The new catalyst 3850s are great! They have a ton of features that come in handy. I recently ran into a bit of a conundrum though. The 3850 was bouncing back my ARP requests from the nexus management interface with the 3850’s MAC address. The nexus management interfaces were on the same subnet in the same vlan, that didn’t even span multiple switches.
I’ve changed the IP addresses to protect the innocent. The packets are out of order, just because it was near impossible to collect amount of packets necessary to make it look nice. This is plenty though. The originator is the owner of the eb:81 MAC. So why is the ARP packet coming back to me with the MAC of the 3850(52:99). Turns out it’s a little feature that Cisco has implemented and is very poorly documented as of 09/2013. Here is the response from TAC –
“Please add the following configuration on the interfaces connected to the Nexus switches: ‘nmsp attach suppress’.
IP Device Tracking (IPDT) is globally enabled by default on the 3850. It can only be disabled on a per-interface level. The 3850 uses a feature called Network Mobility Service Protocol (NMSP) which in turn uses IPDT. The cause of the ARP errors is caused by IP Device Tracking. We’ll want to disable NMSP to prevent the 3850 from sending these packets.
Once the configuration is added, please let me know if the Nexus switch still sees these ARP packets.”