cancel
Showing results for 
Search instead for 
Did you mean: 
cancel

Cisco 871 maximum line speed

chad patterson
Beginner
Beginner

     I'm tryng to determine the maximum speed of some of the routers I have, and of some that I am considering buying. I have reviewed the Portable Product Sheets – Routing Performance many times, and i guess I cannot interperet it very well. I also reviewed this post about a Cisco 871 router speed maxing out at around 12 mbps.

     I maintain a Cisco 871 at my office, and I consistently get around 30Mbps down and 8Mbps up, but that is due to my cable companies speed limitiations applied upon me. However i did plug a Cisco 1721 into the cable connection, and maxed out at around 6Mbps, which is quite consistent with the document. This document however, is dishormonious with all other documents.

     Now all of these documents agree that we are using the Mbps notation as compared to MBps, so the discontinuity does not lie in the notation. What then could be the cause of the discord in the results? are we reading this document wrong? Even if the speeds in the document were stated at half duplex (as I have seen suggested elsewhere), I would then have to max out at around 25Mbps, but I am getting around 30Mbps as I stated earlier.

     Would some kind, enlightened soul help me to understand how to determine the maximum WAN speed that can be acheived with Cisco routers?

2 ACCEPTED SOLUTIONS

Accepted Solutions

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

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

Perhaps the most important thing to understand, required packets per second decreases, for the same rate, as frame/packet increases in size.  Performance numbers (today) are generally provided for minimum size frame/packet.  So when you see the PPS for a 871 is noted at 25 Kpps, and knowing minimum size Ethernet uses 1.488 Kpps per 1 Mbps, 25 Kpps supports about 17 Mbps.  However, maximum size Ethernet only needs 81 PPS per 1 Mbps, so the same 25 Kpps would support about 309 Mbps.  Most routers, though, do not offer the same maximum PPS rate for larger packets, so an 871's worst case (not counting CPU for non-forwarding) should be 17 Mbps, but could range all the way up to 309 Mbps (unlikely it's anywhere nearly that high).

View solution in original post

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

Laugh! I'm not confused, although you appear to be (and I do not mean to seem argumentative when I write that). Your confusion is understandable the way performance specs are often presented.

You write "Well to be fair, 11 to 20 is not close to 30." No you're not being fair because, first you seem to totally overlook my estimation is based on insufficient performance documentation, as I noted, and second, you still don't realize how variable performance can be based on actual traffic mix and device configuration.

Do you know how well your traffic mix conforms to the IMIX standard? I don't. But as the performance documentation we do have appears to support maximum performance of close to 200 Mbps, the fact that ". . . I getting almost 30Mbps on my ethernet WAN interface." doesn't mean your observed 30 Mbps means my estimation is incorrect. (Note: nor does it mean my estimation is totally accurate, which I never claimed. However, I did document how and where my numbers came from.)

You've also now noted, you've found table 6, unlike my usage of using table 5 (which I didn't explicitly note), which does note performance for an 860 and might be more representative of your usage. This is a fine observation!  (In fact, driving in this morning, I was thinking of adding it after making my prior post.)  Again, though, it doesn't invalidate my estimation nor does it prove "So it turns out that the actual maximum speed of a Cisco 871, in a real world environment, is about 27 Mbps."

Basically, when working with router performance, your "mileage may vary". For example, take note that between tables 5 and 6 an 880's performance increases from 45 to 60 Mbps (33% increase), for an 890 performance increases from 64 to 75 Mbps (17% increase) but for a 1921 performance decreases from 110 to 80 Mbps (27% decrease). Think about how different these ratios are for the same traffic but with a different configuration!

How does an 870 truly differ from a 860? Do you know? I don't, although I noted, as you, it might be similar to an 860, but since it's not an 860 is one reason I provided an estimated practical range.

"So it turns out that the actual maximum speed of a Cisco 871, in a real world environment, is about 27 Mbps."  So you're still sure?

You've now also written "I know that 12.8 Mbps != 17 Mbps". Yup!  But the former is based on PPS computation based on the Portable Product Sheets – Routing Performance, while the latter is based on Ethernet. As most of the industry uses, as does the document reference I provided, RFC 2544 for Ethernet, my numbers are correct if that's what Cisco also intended in table 1. As it is a notable difference, it would be nice to know whether the 25 Kpps is truly computed based on what's defined in your Cisco reference or my Cisco reference. (Note: even my first post made it clear I was using 25 Kpps based on Ethernet, so my numbers are correct. So are Cisco's in your reference, for the same 25 Kpps. Basically the conflict is Cisco is providing 25 Kpps [for 860s] within both Cisco documents, which may be correct, or not.  Reason it might be incorrect, it could vary between both based on dealing with actual media traffic or moving packets across internal fabric/bus without L2.  Or, it could be 25 Kpps is correct for both, and just how "bandwidth" is measured, for minimum size packets, can vary.)

"So your first calculation is incorrect." Still think so?  You're really sure?

"Also 25 kpps only applies to small packet sizes, so your second calculation is incorrect, because the kpps is different on packet sizes of 1500 bytes (large packets)." Yes, 25 Kpps noted for minimum sized packets (whatever Cisco is really using), but it can apply to maximum size packets too, and on some devices it does (e.g. 6500 sup720 when in compact mode). You know it doesn't, for an 870, how? (Note: I also noted for this router, it was unlikely to be the same for maximum size packets. But it it was 25 Kpps, my calculation was correct.)

"Both of your calculations are incorrect." What appears to be incorrect is your understanding of what I wrote, but my writing always isn't clear to everyone.  When it's not clear, the fault is mine, but questions and/or discussion are welcome.

"I would still like to know how I can figure this out for any device I come across. Can anybody show me how to figure this out for a Cisco 1760 for instance?" I would have gladly tried to help further, but as you believe my information is incorrect, without even asking for additional explanation, so you might actually learn (as I might too), I don't want to waste your, or my, time further. However, perhaps someone else might still be able to further assist you.

View solution in original post

11 REPLIES 11

Joseph W. Doherty
Hall of Fame Master Hall of Fame Master
Hall of Fame Master

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

Perhaps the most important thing to understand, required packets per second decreases, for the same rate, as frame/packet increases in size.  Performance numbers (today) are generally provided for minimum size frame/packet.  So when you see the PPS for a 871 is noted at 25 Kpps, and knowing minimum size Ethernet uses 1.488 Kpps per 1 Mbps, 25 Kpps supports about 17 Mbps.  However, maximum size Ethernet only needs 81 PPS per 1 Mbps, so the same 25 Kpps would support about 309 Mbps.  Most routers, though, do not offer the same maximum PPS rate for larger packets, so an 871's worst case (not counting CPU for non-forwarding) should be 17 Mbps, but could range all the way up to 309 Mbps (unlikely it's anywhere nearly that high).

     I'm sorry, but it seems that the answer, summed up, is "anywhere between 17Mbps and 309Mbps". Those two values are not in same ballpark, they cannot even belong to the same router. How can I figure the absolute 'realistic', maximum routing speed for a WAN interface with a 10/100 Ethernet handoff, and no ACL's or encryption to hinder the possible speed?

     If you look at this table from Princeton University (ftp://ftp.cs.princeton.edu/techreports/2002/645.pdf)::

                             | Minimum-size Packets   |