QoS Classification and Marking for VoIP

Terry Slattery
Principal Architect

I’m working on a QoS job and we’re not trusting the phones.  I presume that most everyone has heard of the hacking that some fellows did to break into a hotel’s network by using a small program that made their PC look like a Cisco phone.  The switch then trusted the phone-a-like-PC and they were able to gain access to the corporate network.  So why would you trust a phone, particularly if the phone is located in an untrusted location, like a lobby or a classroom?

So the voice network has its own address space and is separate from the corporate network.  That has made life relatively easy for doing QoS because an ACL can be used to determine which packets are likely to be voice without having to run NBAR (Network Based Application Recognition).  The ACL specifies UDP and that both the source and destination addresses have to be in the VoIP address space.  It’s pretty easy in this case.

But in other networks, some people allocate the VoIP subnets out of various chunks of their network address space.  That makes it exceedingly difficult (well, at least tedious) to provide security betwen the VoIP address space and the corporate address space as well as identifying VoIP data packets.

Using a single chunk of address space for VoIP may not jump out at you initially, until after you’ve deployed it and then decide that it needs to be secure or that you need to add QoS (hmmm, you didn’t do QoS up front… welcome to the club – many people put it off in order to get the phones deployed).  Then you find that the access lists you need are very, very long and difficult to maintain.  But readdressing is also a big pain.

Thinking of it up front during the design phase makes a big, big difference in the amount of work that goes into your deployment and into maintaining the system.  My recommendation: go back and readdress the phones.  Sure, it is a hit in productivity to do it, but you’ll have much fewer security and voice quality problems as a result.  (Imagine convincing a SOX auditor that your 5 page access list is correct.)

Plan, plan, plan.  Then build.



Re-posted with Permission 

NetCraftsmen would like to acknowledge Infoblox for their permission to re-post this article which originally appeared in the Applied Infrastructure blog under http://www.infoblox.com/en/communities/blogs.html


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Nick Kelly

Cybersecurity Engineer, Cisco

Nick has over 20 years of experience in Security Operations and Security Sales. He is an avid student of cybersecurity and regularly engages with the Infosec community at events like BSides, RVASec, Derbycon and more. The son of an FBI forensics director, Nick holds a B.S. in Criminal Justice and is one of Cisco’s Fire Jumper Elite members. When he’s not working, he writes cyberpunk and punches aliens on his Playstation.


Virgilio “BONG” dela Cruz Jr.

CCDP, CCNA V, CCNP, Cisco IPS Express Security for AM/EE
Field Solutions Architect, Tech Data

Virgilio “Bong” has sixteen years of professional experience in IT industry from academe, technical and customer support, pre-sales, post sales, project management, training and enablement. He has worked in Cisco Technical Assistance Center (TAC) as a member of the WAN and LAN Switching team. Bong now works for Tech Data as the Field Solutions Architect with a focus on Cisco Security and holds a few Cisco certifications including Fire Jumper Elite.


John Cavanaugh

CCIE #1066, CCDE #20070002, CCAr
Chief Technology Officer, Practice Lead Security Services, NetCraftsmen

John is our CTO and the practice lead for a talented team of consultants focused on designing and delivering scalable and secure infrastructure solutions to customers across multiple industry verticals and technologies. Previously he has held several positions including Executive Director/Chief Architect for Global Network Services at JPMorgan Chase. In that capacity, he led a team managing network architecture and services.  Prior to his role at JPMorgan Chase, John was a Distinguished Engineer at Cisco working across a number of verticals including Higher Education, Finance, Retail, Government, and Health Care.

He is an expert in working with groups to identify business needs, and align technology strategies to enable business strategies, building in agility and scalability to allow for future changes. John is experienced in the architecture and design of highly available, secure, network infrastructure and data centers, and has worked on projects worldwide. He has worked in both the business and regulatory environments for the design and deployment of complex IT infrastructures.