I'm working on a rather small Layer 2 network segment that contains a physical loop. (I have included a picture of the topology below). The core switches are stacked 3850s. The other switches are SG300 series small business switches. The links between all switches are trunks that allow vlans 1,503,508,590 Spanning-tree seems to be functioning as expected and the interface on SW3 that connects to SW4 is put into a blocking state. The problem I have is that the core switch periodically logs a MAC Flap event between the links to SW1 and SW2.%SW_MATM-4-MACFLAP_NOTIF: Host 00eb.d5f2.0b9a in vlan 1 is flapping between port Po1 and port Po2 The offending MAC address seems to be a Cisco MAC (Possible one assigned to a port group?) however I am unable to physically locate the switch to which it belongs.

When I remove the loop, The Core says the MAC is available through Po2 (The link to SW2), however SW2 says the MAC is available through Po1 (The uplink to the Core). My questions are: • How can I determine which switch this MAC address actually belongs too?

Dec 5, 2018 - Software Configuration Guide, Cisco IOS Release 15.2(5)E (Catalyst 2960-L Switches) -MAC Authentication Bypass.

• What might be causing this periodic flap between ports? Here is the current configuration for all switch-to-switch links as well as a the full mac address table for each switch. Core Switch interface configurations: interface GigabitEthernet1/1/1 description Po1 Member1 switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate channel-group 1 mode active end! Interface GigabitEthernet1/1/2 description Po2 Member1 switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate channel-group 2 mode active end! Interface GigabitEthernet2/1/1 description Po1 Member2 switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate channel-group 1 mode active end!

Interface GigabitEthernet2/1/2 description Po2 Member2 switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate channel-group 2 mode active end! Interface Port-channel1 description SW1 Uplink switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate end! Mac os x sound player.

Interface Port-channel2 description SW2 Uplink switchport trunk allowed vlan 1,503,508,590 switchport mode trunk switchport nonegotiate end SW1 interface configurations: interface gigabitethernet51 channel-group 1 mode auto! Interface gigabitethernet52 channel-group 1 mode auto!

Interface Port-channel1 description 'Core Uplink' switchport trunk allowed vlan add 503,508,590! Interface gigabitethernet50 description 'Uplink to SW3' switchport trunk allowed vlan add 503,508,590!

SW2 interface configurations: interface gigabitethernet51 channel-group 1 mode auto! Interface gigabitethernet52 channel-group 1 mode auto! Interface Port-channel1 description 'Core Uplink' switchport trunk allowed vlan add 503,508,590! Interface gigabitethernet50 description 'Uplink to SW4' switchport trunk allowed vlan add 503,508,590! SW3 interface configurations: interface gigabitethernet49 description 'UPLNK to SW4' switchport trunk allowed vlan add 503,508,590!

Interface gigabitethernet50 description 'UPLINK TO SW1' switchport trunk allowed vlan add 503,508,590! SW4 interface configurations: interface gigabitethernet49 description 'UPLNK to SW3' switchport trunk allowed vlan add 503,508,590! Interface gigabitethernet50 description 'UPLINK TO SW2' switchport trunk allowed vlan add 503,508,590! You could start by eliminating the layer-2 negotiations.You can use channel-group 1 mode on to unconditionally channel; this eliminates PAgP and LACP negotiation. Also, you unconditionally set the core ports to trunk, but the interfaces on SW1 are going to use DTP to negotiate a trunk.

This is a bad practice. You should set both sides to switchport mode trunk and switchport nonegotiate, which will eliminate DTP negotiation. The configurations from all the switch-to-switch ports on all the switches would be helpful, as would the output from show mac-address-table on all the switches. Radeon 7970 mac flash. – Jul 20 '16 at 15:33 •.

With much assistance from Ron Maupin in the comments I was able to resolve this by following one of his initial suggestions; Completely disabling DTP. The SG300 switches do not support DTP and all trunks are run with Switchport mode trunk by default. After adding the line switchport nonegotiate to the port groups from the Core down to SW1 & SW2 then the flapping stopped occurring. I tested this by enabling DTP again (By removing the switchport nonegotiate) command, and the flapping returned.