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H.323 QOS Question

Hi there,

My company has some VC equipment connected over a WAN - There are three sites which connect via a mangaged MPLS network.

We've had issues with quality and the supplier recommends trying some QOS.

My question is where to place the QOS?

Each set up goes Cat Switch (3700 series) > WAN router. Should the qos ideally be placed on the router or the switch before it?

Any advise would be much appreciated.




Actually i would suggest you do Qos at both the devices

On the edge cat switch identify the VC traffic and mark it to a specific DSCP bit.

Check with your Managed SP on the DSCP bit/precendece value that would be prioritised in their cloud and use the same on the switch as well

On the WAN router just match this DSCP bit and prioritise.



Joseph W. Doherty
Hall of Fame Expert

Where you need special traffic treatment, i.e. to provide your VC flows with their necessary bandwidth, would be at the bandwidth bottlenecks, often CE to PE and PE to CE when working with MPLS.

PE to CE depends on MPLS vendor's QoS offering. You insure your VC traffic is marked correctly, and work with the vendor to insure there's sufficient bandwidth for the class.

For CE to PE, since you noted its a "managed MPLS network", i.e. assuming the MPLS provider controls the CE, you do about the same as the above for the PE.

Once you know the markings to use, and you're sure you'll be guaranteed the bandwidth by the MPLS network, you need to insure the VC traffic is correctly marked and those marking get to the MPLS provider. VC ToS marking might be possible on the VC equipment itself. If it is, you need to insure this ToS marking gets to the provider. If the VC doesn't mark or marks incorrectly, you'll need to either mark or remark the correct values before the traffic is handed off to the MPLS provider.

The WAN, usually because of much less available bandwidth than a typical LAN, often requires QoS support for traffic like VC. However, you might need to also deal with QoS on the LAN. However, see if having the WAN QoS support your VC resolves your quality issues first.


The reason I didn't address your question of whether to place QoS on the 3700 series (3750 series?) or WAN router is because I assume the WAN router is managed by the MPLS provider. If it is, you might not need to do anything beyond have the VC equipment mark ToS (and insure the LAN equipment doesn't reset it).