Cisco IOS 12.4(20)T Packet Capture Feature

Terry Slattery
Principal Architect

Jamey Heary, CCIE No. 7680, who writes for Network World’s Cisco Subnet, recently wrote about a set of new features in Cisco’s IOS 12.4(20)T release.  One of the features he describes is pretty neat:   Packet Capture.

At times, the only way you can troubleshoot a network problem is to get a packet capture of some application or of the traffic on a suspect link.  So you need to take your packet analyzer out to the site and capture data for this analysis.  Distributed Sniffer and similar systems were created to allow network engineers to not always have to go into the field to capture data.  With technologies like MPLS, remote sites will often communicate with one another without the data transiting a central facility where an expensive packet capture probe can be located.

So Cisco’s new feature that allows packet capture within the router will help the network engineer perform diagnosis and analysis without having to go onsite.  Also cool is the fact that this packet capture supports both CEF and process switched packets. One of the neat features is that the packet capture data is exportable in PCAP format, so analysis tools like Wireshark can import the capture data.

As with any new feature, it is only available on a subset of routers – the ISR and 7200 series routers, which are software based.  I’m sure that the hardware based routers will follow in the future as new ASICs support packet capture.

Take a look at Jamey’s article and its links to the relevant documentation for details on how it works.



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

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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.