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Partial T3 on a 2811 with NM-1T3/E3

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Sep 11th, 2013
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Hello all,


Thank you for reading. I have a 2811 with an NM-1T3/E3. I know it can't support a full T3 but does anyone know if I can get away with using this on a 15Mb partial DS3? This is just temporary. Cisco only says it supports it "Subrate T3/E3 with concurrent services" but doesn't specify what that subrate is.



Thanks

Correct Answer by Joseph W. Doherty about 3 years 11 months ago

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Process switching means, for some reason, the packet needed to be handled by a complex set (i.e. slow) set of code on the router.  CEF/Fast switching, means the packet was able to be forwarded by some very optimized code.  All packet forwarding on a 2811 involves using software, but how the code does what it does can sometimes have huge performance implications.


Enabling CEF does not guarantee all packets will be CEF switched (just hopefully most).


The 61.44 is a measure of how fast that system's CPU can make a L3 forwarding decision for a L3 packet.  Real packet forwarding involves more overhead, sometimes much, much more overhead, and the CPU has other "stuff" to process too (e.g. keeping all the stats current).


I'm attaching a later ISR performance document.  It doesn't cover the 2800 series, but take note the difference between raw performance metrics at the beginning of the document and recommended performance usage at the end of the document.  You should see quite a difference.

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jomar050485 Wed, 09/11/2013 - 14:13
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Thanks. Paolo.


Can you you further explain the difference between processes switchand and CEF/Fast Switching? As long as I have "ip cef" enabled, I can do 61.44 Mbps? If so, why doesn't it support a full DS3?

paolo bevilacqua Wed, 09/11/2013 - 14:41
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There are many documents that explain that all figures are bidirectionsal, so you have to divide by two.

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Joseph W. Doherty Thu, 09/12/2013 - 03:29
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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


Process switching means, for some reason, the packet needed to be handled by a complex set (i.e. slow) set of code on the router.  CEF/Fast switching, means the packet was able to be forwarded by some very optimized code.  All packet forwarding on a 2811 involves using software, but how the code does what it does can sometimes have huge performance implications.


Enabling CEF does not guarantee all packets will be CEF switched (just hopefully most).


The 61.44 is a measure of how fast that system's CPU can make a L3 forwarding decision for a L3 packet.  Real packet forwarding involves more overhead, sometimes much, much more overhead, and the CPU has other "stuff" to process too (e.g. keeping all the stats current).


I'm attaching a later ISR performance document.  It doesn't cover the 2800 series, but take note the difference between raw performance metrics at the beginning of the document and recommended performance usage at the end of the document.  You should see quite a difference.

jomar050485 Wed, 09/11/2013 - 14:30
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wwops. i guess i read your response wrong. so does that mean it only have 1.5Mb throughput??

Joseph W. Doherty Thu, 09/12/2013 - 03:12
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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.


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I actually saw a case where a (full/clear) T3 was "accidentally" ordered for a 2811.


That 2811 was able to deal with up to about half the T3's capacity (duplex) before the CPU maxed out.  I.e. it seemed able to forward about 20 Mbps duplex.


Typically, that particular link only averaged about 10 to 15 Mbps (duplex), so the 2811 usually was able to manage.

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