Router throughput !

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Oct 14th, 2009
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hi all. I am not a presales person but i think i lack the knowledge of what exactly might be a throughput of router interfaces. I have a 3845 with 2 2FE2W NM module. now on this forum, i read someone telling that although the port speed is 100 mbps but the throughput might be somewhere like 30 mbps ? this has to do something with the backplane of router ? can someone pls refer me to a doc plus explain a bit as to what this is all about ?

Pls guide me

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Giuseppe Larosa Wed, 10/14/2009 - 23:14
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Hello Ovais,

performances of a software based router like C3845 is strictly related to cpu speed and capabilities to move packets.

Even using CEF switching the device still uses its main cpu to forward packets: no ASICs or linecard cpus are there to help.

in the attached file you can find a pdf version of Portable router performance data sheet.

Also consider that traffic is birectional so we say a router can handle 50,000 pps for example it means is able 50,000 pps from interface 1 to interface2.

so when you see a throughput in Mbps you need to divide by two in real world.

Also the document provides raw data without any feature applied to the router: ACLs, QoS and other features subtract cpu resources.

Hope to help


illusion_rox Thu, 10/15/2009 - 21:30
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Dear Sir, i am a bit more confused. I thought fast switching like cef uses special hardware (ASIC) to forward the packet. How shall i know that which router uses asics and which not ? i am from an enterprise means a customer but i think i should have some knowledge of this stuff as well. I will be grateful if you could help me get on the way. Sir how shall i determine the actual throughput of a interface. If its 100 mbps, does it mean that i can reach this max throughput ? we have observed on numerous occasions that, when the same radio link is terminated on pc at both end, the file transfer rate reaches 20+ Mbps, but when we terminate the same link on router Fe interface, it drops to 14-15 Mbps and never reaches any high. How shall i know its a radio issue or interface throughput problem ? i am still confused as to how exactly shall i determine the bandwidth (or throughput ?) of an interface so that my link testing is successful ?

Pls sir guide me

Giuseppe Larosa Thu, 10/15/2009 - 23:51
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Hello Ovais,

all software based routers miss the ASIC chips for hardware based forwarding.

That is:

C2600, C3600, ISR 1800,2800, 3800 and also C7200 are classified as SW based routers in that they rely on main cpu for forwarding purposes.

a multilayer switch like C3550, C3560, C3750 or C6500 have ASICs for HW based CEF forwarding.

For your issue, you should count all traffic on the routers in all interfaces: the performance figures are given for the whole box.

If a device is already loaded by handling traffic between two FE, adding the radio link on third FE could make it suffer some limitations.

wire speed capability on an FE is 1488000 that is given by:

considering smallest ethernet frames:

64 bytes / frame

including preamble and inter-frame-gap that counts for 20,1 bytes

frame rate = 100000000/(8*(64+20))= 148800

! 8 here it is for 8 bits/byte

clearly as Joseph has noted this is a worst case.

with the biggest frames that are 1518 byte in size

frame rate = 100000000/(8*(1518+20))

8127 pps

you can check the current load offer using simply

sh interface fas0/0 | inc rate

note that it is better to use

load-interval 30

on each interface configuration

because values that are shown are averages and not instantaneus values

with default values of load interval 5 minutes it takes 20 minutes to show the real traffic rate.

Hope to help


illusion_rox Sun, 10/18/2009 - 23:26
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Dear Sir, kindly pardon me for queries over queries but i am newbie to this stuff of calculating throughput. So i am still confused. I have over 4 FEs already connected to different WAN links. Then i have 9HWIC-ESW with 6 of them being used. Now i want to add Radio link. How should i calculate the accurate throughput of this port now ? because there is a clear difference in throughput when the file transfer is done between pcs versus through routers.

Since i dont know what throughput shall i be getting since on this router, i am using extensive acl filtering, natting, so what throughput shall i be expecting ?

Is there any doc or other method that i shall use ?

you can see i am totally blind in what to do ?

Giuseppe Larosa Mon, 10/19/2009 - 05:33
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Hello Ovais,

my suggestions may have been misleading.

interfaces of the 9HWIC-ESW are actually switch ports and don't need to be counted as standard routed ports.

You say you have 4 FE routed ports connected to WAN links and this has to be taken in account.

to know how much you are using is enough to count pps on the 4 FE ports using show interface.

However, there may be other aspects that can lower the performance of the radio link like an mtu mismatch or errors on the port where the radio link device connects.

Hope to help


Joseph W. Doherty Thu, 10/15/2009 - 06:45
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Most software routers require the main processor to actually accept a packet received on an interface, decide what to do with it, and then do it (the latter often being sending the packet out another interface). Router CPU performance, then, is often the major factor in determining the throughput of the router. What we look for, as a measure, is the router's packets per second rating. However, since packets can vary in size, we need a rate for various packet sizes, yet generally only a PPS rate for one size is often documented, usually for minimum size packets. (NB: BTW, PPS rates often don't correspond directly with packet size, i.e. given PPS for size x, it's not guaranteed 2x will allow 2x performance.)

The document Giuseppe posted provides various forwarding performance numbers, often worst case and best case. Usually most traffic is "best case", but when it isn't, it quickly limits performance. Also, and as noted in the document, routers can do other things that the CPU supports which limits their packet forwarding performance.

Correctly sizing a router can be a bit complex; assuming you don't "play safe" and size for just-in-case. Correct sizing, I suggest, is best accomplished with someone with expertize who has no financial interest in the equipment you might purchase.


A 3845 usually should be able to support a single 100 Mbps link, full duplex, full rate. However, when working with Ethernet, depending on other requirements, a L3 switch might be a better choice.


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