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The scenario is like, in my VLAN 30 network, randomly one device stops communicating and when I check the Cisco L3 Stack switch, it shows the following error messages:

Jul 27 13:59:55: %SW_MATM-4-MACFLAP_NOTIF: Host 009c.02c3.cccc in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1
Jul 27 13:59:56: %SW_MATM-4-MACFLAP_NOTIF: Host 8038.9690.dddd in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1
Jul 27 14:00:00: %SW_MATM-4-MACFLAP_NOTIF: Host 8038.9690.dddd in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1
Jul 27 14:00:01: %SW_MATM-4-MACFLAP_NOTIF: Host 009c.02c3.cccc in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1
Jul 27 14:00:38: %SW_MATM-4-MACFLAP_NOTIF: Host 009c.02c3.cccc in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1
Jul 27 14:00:52: %SW_MATM-4-MACFLAP_NOTIF: Host 009c.02c3.cccc in vlan 30 is flapping between port Gi1/0/1 and port Gi4/1/1
Jul 27 14:01:07: %SW_MATM-4-MACFLAP_NOTIF: Host 009c.02c3.cccc in vlan 30 is flapping between port Gi4/1/1 and port Gi1/0/1 

Cisco Switch shows MAC address 009c.02c3.cccc on both interface Gi4/1/1 and Gi1/0/1, however I have been running a wireshark capture on Router 1 and couldn't find the ARP reply from Router 1 stating the MAC address 009c.02c3.cccc as a source, but for some reason it is in the MAC address table of Cisco L3 switch and Cisco switch does forward the traffic for the MAC address to Router 1.

The strange thing is, this happens intermittently to a random end-device and it starts working automatically after 10 mins. I have a spanning-tree enabled on my Cisco L3 so when I check for a loop, it shows nothing.

interface GigabitEthernet1/0/1
description Uplink
switchport access vlan 30
spanning-tree portfast
end
! 
interface GigabitEthernet4/1/1
switchport access vlan 30
trust device cisco-phone
spanning-tree portfast
end

enter image description here

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    We really need to see the full (sanitized) configurations for all the network devices. Sinc you have a layer-3 switch, you really should use routed links between the switch and routers. Do all the LAN routing on the layer-3 switch. – Ron Maupin Jul 28 at 15:08
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    You have different physical connections than you think you have or R1 is configured differently than you think – Ron Trunk Jul 28 at 15:28
  • Thank you for your response, unfortunately, I won't be able to provide a full configuration of the Cisco L3 switch because I am not the owner of it. I am responsible for the R1 and R2. I understand it'd be challenging to provide a solution with limited information, but that's all I have. :-( Yes, My next step is to create a separate VLAN for the uplinks and segregate the end-connections. But I am curious to know what's causing it? – Jenil Shah Jul 28 at 15:39
  • Seems to be your network is facing looping issue . Please check where is looping – Sagar Uragonda Jul 28 at 16:35
  • The port config suggests it's a phone. So there's a fair chance someone has connected it to the network twice. But I don't see how the router could be sourcing traffic for that MAC without some serious misconfiguration. – Ricky Jul 29 at 2:33
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Need to check physical connectivity of cable to address this issue or SFP module .

General this type of link flàgging accours when there is physically connectivity issue or SFP module issue connect ing both end switch ports ..

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