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I am researching the throughput of my Cisco routers. For now I know that throughput depends from packet size and configured services like NAT or ACL filtering. My question is about packet size. For example throughput for Cisco 2911 router depending the packet size (64-1500 bytes) is from 180 to 4236 Mbps, which is very big span. (Cisco 2911 PPS is 353.000) My question is how can I know how big are packets in my network? From which factors depend the size of MTU? And size of packets itself? Is it the application that generates the data? Is it the operating system? Is it the NIC on my computer? Because it is directy in relationship with throughput on my router.

One more question. Does anyone have the latest comparsion of Cisco routers in Packets Per Second category?

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  • So, after all this informations, what is the criteria for determing the appropriate router? Is it the WAN link speed, type of traffic, functions enabled...is there some formula? When calculating the possible throughput, which pacet size should I take?
    – John
    Commented Jan 22, 2018 at 10:32
  • The question in your comment is very broad. You need to analyze or predict what the traffic actually is, and then you can look at the factors to determine the appropriate equipment. NetFlow, which is built into a lot of business-grade equipment, is a great tool for this. It can also be worth hiring a consultant to help with this. Unfortunately, the question is too broad for this site.
    – Ron Maupin ♦
    Commented Jan 22, 2018 at 15:59
  • Did any answer help you? If so, you should accept the answer so that the question doesn't keep popping up forever, looking for an answer. Alternatively, you could provide and accept your own answer.
    – Ron Maupin ♦
    Commented Apr 1, 2018 at 20:06

2 Answers 2

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When calculating the possible throughput, which pacet size should I take?

Short answer: search the internet for "IMIX" or "Internet Mix"

The basic issue is this: network devices process and forward packets irrespective of how big the packets are. A device can be software-based or ASIC-based or NPU-based, but in all cases the most accurate measure of the device's performance can only be expressed in terms of packets per second (pps).

Now, a pps number is pretty much useless for a network planner / designer. A designer needs answers for things like "how many laptops, wireless access points and IP phones can I attach to this device?", and a better picture emerges only when the number is expressed in K/M/G bits per second.

As you noticed, however, the bps number varies widely based on the packet size.

IMIX is an industry standard practice to resolve this "bps vs. pps" issue when comparing the performance of devices from different vendors. Given that, at the end of the day, any real-world device needs to forward real-world traffic, the idea behind the IMIX concept is to take a standard pattern (i.e. a predfined proportion of small-, medium- and large-sized packets) and use the same pattern to compare between different vendors.

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  • So, after all this informations, what is the criteria for determing the appropriate router? Is it the WAN link speed, type of traffic, functions enabled...is there some formula?
    – John
    Commented Jan 22, 2018 at 15:09
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    Well, no there is no formula for determining the appropriate router, just as there is no formula for determining what car to buy. The IMIX data will tell you the data forwarding performance comparison between model A and model B, but that is just one of the several factors. You need to look at cost, port density, tech support, power consumption, software features, interface types supported, scalability, future upgrades .... and so on.
    – mere3ortal
    Commented Jan 22, 2018 at 17:47
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The MTU is dictated by the link. For example, ethernet has an MTU of 1500, while frame relay on a serial interface has a much larger MTU.

Other factors can reduce the MTU. For example, a tunnel will encapsulate your packets inside other packets, so the tunnel MTU will be reduced by the size of the encapsulating protocol header. The tunnel is a virtual link, so the MTU is still dictated by the link MTU.

The MTU is only the maximum packet size, and not every packet will be that size. For example, VoIP will use a lot of very small packets, regardless of the MTU.

Cisco has documents that explain the PPS for the different routers with different options and licenses. I don't believe there is a single document that gives this for you.

Edit:

The size of your packets is determined by how much data your application sends at any one time. If the application is trying to send an image, it is likely that the amount of data exceeds an amount that would cause packets to use the minimum path MTU, but if you are using telnet, you would only be sending a byte (or few) at a time, so the frames may be padded to the minimum frame size.

Because of the specific application mix and usage, there is no way for us to tell you have to calculate the packet size. That is what tools, e.g. NetFlow, are for. You can use that to maintain data on all your traffic flows.

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  • MTU is a maximum packet size. Maximum is 1500 bytes?
    – John
    Commented Jan 21, 2018 at 19:46
  • Testing with very small packets, the worst case scenario, allows you to measure the maximum packet rate a router can achieve. Note that 64*8*353,000=180M and 1500*8*353,000=4,236G.
    – Zac67 ♦
    Commented Jan 21, 2018 at 19:49
  • That depends on the link. It is 1500 for ethernet, but other link types have other MTU sizes. Wi-Fi is larger than ethernet. Packets can pass through many links with different sizes, so we use PMTUD.
    – Ron Maupin ♦
    Commented Jan 21, 2018 at 19:49
  • So Ron, you think that with my Internet HTTP and HTTPS traffic over ethernet link I will have full throughput on Cisco router? Because MTU will be always 1500 bytes?
    – John
    Commented Jan 21, 2018 at 21:15
  • It is impossible to say without a lot more information. Also, not every packet will be 1500 bytes. What you seem to want is a wirespeed router. That is easy for switches, but much more difficult and expensive for routers. Also, traffic on the same LAN would not even use a router, only a switch.
    – Ron Maupin ♦
    Commented Jan 22, 2018 at 2:10

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