Host is flapping between Port channel and physical port

[as00001111]

Hey all!

I have a problem where I don't know how to solve it.

I have two Catalyst 4500 as a VSS.

Furthermore, I have a stack of two Catalyst 3850 switches connected to the VSS:

the 3850 stack is connected with two trunk interfaces, as a port-channel.

configuration on the VSS:

interface TenGigabitEthernet1/1/9
 switchport mode trunk
 switchport nonegotiate
 channel-group 22 mode active

interface TenGigabitEthernet2/1/9
 switchport mode trunk
 switchport nonegotiate
 channel-group 22 mode active

configuration on the stack:

interface TenGigabitEthernet1/1/1
switchport mode trunk
switchport nonegotiate
ip arp inspection trust
storm-control broadcast level 30.00
storm-control multicast level 20.00
storm-control action trap
channel-group 22 mode active
ip dhcp snooping trust

interface TenGigabitEthernet2/1/1
 switchport mode trunk
 switchport nonegotiate
 ip arp inspection trust
 storm-control broadcast level 30.00
 storm-control multicast level 20.00
 storm-control action trap
 channel-group 22 mode active
 ip dhcp snooping trust

Every minute, I get a syslog message with mac flapping:

Host 685b.3584.bc41 in vlan 20 is flapping between port Te2/1/1 and port Po22 (Access-2-2-2)

That's just an example, there are other message with other mac addresses and other vlans.

Is someone able to help me?

Thanks!

[willwetherman]

Hi,

Can you post the output of 'show etherchannel summary' from the Cisco 4500 and 3850 switches?

Thanks

[as00001111]

Hi,

that's normal:

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
22     Po22(SU)        LACP      Te1/1/1(P)  Te2/1/1(P)

They are both bundled.

[willwetherman]

This looks like a bug with the Cisco Catalyst 3850 switch

MAC address being learnt on an individual Port-channel member interface

https://bst.cloudapps.cisco.com/bugsearch/bug/CSCut87285  

[as00001111]

Oh okay thanks!

I will try an ios update and keep you posted.

[Julio E. Moisa]

Hi

It could be generated by a loop on somewhere, you should identify what kind of device are associated to the MAC addresses displayed on the logs, you can use:

show mac address-table | include <mac address or at least the last 4 values>

Also it could be a wireless client. 

This link could be useful:

https://blogs.it.ox.ac.uk/networks/2011/02/04/mac-flaps-why-are-they-bad/

:-)

[as00001111]

There is no loop.

And it isn't a wireless client.

Our VSS is in the middle of our star toplogy and all access switches around it.

I have those mac flappings on all access switches.

But the weird thing is: Why is it flapping between the portchannel interface and the psyhical portchannel interface. That makes so sense.