sizes, we cannot say what the maximum actually is.)

Also, again, additional configuration options may reduce performance, but many configuration options are supported in L3 switch hardware, so while an 1811's performance is likely to degrade as you add "services" (e.g. ACLs), the 3850's might not.

[Jon Marshall]

The 1811 has 70,000 packets per second not million packets per second.

The 3850 you are quoting has 68.4 million packets per second.

There is a big difference between the two and this is because the 1811 is a software based router and the 3850 is a hardware based switch.

Hardware devices will always have much larger forwarding rates than software devices.

So in terms of performance there really is no comparison between the two but L3 switches do not support all the features routers do so it is not always a choice simply between forwarding rates.

Jon