l2ptguard error

Hello,

--]I was working to one of INE labs and i had some problems regarding "l2ptguard error " with ports going into err-disabled because of "l2ptguard error ". After i finished the lab i tried to emulate that issue with another gear(real device) and i had the same problem.

Topology looks like this:

SW1--(f0/21,f0/22)---------(g0/21,g0/22)--SW4--(g0/23,g0/24)---------(g0/23,g0/24)--SW3

--]A portchannel between Sw1 and Sw3 is configured

Sw1:

int f0/21, f0/22

 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
 channel-group 1 mode active

Sw3:

 int g0/23, g0/24

 switchport trunk encapsulation dot1q
 switchport mode trunk
 switchport nonegotiate
 channel-group 1 mode active

--]On Sw4 tunneling is configured
 
Sw4:

 g0/21, g0/23

 switchport access vlan 121
 switchport mode dot1q-tunnel
 l2protocol-tunnel stp
 l2protocol-tunnel point-to-point lacp

g0/22, g0/24

 switchport access vlan 122
 switchport mode dot1q-tunnel
 l2protocol-tunnel stp
 l2protocol-tunnel point-to-point lacp

--] Everything looks fine. However, on Sw4 i have all the time either g0/21, g0/22 or both of them into errdisable

01:31:35: %PM-4-ERR_DISABLE: l2ptguard error detected on Gi0/22, putting Gi0/22 in err-disable state
01:31:36: %LINEPROTO-5-UPDOWN: Line protocol on Interface GigabitEthernet0/22, changed state to down
01:31:37: %LINK-3-UPDOWN: Interface GigabitEthernet0/22, changed state to down

--] I was reading about this topic several forums but i never really found an answer for this problem.