Limiting the backup traffic on WAN link

Answered Question
Dec 8th, 2011

Dears,

I have a scenario where i have two datacenters with two core switches each. Backup solution is hosted in one of the DC. Eventhough i have 1Gbps dark fiber links, it is almost utilized to 980Mbps and we are facing serious performance issues with bussiness applications.

I would appreciate if someone can help me with a solution to limit the backup traffic on the WAN links.

I can do class-map for identifying the backup traffic based on the destination IP address.

Would "priority" MQC command help in this regard? I can give a priority of say 750Mbps to the genuine traffic , so that it is always guaranteed. Whenever genuine traffic is not much backup can takeover the free bandwidth.

Please help me.

Regards,

I have this problem too.
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Correct Answer by JosephDoherty about 2 years 4 months ago

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Posting

No, the backup-traffic class bandwidth set to 20 percent, in theory, will guarantee a minimum of 20%.  More can be used if it's not otherwise being used (which is often a good thing, otherwise available bandwidth goes unused).

Because unused bandwidth can be used with such a configuration, you might even decrease the bandwidth percentage for the backup traffic.  You might go to a little as 1%.  You do need to consider whether the backup traffic obtains enough bandwidth to complete in a required time window, if there's one.

I would also recommend enabling FQ in class-default, and if there's more than one backup flow, enabling FQ in its class too (if supported by the IOS version).

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Marwan ALshawi Thu, 12/08/2011 - 11:51

You can shape/police the backup bandwidth to a max amount as desired to avoid being over utilizing the link bandwidth

HTH

Sent from Cisco Technical Support iPhone App

thameemyousuf Wed, 12/14/2011 - 00:06

Thanks @marwanshawi.

Can you please provide a sample script?

Would it look like this?

class-map Backup-Traffic

match access-group 101   <- ACL for identifying backup traffic.

policy-map WAN-INTERFACE-QOS

class Backup-Traffic

   bandwidth percent 20

interface Gig7/1

service-policy out WAN-INTERFACE-QOS

In this case, am planning to reserve 20% of bandwidth for backup traffic. Does that also means during congestion, class Backup-Traffic will be limited to 20% of bandwidth and class class-default will share the remaining 80% ?

Thanks in advance.

Correct Answer
JosephDoherty Wed, 12/14/2011 - 05:51

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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

No, the backup-traffic class bandwidth set to 20 percent, in theory, will guarantee a minimum of 20%.  More can be used if it's not otherwise being used (which is often a good thing, otherwise available bandwidth goes unused).

Because unused bandwidth can be used with such a configuration, you might even decrease the bandwidth percentage for the backup traffic.  You might go to a little as 1%.  You do need to consider whether the backup traffic obtains enough bandwidth to complete in a required time window, if there's one.

I would also recommend enabling FQ in class-default, and if there's more than one backup flow, enabling FQ in its class too (if supported by the IOS version).

thameemyousuf Thu, 12/15/2011 - 07:57

@Joseph

Many thanks for the nice explanation.

Can you please look at my command script in above thread and verify if that is correct. Here I am reserving a 20% bandwidth for backup when there is a congestion on the WAN interface.

Also, can you explain why you would prefer to have FQ on the class-default?

Also I believe we have to enable "mls qos" globally on the switch for this command to work. Is there any other hidden impact on the switch if i enable this ? Ours is a flat network now without having any issues, except this WAN link congestion due to Backup traffic.

Regards,

Thameem

JosephDoherty Thu, 12/15/2011 - 10:10

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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

As to verification of your script, looks about right, but what platform/IOS are you using?

I like FQ because it mitigates individual bandwidth consuming flows from adversely impacting other less bandwidth demanding flows.  I assume there are many other flows that might be running concurrently besides the backup.

Hmm, "mls qos", that is normally needed on L3 switches, and if we're dealing with such rather than a router, good chance your CBWFQ can't be used.  L3 switches generally have a much less featured QoS and capabilities are very tied to the hardware, on some platforms even the line cards.

thameemyousuf Fri, 12/16/2011 - 22:36

Hi Joseph,

Thanks again.

I am planning to do it on our 6513 sw. Interface is on the SUP-720-3B.

"  I assume there are many other flows that might be running concurrently besides the backup."

Yes, this is a core router and there are too many flows on the WAN interfaces.

My understanding is all interfaces are FIFO by default , unless enabled by a policy-map. Is that correct?

Also, I am noticing "Total output drops" counter incrementing over this WAN interface, eventhough BW consumption is reaching only 930Mbps. If we implement this policy-map would we be able to limit this drops?

Warm regards,

JosephDoherty Sat, 12/17/2011 - 05:23

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

Few 6500 line cards support output policy maps.  What specific line card hosts this interface?

"My understanding is all interfaces are FIFO by default , unless enabled by a policy-map. Is that correct?"

Usually, but most 6500 line cards support multiple hardware queues, each often also FIFO.

"Also, I am noticing "Total output drops" counter incrementing over this  WAN interface, eventhough BW consumption is reaching only 930Mbps. If we  implement this policy-map would we be able to limit this drops?"

Maybe, maybe not.  It's natural for many high bandwidth flows to attempt to use all available bandwidth, and drops often indicates to such flows what the available bandwidth is.  For such flows, drops are to be expected although you can work to optimize the drops such that only the minimum number are dropped to provided congestion feedback to the source.  Such drop optimization is difficult to do.  However, often easier is to limit drops to just the high bandwidth flows and avoid them for low bandwidth flows.  The latter, without a QoS policy, often suffer drops too.

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