Multicast with a Disaster Recovery Site

Terry Slattery
Principal Architect

Pete Welcher at Chesapeake NetCraftsmen recently told the interesting story of a customer who added a backup data center.  Their network was configured to support multicast.  Pete describes the problem very succinctly:

If you have a customer doing multicast, and they add a 2nd Data Center or DR Site at L2, they’ll probably give the new router/switch the next higher IP on each VLAN. The problem with that is that it shifts the PIM DR from the main campus to the DR Site. It may be somewhat invisible and work, unless they don’t enable IPmc on say the links back to the main campus core from the DR Data Center.

In the case I just ran into, there was a new DR switch with an IP on exactly one VLAN, PIM enabled on the VLAN but no other interfaces. It took a while to figure out why the main campus Data Center server switch wasn’t sending PIM Registers for the one IPmc server on that VLAN. The answer turned out to be, the other one had the higher IP, it was PIM DR, so it was trying to do the Registers but did not have PIM enabled on the links on the dual unicast path to the RP…

What Pete describes is one of the reasons that I don’t like to do L2 across the WAN.  There are numerous failures that can occur and such failures are often very difficult to troubleshoot, especially with a transparent bridged L2 infrastructure.



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.