Prioritizing Voice Traffic – But Is That All?

Terry Slattery
Principal Architect

Cisco has implemented a number of features that allow us control over packet prioritization, which allows us to give priority treatment to specific applications. One of these features is the command

ip rtp priority starting-rtp-port-number port-number-range bandwidth

(There’s a related Frame Relay version of the command.) RTP is encapsulated in UDP packets and the port numbers are what are used to determine packets for the priority queue. This is a good control and is easily understood.

There is a problem with this command. It prioritizes any packets that fall into the specified UDP port range. What’s to keep other applications (Skype comes to mind) from gaining access to higher priority for its packets by finding and using these UDP ports? Unlike the CBWFQ configurations, there is no access list or NBAR-based filter to identify which packets should receive high priority treatment.

Cisco’s documentation also notes that we must monitor the amount of bandwidth being used by the priority packets because no admission control filtering is specified. (Question that I don’t yet have answered: Does call admission and RSVP know the bandwidth configured for priority queues and do the right thing?) Monitoring queue utilization and queue drops seems like the appropriate way to anticipate unplanned congestion and the resulting packet loss on interfaces configured with RTP priority queueing.



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


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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.