I am posting a resolution to a problem that I have encountered only once before, about two or three years ago. The issue arose yesterday and resulted in hours of troubleshooting until a coworker recalled the previous occurrence:
I am posting a resolution to a problem that I have encountered only once before, about two or three years ago. The issue arose yesterday and resulted in hours of troubleshooting until a coworker recalled the previous occurrence:
SYMPTOM:
Multicast packets were not reaching a client attached to a Cisco 9200L network switch connected to a next hop router, an ISR4331. The next hop router was connected to the local router (also an ISR4300) via a GRE tunnel.
TROUBLESHOOTING:
All network appliances involved had recently replaced older hardware (a pair of Cisco 2921's and a Cisco 3560X), so our first impression was that perhaps, we had overlooked new global command requirements or perhaps an additional sub-interface command? Such is the case with the "ip network-broadcast" command requirement in addition to the "ip directed-broadcast" command in IOS-XE 17.3.1 and higher (Cisco Bug: CSCvy85946), where directed broadcasts are concerned.
Several steps were taken to verify router and switch configurations at both ends and to confirm routes and traffic flow. We compared command outputs to a third router, also connected via a GRE tunnel. It was managing multicast packets as intended, so we used it as our "known good" reference. After ruling out network configuration issues, we directed our attention to the Microsoft Windows client running third party application software.
RESOLUTION:
Post Windows 7 era machines (the client in question runs Windows 10) no longer process multicast packets on dual-homed NIC's. If a soft loopback adapter or a secondary hardware NIC is present, the loopback or the other hardware NIC will most likely take priority over the other connection(s) and will block multicast on those adapters. As soon as we disabled the work station's soft loopback adapter, the application software populated the display with intended data. "Show ip mfib active" on the next hop router confirmed the presence of the intended multicast route. Aside from the obvious visual output on the computer's display, the "show interface [interface number]" command on the associated switch port confirmed increasing multicast packets.
