Troubleshooting EtherChannel

The checklist

1. show etherchannel summary on both switches: which flags?
2. Protocol column: LACP on one side and PAgP on the other never bundles.
3. Modes: passive/passive, auto/auto and on/active don't bundle.
4. Suspended member: compare its speed, duplex, and switchport settings with the others; look for %EC-5-CANNOT_BUNDLE2 log messages.
5. Settings belong on the port-channel interface, so the members stay identical.

Here SW1 uses PAgP and SW2 uses LACP. Neither side bundles, and the ports stay stand-alone:

In NetLab

▶ SW1
SW1>enable
SW1#show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         PAgP      Gi0/1(I)    Gi0/2(I)    
▶ SW2
SW2>enable
SW2#show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SD)         LACP      Gi0/1(I)    Gi0/2(I)    

And here one member on SW2 has a different access VLAN from the port-channel, so it's suspended:

In NetLab

▶ SW2
SW2>enable
SW2#show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi0/1(P)    Gi0/2(s)    
SW2#show interfaces g0/2 switchport | include Access
Access Mode VLAN: 30 (VLAN0030)

Common mistake: Changing a member's mode from on to active is refused while it's in the group. Remove it first with no channel-group, then add it back with the new mode.