CoS on Layer 2 metro ethernet

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Aug 7th, 2008
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I'm working on possibly utilizing a carrier that provides a metro-ethernet type service. One issue I have is the question of Class of Service / Quality of service on the layer 2 network.

The carrier transports the service from the customer site to their switching facility over T1. Each site will have a full T1's worth of bandwidth, except for our main location, which will have a 4xT1 connection to this metro-ethernet service. The whole ethernet is straight layer 2 from one site to another. at the hub the T1's connect to a VLAN on a switch.

The issue I'm having is the carrier says we don't have to worry about CoS/QoS because its layer 2. I don't buy this because if any of the two locations start sending more than a single T1's worth of traffic to a site that only has a single T1, how can the switch/vlan at the carrier prioritize traffic on the egress of their switch port? They say they don't need to prioritize the traffic, we only need to prioritize on the egress from our network onto the metro-ethernet.

Am I correct that they need to look at the frames and prioritize based on the tags I set, or do they not need to do anything?



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Joseph W. Doherty Thu, 08/07/2008 - 10:40
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If you have a mixture of delay or drop sensitive traffic and bulk traffic, I believe the vendor should support some type of CoS or ToS.

What you descibe isn't much different from a single L2 LAN switch with several 10 Mbps ports and one 100 Mbps port. Depending where ports send their traffic, congestion can easily form on any of the 10 Mbps egress ports.

matt.goli Thu, 08/07/2008 - 12:10
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That's my thought exactly. I spoke w/ the carrier this afternoon and they believe that shaping and policing on the egress towards the network can resolve this, but as this network grows it could potentially become more and more complex w/ an any-to-any topology.


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