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New Member

Branch office VoIP/QoS architecture

Background: we are in the process of re-designing our 20 site European WAN. Our current arrangement is hub/spoke to London, connections varying between 2Mb - 10Mb. The offices vary in size from 5 people to 150, and will have new Cisco 2821 routers.

At many of the smaller offices, we are deploying (non-Cisco) IP phones, which will connect directly back to London. My question is regarding how we configure QoS at these sites.

Our IP phone supplier has advised us to run 2 VLANs at each site, one for voice and one for data. This would require a more expensive switch option, and in most cases a 3-ethernet port router (again, more expensive). DHCP config would be more complex (don't ask!). The QoS config would be based on VLAN membership. My aim is to simplify this as much as possible, in the interests of management and future expansion.

My question: how would you configure QoS in this environment? My preferred (most simple) route would be to stick to a flat layer 2 LAN, and perform QoS on the router via access lists/policies.

I must confess to having no previous VoIP experience. Am I being naive?



Super Bronze

Re: Branch office VoIP/QoS architecture


I think you should stick to the suppliers' recommendations if at all possible - what kind of switches are you running that don't support VLANs? I take if from the way you've written your post you haven't bought them yet, there are manufacturers that produce cheap PoE/VLAN/QoS (HP for example)capable switches that should be good enough for what you're proposing - they don't need to be Cisco if you're running another manufacturers' IP handsets (although you should probably consider something like the new Cat 500 series?).

The 2821 routers already have 2 gig ethernet interfaces - presuming one of these is your WAN connection you should be able to run the other as a dot1q trunked interface to serve both VLANs.

DHCP config really doesn't get complicated - ever!! Just add the scope to your server(s), set an IP helper...

Finally with regard to QoS, if you have your phones in a seperate VLAN this makes your ACLs much simpler and more reliable.

If you use a single VLAN, it will be a headache to keep all your phones/PCs in seperate IP ranges in the subnet - and if you create an ACL to match voice traffic there's no guarantee that traffic sourced from a PC or server could be matched accidentally (VoIP typically uses large ranges of UDP ports, e.g Cisco use 16383-32767 or something like that) and fill or at least oversubscribe your voice LLQs.

Hope this helps


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Aaron Please remember to rate helpful posts to identify useful responses, and mark 'Answered' if appropriate!
New Member

Re: Branch office VoIP/QoS architecture

Many thanks Aaron - that was very helpful.


New Member

Re: Branch office VoIP/QoS architecture


Having separate data/voice vlans is NOT mandatory but highly recommended. You can configure IP phones with static IP addresses(not recommended). Your biggest challenge is going to be VoIP over WAN and you must use appropriate queuing methods(LLQ recommended) to prioritize voice over data over WAN links.



New Member

Re: Branch office VoIP/QoS architecture

Many thanks for the reply, Tapan.

Could you tell me *why* VLANs are highly recommended? To improve available bandwidth, or as part of QoS identification?

Also, do you recommend performing QoS on the switch as well as the router, or can I get away with just the router?

Thanks once again


Hall of Fame Super Silver

Re: Branch office VoIP/QoS architecture

Seperate VLANs are helpful in seperating broadcast domains, helpful in QoS configuration as well as provide security barrier where you can configure ACLs to not allow other non-voice traffic to cross into voice networks.

As far as QoS on the switches, there are several QoS toolsets some applicable at access layer switches, others at the WAN edge routers. For the switches it all depands what type of switches you have, and this is where your traffic classification, trusting, priority queing, Weighted Random Early Detection (WRED) and policing is ideal.

At the WAN router the most critical toolset is Low Latency Qeueing (LLQ, think prioritinzing and sending out voice traffic out first), traffic-shaping, posibly policing, compression and fragmentation and interleaving.

As I said eariler the switch QoS configuration depends on the switch model, Cisco offers a variaty of different tollsets.

Check out for latest QoS SRND document for more details.

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