Layer 2 QoS? And 2960 for access?

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Jun 29th, 2008
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Hi folks,

I'm designing a new site for my employer and this is the first time I've been asked to do so since my predecessor left. They are aware of my skills but I'm unsure about 2 things and was wondering if you experts could help me?

1. The new site will be connected to an existing site via Microwave (all layer 2), we're running VOIP phones over the link to a PBX in the other site. Knowing the phones are configured correctly and were using a voice vlan then can I simply use a 3560 or 3750 for the link trusting COS? On the other side is a 4006 running CatOS.

2. The access switches were marked as 3560's by my predecessor, however I'm sure 2960's would do, we only have need for Layer 2 QoS and POE on 24 ports, am I correct in my assumption? I know there are many other factors like port density, SFP compatibility, layer 3 MLS, etc but these are simple access switches with VOIP phones, connected to the core switch (the 3560 or 3750)? via Single Mode Fibre all at Layer 2.



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ccannon88567 Mon, 06/30/2008 - 02:10
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I am pretty sure that all though the Catalyst 2960's are L2 Switches, they still have the ability to perform QOS functions at L3 DSCP level aswell as L2 COS QOS.

With the 2960, if you are using Cisco VOIP phones the phones already set the DSCP and COS values so all you need to do is "Trust COS" and "Trust DSCP".

I'm not an expert on QOS but with Cisco VoIP you can't go wrong by issuing the interface cmd "Auto QOS VoIP Trust".

This will allow the ports to trust what the phones mark COS/DSCP and automatically configure bandwidth allocatiion.

Bottom line - You will be fine using 2960's as they are more cost effective and do the job you require in terms of QOS. However I'm not sure if 2960 provide for POE?

Hope this helps you out.


Joseph W. Doherty Mon, 06/30/2008 - 03:24
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Will the microwave link support full Ethernet link speed, or does it actually run at a lower rate? If the latter, you could see L2 congestion at the Ethernet to microwave physical conversion and good chance any such buffering might only be FIFO which isn't ideal for VoIP. If this can happen, you'll want to shape the outbound Ethernet traffic before it gets to the microwave device. I know the 3560 series (and 3750 series?) with the latest software get do a gross shaping by idling the Ethernet port. Unsure of the 2960 or 4000 series shaping features. (Small router would work too.)

d.hodgson Mon, 06/30/2008 - 15:20
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Microwave is full Ethernet running at 100Mb full duplex.


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