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SIP Priority / VOIP QoS issues

Adam Hudson
Level 1
Level 1

Currently at my company we have three locations with Cisco VOIP phones running on an Asterisk system. Referring to the diagram below, the Asterisk server sits at the home location and all phones communicate back to it. From time to time I'm alerted to call quality issues from users at the two remote locations. What I've been tasked to do is ensure some kind of priority/QoS for phone traffic to eliminate these issues.

My question is, what's the best approach to tackle this? I've done some research and found articles on things like policy mapping to prioritize SIP traffic and an article on Auto QoS that involves Cisco vOIP phones. Overall I'm not sure exactly what I need.

Below is the network diagram with all of the devices involved, their license levels, the links between facilites and their speeds.

(Location 3) 1841 Router-adv ip services(Gateway)>-->3750 Switch-ipservices >--40 MBps MPLS--> (Location 2)1921 Router-ipbase >-->3750 Switch-ipservices (Gateway)>--100 MBps Fiber--> (Home Location) 3750 Switch-ipservices (Gateway)

Any help is appreciated.

15 Replies 15

Adam Hudson
Level 1
Level 1

I found this page: http://www.cisco.com/c/en/us/support/docs/switches/catalyst-4500-series-switches/69632-configuring-cat-ip-phone.html#topic3 From that page it looks like 3 different command groups and I could be done:

auto qos voip cisco-phone for the port the phone is on

auto qos voip trust for layer 2 uplinks

auto qos voip trust for layer 3 uplinks

Is it really that easy?

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Posting

"Is it really that easy?"

Yes, if you want to say you've enabled QoS.

No, if you want effective QoS.

 

I'll bite, what is effective QoS to you?

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Posting

Effective QoS is QoS that meets your traffic's service requirements.  For example, yours is that VoIP traffic isn't degraded by other traffic, correct?

From what you've described, if I understand it, auto QoS will likely not meet such a requirement.  Why?  Well if you have links that have logical bandwidth limits less than physical port bandwidths, or asymmetrical bandwidth, or multipoint, your QoS won't "trigger" until the port congests, which it may never, because congestion will form inside the WAN provider's cloud.

Joseph, forgive my ignorance since I'm new to this subject but more to the point I was asking, but configuration wise what would be good QoS in this situation? I'm guessing I'm shooting for specific reserved bandwidth numbers over my links, etc.

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Posting

Ah, well I'm a bit confused by your topology, as in what actually connects to what.  Could you further clarify that.  Also, what IOS versions are on the routers?

The 40 Mbps MPLS is Ethernet hand-offs; 100 Mbps?  Are your 3750s running as L2 or L3?

"Good" QoS would be a configuration that protects your VoIP traffic when there's congestion.

What specifically needs clearing up in the topology? Location 3 is the farthest location away, it connects into location 2, which then connects back to the Home location where the Asterisk server sits.

I believe both the 40 Mbps and 100 Mbps connections are ethernet hand-offs.

The 3750s marked (gateway) are L3.

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Posting

So the location 3 3750 is only L2?  The 3750 is what has the physical connection to MPLS, correct?

The bandwidths between the locations 3 and 2 router and switch are?

What else is attached to locations 3 and 2 devices?

What actual model of 3750s are being used?

What IOS versions are being used on all devices?

Location 3 is a p2p to Location 2, while location 2 is a p2p to Location 1, correct?

 

Joseph, I can answer your questions if needed, but I think at this point I'm just interested in prioritizing traffic from the vlan the phones are on across my links. I don't think it's a bandwidth issue since all of the interfaces involved are 100 Mbps if not Gigabit interfaces, and the two links between locations are 100 Mbps and 40 Mbps.

Ah, okay.  Well in that case, go ahead and try auto-QoS.  It should prioritize your VoIP traffic across your site links.

Let us know how it works out for you.

Correct me if I'm wrong, but from what I've seen I can't apply auto-qos just on a vlan, I'd need to apply port by port. I would like to avoid this if possible.

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In no event shall Author be liable for any damages whatsoever (including, without limitation, damages for loss of use, data or profit) arising out of the use or inability to use the posting's information even if Author has been advised of the possibility of such damage.

Posting

I don't believe you're wrong.  I.e. you'll need to apply some QoS per port, such as telling the port to use VLAN based QoS.

(NB: this is when the "range" command is your friend.)

That's confusing to me. Why would I need to set QoS by port if I'm already specifiying the port by VLAN? I can't set up a policy map at my egress points to check the VLAN tag and prioritize from that?

Also, is there a way to prioritize traffic without putting a bandwidth number on it?

I don't know why.  For that you would need to discuss with Cisco.  But if you want to use VLAN based QoS, on a 3750, I believe the interface needs the command mls qos vlan-based.

The 3750 doesn't support interface egress policies, as it has 4 hardware queues per port (when QoS is enabled).

"Also, is there a way to prioritize traffic without putting a bandwidth number on it?"

Sure, but it depends on the platform.  Your 3750 egress queues don't take bandwidth values.  Your ISRs depend on the IOS version (and sometimes feature set). Later versions support bandwidth percentage and bandwidth remaining percentages.

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