LFI vs QoS

[Balajee Muggalla]

Hi,

   I am pretty new to wan technologies and came across this question while reading QoS book. LFI(Link Fragmentation and Interleving) solves the problem of delaying real time application data such as voice,video etc on PPP links. Same thing can be achieved with modular QoS commands by classifying and policing voice traffic with service-policy commands. Is there any difference between these two approaches? If no, why to have redundant menthols for the same thing?

Thanks,

Balajee

[Joseph W. Doherty]

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Posting

They are not redundant.  The purpose of LFI is to deal with serialization delay on low bandwidth links.

MQC QoS deals with traffic treatment.

In some situations you need both.

For example, on a 128 Kbps link, as you might with a 1.5 Mbps link, you might use MQC QoS to place your VoIP traffic into LLQ.  The problem on a slow link, a MTU packet might have just started to transmit right before your VoIP packet arrives. For 128 Kbps, it will take about 94 ms to transmit 1500 bytes.  This delay might bust your end-to-end time delay for VoIP.

To allow your newly arrived VoIP to transmit sooner, LFI will fragment larger packets.  Then your VoIP packet can be inserted (sooner) into the stream of the fragmented larger packet.

[John Blakley]

Like Joseph said, LFI and QoS are 2 different technologies, but they can work together. LFI is usually used on 768k link speeds or lower. In fact, Cisco doesn't recommend enabling it on links higher than that.

John

[Peter Paluch]

John,

LFI and QoS are 2 different technologies

Ummm... I don't think so. QoS encompasses various technologies and tools (classification, marking, congestion avoidance, congestion management, policing and shaping and link efficiency mechanisms), of which the LFI is one component belonging into the link efficiency mechanisms toolset.

With regard to MQC - MQC is a way of configuring these tools. It is not a QoS tool or technology per se, only a style of configuring and controlling the QoS tools.

Best regards,

Peter

[John Blakley]

Peter,

I think I meant to say that you don't have to enable one to have the other. You can enable only LFI on the interface and not have QoS policies or vice-versa. Isn't that correct?

John

[Peter Paluch]

Hi John,

Yes, you are correct. But from a strict terminological viewpoint, as soon as you are configuring the LFI, you are already deploying a particular QoS policy saying "large packets must be fragmented and reassembled so that the short packets do not experience excessive delays and jitter". It is also noteworthy to say that configuring the LFI alone without any prioritizing queueing mechanism has no effect - you will only chomp and reassemble the large packets but the interleaving will not be performed because there is no key to say why would a particular short frame be put in front of a large packet's fragment.

Best regards,

Peter

[bmuggall]

Thanks for your explanations. Here is my question on the same line. For Eg: I have a low speed link..let us assume it is less than 768 kbps. I want voice packets to have less delay than any other data packets. So, here are the two approaches I can think of as mentioned in my original post.

   1. Configure class map to match the voice packets and put them in priority queue. Since default MTU is 1500, packets with size larger than 1500 will be fragmented so that voice packets can be interleaved.

   2. Configure LFI on interface as per

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcflfi.html#wp1000907 . Here , voice packets are getting prioritized and transmitted before the other fragments of data packet.

So, could you explain what exactly is happening in both the cases?

Thanks,

Balajee

[Balajee Muggalla]

Guys,

    any thoughts on this?

Thanks,

Balajee

[Joseph W. Doherty]

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.

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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

Balajee Muggalla wrote:
Guys,
    any thoughts on this?
Thanks,
Balajee

Such as?  What's not clear?  What else are you looking to determine?  I thought your two original questions were answered.  I.e. They are different and they are not redundant.

[Balajee Muggalla]

Hi Joseph,

    Thanks for replying. Please look at my March 22 nd post and let me know your thoughts.

- Balajee

[Joseph W. Doherty]

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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.

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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

For a link of that bandwidth, you do both.  LFI does not prioritize, it only splits larger packets so you can interleave (insert) other smaller higher priority packets.

[Balajee Muggalla]

Hi Joseph,

    In the link I gave, it is shown that you can prioritize the traffic with "ip rtp" command along with LFI. Please refer to

http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcflfi.html#wp1000907 .

- Balajee

[Joseph W. Doherty]

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

Ah, now I understand your or my confusion!  That feature isn't unique to LFI (see http://www.cisco.com/en/US/docs/ios/12_2/qos/configuration/guide/qcfconmg.html#wp1001245), it really part of WFQ or CBWFQ.

Again for something like 768 Kbps, you want to both LFI and to prioritize VoIP; IP RTP reserve is a method to prioritize.