MACFLAP between 8 connected switches

bancaintesaru · ‎01-20-2015

Hi All,

Two weeks ago we have some trouble with one user PC, it had received IP address via DHCP but don't ping default gateway, DHCP is behind gateway and use ip-helper. We ask user change MAC address on PC, he change last 2 digits, received new IP address and ping was successful.

But then we see CPU utilization 95-97% on all Catallyst2960 and notification about mac flapping between ports that they are connected. By the way MAC address in this notification is old MAC address of PC.

How can I troubleshoot on this?

Output from LSW1:

Jan 20 2015 13:56:57.416 MSK: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi1/0/3 and port Gi2/1/1
Jan 20 2015 13:57:12.616 MSK: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi1/0/3 and port Gi2/1/1
Jan 20 2015 13:57:27.825 MSK: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi2/1/1 and port Gi1/0/3
Jan 20 2015 13:57:42.933 MSK: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi1/0/3 and port Gi2/1/1
Jan 20 2015 13:57:58.133 MSK: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi2/1/1 and port Gi1/0/3

Output from LSW-3-55:

.Jan 20 09:04:39.244: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/1 and port Gi0/3
.Jan 20 09:04:54.595: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/1 and port Gi0/2
.Jan 20 09:05:09.980: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/2 and port Gi0/3
.Jan 20 09:05:25.491: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/2 and port Gi0/3
.Jan 20 09:05:40.817: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/1 and port Gi0/2
.Jan 20 09:05:56.285: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/1 and port Gi0/2
.Jan 20 09:06:11.603: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/2 and port Gi0/3
.Jan 20 09:06:27.013: %SW_MATM-4-MACFLAP_NOTIF: Host 7446.a09b.fe44 in vlan 10 is flapping between port Gi0/2 and port Gi0/1

Jon Marshall · ‎01-20-2015

It sounds like the user originally had a mac address that was already in use, it can happen.

Which may explain why he got an IP but then couldn't use it.

He may have changed it but perhaps it has gone back to the original mac address.

Have you tried looking on LSW-3-58 as that seems to be the problem switch ?

Jon

bancaintesaru · ‎01-20-2015

For that moment user connected to FastEthernet0/26 on LSW-1-56 with MAC address 7446.a09b.fe42. Problem switches is LSW-2-53, LSW-1-54, LSW-3-55, LSW-1-56 they all have CPU utilization 97%, on LSW-2-50, LSW-3-58, LSW-3-59 CPU utilization is 80-85%.