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1921 Throughput

Answered Question
Sep 25th, 2014
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I'm experiencing some confusion about the throughput of the 1921 series routers. I see a lot of people mention throughput of 15Mbps but then other documents such as the whitepage, https://supportforums.cisco.com/sites/default/files/attachments/discussi..., mention RFC 2544 Based Performance of ~2700Mbps.

Is that 15Mbps throughput for the router being hooked directly to the ISP through a leased line and not going through a provider modem?

Lets say I'm purchasing 150Mbps from an ISP and they supply a cable modem and I hook the 1921 up to that modem, will I get the full 150Mbps?

Correct Answer by Joseph W. Doherty about 2 years 10 months ago

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Posting

Oops, sorry, that's what I get when I don't read the actual reference.  I had thought it the more commonly referenced router performance paper (attached).

A "WAN" circuit, more or less, just means a non-LAN circuit.  It's expected to be full duplex.

For a 1921, the 15 Mbps means that Cisco recommends this router for up to 30 Mbps of aggregate bandwidth going to/from a "WAN".  (A "WAN" may impose additional feature overhead beyond interior LAN routing due to typical [Internet] WAN features such as NAT or firewall, and/or different media, etc.)

The only usual difference a cable modem (or ADSL modem) imposes, is asymmetrical bandwidth.  If you had a cable modem like 150/20, the "WAN" bandwidth equivalent would be 85 Mbps ((150+20)/2).

Back to the 1921, yes it's documented providing up to 2.77 Gbps, but that's best case.  1500 byte packets, and just packet forwarding.  Also that's not 2.77 Gbps, duplex.

But for 64 byte packets, it's documented at 290 Kpps.  As wire-rate for such sized Ethernet packets is 1,488 Kpps per gig, the 290 Kpps supports about 195 Mbps.  That's still for just packet forwarding, but quite a drop from 2.77 Gbps.  (It does show the impact of packet sizes.)

Once you start to add features, performance drops even more.

In your reference, Table 8 might be used to support "typical" usage.  In that table, a 1921 is listed at 68 Mbps (aggregate) but that's much better than Figure 1's 15 Mbps (WAN - or 30 Mbps aggregate).

Cisco's recommendations are conservative, to insure the recommend router's performance doesn't disappoint, but real world often doesn't load links to 100% all the time, or use as many IOS features.  Often you can "downsize" one two levels and not have any performance issues.

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Joseph W. Doherty Thu, 09/25/2014 - 09:01
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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

See if the attachment helps.

Lurickin77 Thu, 09/25/2014 - 09:38
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That white paper is the exact one linked to in the original post. That doesn't exactly answer the question since my question was related to that exact paper. What exactly does the paper mean when it says a WAN Circuit, are we talking about Ethernet to cable modem being a WAN Circuit or a T1 line?

Correct Answer
Joseph W. Doherty Thu, 09/25/2014 - 10:52
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  • Super Bronze, 10000 points or more

isclaimer

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

Oops, sorry, that's what I get when I don't read the actual reference.  I had thought it the more commonly referenced router performance paper (attached).

A "WAN" circuit, more or less, just means a non-LAN circuit.  It's expected to be full duplex.

For a 1921, the 15 Mbps means that Cisco recommends this router for up to 30 Mbps of aggregate bandwidth going to/from a "WAN".  (A "WAN" may impose additional feature overhead beyond interior LAN routing due to typical [Internet] WAN features such as NAT or firewall, and/or different media, etc.)

The only usual difference a cable modem (or ADSL modem) imposes, is asymmetrical bandwidth.  If you had a cable modem like 150/20, the "WAN" bandwidth equivalent would be 85 Mbps ((150+20)/2).

Back to the 1921, yes it's documented providing up to 2.77 Gbps, but that's best case.  1500 byte packets, and just packet forwarding.  Also that's not 2.77 Gbps, duplex.

But for 64 byte packets, it's documented at 290 Kpps.  As wire-rate for such sized Ethernet packets is 1,488 Kpps per gig, the 290 Kpps supports about 195 Mbps.  That's still for just packet forwarding, but quite a drop from 2.77 Gbps.  (It does show the impact of packet sizes.)

Once you start to add features, performance drops even more.

In your reference, Table 8 might be used to support "typical" usage.  In that table, a 1921 is listed at 68 Mbps (aggregate) but that's much better than Figure 1's 15 Mbps (WAN - or 30 Mbps aggregate).

Cisco's recommendations are conservative, to insure the recommend router's performance doesn't disappoint, but real world often doesn't load links to 100% all the time, or use as many IOS features.  Often you can "downsize" one two levels and not have any performance issues.

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Lurickin77 Thu, 09/25/2014 - 11:15
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Thank you, I've got TWC Business Class and their supplied modem and with the talk of up to 300/50 coming out in another next year I want to make sure I have a router that would get close to those speeds.

I won't be using most of the advanced QoS/firewall/encryption features so it looks like the 1921 should probably handle those speeds without much issue but I might look at the 2901 as well since that has higher throughput compared to the 1921.

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