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Sysxp
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Enthusiast

Host is flapping between port Gi1/0/1 and port Po1

Hello! 

I really need help with troubleshooting an error message on my cisco:

%SW_MATM-4-MACFLAP_NOTIF: Host 0050.56a9.6656 in vlan 1 is flapping between port Gi1/0/1 and port Po1 

I have an ESXi host with 4 NICs, each NIC connected to a Cisco switch (Bl 3120X), and all the switches are stacked. So, uplinks 1,2,3,4 are looking into Gi1/0/1, Gi2/0/1, Gi3/0/1 and Gi4/0/1.

Then, I have Link Aggregation to clients on this switch, ports Te1/0/1 and Te3/0/1 forms a Port-channel Po1. (no matter static or LACP it acts the same, regarding to the error). This Po1 is connected to Mikrotik326-24.

The ONLY thing that helps to get rid of that flapping error is to remove both Te ports from Port-channel, this way STP blocking one port and flapping stops.

What exact packets ESXi is using to "advertise" the MAC of a VM on a port? I assume that I'm having a bug, when this packet is recieved by Mikrotik on one port and it is then sending it back into another port of Po1, this way the packet get back to Cisco and flapping occurs. But this should not be happening, because these ports should act like one, they are aggregated together, apparently this is not working correctly.

Can you please shed a light on this? What packets should I look for? Maybe if I drop them on Mikrotik ports of Po1 it will stop sending them back.

Thank you!

P.S. I read and tried everyting I could find about port flapping, including isolating vmk mgmt interface in another VLAN, it doesn't help.

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Sysxp
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Well, I guess any packet with SRC MAC address coming from my LAN back to Cisco can cause this flapping. Lets suppose, that deep down in the network there is a hub, or something bad happening and packet with SRC MAC of the VM comes back.

This way, all I have to do is to filter all packets with VM MAC coming from LAN ports. I will check this once I have the hardware to do it. Still, the flapping stops if no etherchannel is configured between Cisco and Mikrotik, this is strange bc packets can come back in this only working port, but it never happened and flapping only occurs if etherchannel is active.

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