PortChannel packets output issues

[Juraj Papic]

We have a customer that has a 3800 port-channel (MTU 1500)  connected to a ISP, this ISP is sending MTU 9200. We only have one (1) interface enabled on the port-channel , when we enable the second interface we start getting output errors , and start with traffic issues.

Can this be because the MTU?

[MHM Cisco World]

there is absolute issue of MTU mismatch 
but the OUT error and traffic issue you need to check the STP. 
share the output error you get 

[Juraj Papic]

!
interface GigabitEthernet1/0/48
switchport trunk allowed vlan 14,16,20,21,61,108-110,160,221,801
switchport mode trunk
switchport nonegotiate
shutdown
channel-group 3 mode active

interface GigabitEthernet1/0/32
switchport trunk allowed vlan 14,16,20,21,61,108-110,160,221,801
switchport mode trunk
switchport nonegotiate
channel-group 3 mode active


interface Port-channel3
switchport trunk allowed vlan 14,16,20,21,61,108-110,160,221,801
switchport mode trunk
switchport nonegotiate

Sorry posted a wrong interfaces.

[MHM Cisco World]

spanning-tree portfast <<- this issue 
also this which side ? and how you config port member of Port-channel ?

[Juraj Papic]

sorry posted the wrong interfaces I just did the update.

[MHM Cisco World]

do you connect to only one ISP router o multi ISP routers ?

[Juraj Papic]

Only 1 ISP

[MHM Cisco World]

the ISP add two port to port-channel and when from your side try to enable the second port the issue arise ?

[MHM Cisco World]

how your SW support this command 
SW#show lacp neighbor detail
Flags:  S - Device is requesting Slow LACPDUs
        F - Device is requesting Fast LACPDUs
        A - Device is in Active mode       P - Device is in Passive mode
Channel group 1 neighbors
Partner's information:
          Partner               Partner                     Partner
Port      System ID             Port Number     Age         Flags
FaX     MAC of ISP SW/R  0x8894           11s        FA
 !
Port      System ID             Port Number     Age         Flags
FaY    MAC of ISP SW/R  0x8893           11s        FA

Your Sw must see same MAC of ISP SW/R in both port otherwise the SW not recognize the PO.

[Juraj Papic]

[MHM Cisco World]

If above is from your ISP side then 
I see port g1/0/48 as passive but it config as active !!

interface GigabitEthernet1/0/48
switchport trunk allowed vlan 14,16,20,21,61,108-110,160,221,801
switchport mode trunk
switchport nonegotiate
shutdown
channel-group 3 mode active <<<--can you try add this command one more time.

[Juraj Papic]

The screen capture is from my side, I waiting to the ISP to send the config they have. 

[MHM Cisco World]

OK, port g1/0/48 connect to ISP, the ISP side use passive but ACTIVE->PASSIVE can work no problem 
the issue is Your PORT not SYNC with ISP PORT. 

[Juraj Papic]

What I have from the ISP port-channel is

load-interval 60
no lldp transmit
no lldp receive
no cdp enable
no vtp