auto qos and Metro Ethernet point-to-point

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Feb 7th, 2009
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We currently have 2 locations that are connected via 100Mbps metro ethernet (Cisco 3550 on each side) that is rate limited at 15 Mbps by the provider. On our network, we currently use "auto qos voip trust" for all trunk ports and "auto qos voip cisco-phone" for all access ports.


What is the best way to configure the ports that terminate our metro ethernet to ensure that VoIP signalling/traffic have sufficient bandwidth? I assume that "auto qos" won't work since both sides think there is a full 100Mbps available. Can anyone provide a config example?


Correct Answer by Joseph W. Doherty about 8 years 3 months ago

If you slow your outbound connection to 10 Mbps, it would work since you'll now will not exceed the provider's 15 Mbps. If your outbound connection remains at 100 Mbps, yes you'll insure your VoIP is protected if there's transient congestion at 100 Mbps, but the issue is what does your provider do with the 100 Mbps? (Even though your VoIP won't exceed 100, unless the provider treats traffic differently they can still drop VoIP when all your traffic bandwidth exceeds 15.)

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Joseph W. Doherty Sun, 02/08/2009 - 04:43
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What you logically want to do is shape your egress to match the available bandwidth (in your case, 15 Mbps) and prioritize VoIP within the shaped traffic. However, looking at the QoS for the 3350, I don't see something like the "srr-queue bandwidth limit" command found on the later 2960/3560/3750.


If I'm correct that the 3550 doesn't support any type of shaping, you have a couple of options. You might check whether your provider supports any kind of ToS markings or you might place a network device with better QoS support between the 3550 and the MetroE. For the former, you'll want to determine whether the provider will prioritize VoIP marked packets and avoid discarding them if your total bandwidth exceeds 15 Mbps. For the latter, you could use a low end ISR that supports 15 Mbps (don't forget to size larger if you expect future bandwidth increase), or a small L2/L3 switch that supports some form of shaping (e.g. srr-queue bandwidth limit) and an Egress Expedite Queue (e.g. 2960/3560).

jordan.bean Sun, 02/08/2009 - 19:14
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Assuming that our voice traffic never exceeds the link bandwidth and considering the fact that we are using 3550's on each end, could I use the "wrr-queue bandwidth" command along with "priority-queue out" and then classify all VoIP traffic to queue 4? That would guarantee VoIP traffic would get delivered even if the link were saturated with data (DSCP 0) traffic, correct?

Correct Answer
Joseph W. Doherty Mon, 02/09/2009 - 03:47
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If you slow your outbound connection to 10 Mbps, it would work since you'll now will not exceed the provider's 15 Mbps. If your outbound connection remains at 100 Mbps, yes you'll insure your VoIP is protected if there's transient congestion at 100 Mbps, but the issue is what does your provider do with the 100 Mbps? (Even though your VoIP won't exceed 100, unless the provider treats traffic differently they can still drop VoIP when all your traffic bandwidth exceeds 15.)

jordan.bean Mon, 02/09/2009 - 05:27
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Thanks. I think we have some 3650's somewhere on the network. I'll free up 2 of them so I can implement it the recommended way. I'll mark this one as resolved.

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