We recently upgraded our ISR routers to two ASR 1001 routers, and then experienced a multicast storms of Microsoft NLB packets. The setup is two ASR routers and a Microsoft NLB connected to the same Layer 2 VLAN. The NLB packet is broadcasted by the switch to all switch ports (expected behaviour for a packet using a multicast MAC), and therefore also reaches both ASR routers. What now happens, is that the ASR router (instead of dropping the packet), routes the packet back out the same interface it was received. The switch, in turn, broadcasts this packets as well, as this is now a "new" multicast packet. This new packet reaches the 2nd ASR, and the process repeats itself, creating a packet storm.
The ASR routers have static ARP entries for resolution of the unicast NLB IP to multicast mac.
I know that the problem can easily be solved by simply configuring the switch with static MAC address entries for the ports attached to the NLB, thereby preventing the NLB packets from reaching the routers in the first place, but I am concerned that something else is wrong. The ISR routers did not behave in this way.
Any ideas?
