QoS on MAN connection

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Aug 15th, 2007
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Hi, I have a situation where we are paying for 100mb MAN connection. Our uplink to the MAN is a gigabit connection. So I'm guessing they will police our traffic to 100mb.

Voice will be running over this link along with SAN traffic, so we will be applying the below QoS policy:

class-map match-any SAN-TRAFFIC

match access-group name SAN-Traffic

class-map match-all VOICE

match ip dscp ef

class-map match-any CALL-SIGNALING

match ip dscp cs3

match ip dscp af31



policy-map DC-DC-EDGE

class VOICE

priority percent 5


bandwidth percent 5


bandwidth percent 60

class class-default

bandwidth percent 5

ip access-list extended SAN-Traffic

permit ip host host

Is it recommended that I police the traffic before it gets to the Provider or should I just stick with my percentages and let the provider police the traffic?



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Overall Rating: 4.2 (5 ratings)
a.cruea1980 Wed, 08/15/2007 - 08:51
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Let your provider police it for you. . .just remember to tell the WAN interface that it only has 100Mb to work with, not the Gb it auto-negotiated, that way your queues will get used.

melchib Wed, 08/15/2007 - 09:01
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So if set the bandwidth on the interface to 100000 - it will know that 5% for Voice is actually 5 meg and not 50 meg?



a.cruea1980 Thu, 08/16/2007 - 07:45
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Correct about the bandwidth. Think of it as creating a false congestion so the router knows about it.

And as was stated before, QoS on this link might not even really be needed. I don't think it would HURT to implement it, just in case, but you really should baseline your connection (possibly with AutoQoS) and go from there.

jwdoherty Fri, 08/17/2007 - 06:35
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Adam does touch upon an interesting point, if you can (physically) run the WAN interface at 100 Mbps, you wouldn't need to use a shaper. However, using the shaper you can easily contract with the provider to increase your effective bandwidth at any value between 100 Mbps and gig.

paolo bevilacqua Wed, 08/15/2007 - 08:53
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Hi Brad,

Suggest that you throughly test the circuit for voice in presence of intensive data traffic.

You may find that considering the speed you have purchased there is no need for any QoS setting whatsoever.

That would be the best situation, otherwise you may need to shape (that is better than policing) traffic at the actual speed contracted with the SP.

Hope this helps, please rate post if it does!

jwdoherty Wed, 08/15/2007 - 10:13
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If the provider is actually policing your traffic, then it's fine not do anything if you don't mind them discarding your traffic when you go above your contracted rate or if you know that your traffic would never go above your contracted rate. An example of the latter would be a front side 100 Mbps (LAN) connection with your backside gig (MAN) connection that's policed to 100 Mbps.

If it's possible that you can oversubscribe the 100 Mbps MAN, and the oversubscription can be due to the SAN traffic, and considering you want to protect your voice, then QoS is a must.

If you only have voice and SAN, you only need to provide the two classes for voice (as you done); also assuming you've allocated enough bandwidth. You can leave off SAN-TRAFFIC, i.e. let it fall into the default.

If you might have other traffic other than voice and SAN, you can go with a policy much like what you have except you can set the SAN bandwidth to a setting to split the bandwidth with your other traffic as you like. The 60% setting doesn't limit SAN to 60% it means it gets 60% if the other 40% is in demand. (Except for LLQ, bandwidth is a floor, not a maximum.) If this is what you intend, that?s find.

However, your policy would assume all the bandwidth of the gig is available so you need to create a hierarchical policy that limits the total to 100 Mbps.

e.g. (12.4 syntax - assumes gig limited to 100)

policy-map DC-DC-EDGE-shape

class class-default

shape average percent 10

service-policy DC-DC-EDGE

melchib Wed, 08/15/2007 - 12:27
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Won't the shaping add a lot of delay which will impact voice? (Using a 2851 router).

paolo bevilacqua Wed, 08/15/2007 - 12:41
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not at all. Voice traffic goes into priority queue, so chances are that is never shaped.

True is that with a nested policy-map, one or two voice packets it could be subject to shaping if the gap with the packet that has been transmitted immediately before is small.

But even if was shaped, delay at speed of 100 mbps is of a sub-millisecond order of magnitude, so never an issue.

Shaping is better than policing because it allow you to not drop traffic, still respecting a traffic contract.

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