×

Warning message

  • Cisco Support Forums is in Read Only mode while the site is being migrated.
  • Cisco Support Forums is in Read Only mode while the site is being migrated.

Fractional Fast-E and QoS

Answered Question
May 1st, 2014
User Badges:

Hello everyone.

I have a question regarding Fractional Fast-E and QoS. If you have a router subscribed to say a 20Mb MPLS circuit, it is advisable (even necesary) to use a FE interface for connectivity versus GE interface when you are running VoIP or any delay-sensitive apps. It seems that QoS is not enough to combat the discrepancy between the higher clock-rate  that GE sends traffic on the wire in  spite of a QoS/Shaping policy applied to the WAN interface. I have  run into this situation in the past where VoIP quality was very poor until I switched the WAN interface to FE. My rule of thumb was if you subscribe to 100Mb or less, use FE ports.

 

If someone can point me to a URL/resource on CCO that explains/confirms this I would be very grateful for your help.

Thanks in advance,

Mike G.

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

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

More likely, you're QoS isn't configured to account for the subtle "gotchas" that aren't normally discussed in most QoS text books.

 

In your other posting, I mention the importance of Tc, when shaping, and the fact (I believe) many shapers don't account for L2 overhead.  Unless these are configured correctly, shaping behavior may not behave as you expect.

 

Shaping doesn't truly mimic an interface of the same bandwidth, even with optimal settings, because it works with average transmission rates over time.  For example, a gig interface shaped at 100 Mbps will not be exactly like a FE interface at 100 Mbps.  However, it can generally come close enough that most traffic will meet its service requirements, including VoIP.

 

BTW, even when doing QoS on physical interfaces, there are "gotchas".  For instance, interfaces maintain a physical transmission queue (tx-ring) which is FIFO.  Only when this queue overflows, is your QoS policy applied.  So, to reduce the impact of a bunch of large bulk traffic packets in this queue when a VoIP packets arrives, you may need to configure a smaller tx-ring limit.

  • 1
  • 2
  • 3
  • 4
  • 5
Overall Rating: 5 (1 ratings)
Loading.
Correct Answer
Joseph W. Doherty Thu, 05/01/2014 - 06:50
User Badges:
  • Super Bronze, 10000 points or more

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

More likely, you're QoS isn't configured to account for the subtle "gotchas" that aren't normally discussed in most QoS text books.

 

In your other posting, I mention the importance of Tc, when shaping, and the fact (I believe) many shapers don't account for L2 overhead.  Unless these are configured correctly, shaping behavior may not behave as you expect.

 

Shaping doesn't truly mimic an interface of the same bandwidth, even with optimal settings, because it works with average transmission rates over time.  For example, a gig interface shaped at 100 Mbps will not be exactly like a FE interface at 100 Mbps.  However, it can generally come close enough that most traffic will meet its service requirements, including VoIP.

 

BTW, even when doing QoS on physical interfaces, there are "gotchas".  For instance, interfaces maintain a physical transmission queue (tx-ring) which is FIFO.  Only when this queue overflows, is your QoS policy applied.  So, to reduce the impact of a bunch of large bulk traffic packets in this queue when a VoIP packets arrives, you may need to configure a smaller tx-ring limit.

Michael Grann Thu, 05/01/2014 - 07:42
User Badges:

Ah, yes, my old friend "tx-ring". I used to tune this on ATM interfaces; i didn't realize this was my issue on Gig-E interfaces. Thanks for pointing this out.

Regards,

Mike

Actions

This Discussion