Octet

Say this first

All members must agree with each other and with the port-channel: speed and duplex, layer 2 or layer 3, access or trunk mode, the same access VLAN or the same allowed and native VLANs, and the same channel protocol and partner. A member that disagrees is suspended (s in show etherchannel summary) with a log message giving the reason, while the rest of the bundle keeps working. I make every change on the port-channel interface, which applies it to all members.

Go deeper

What a suspended member looks like

Output

ASW2# show etherchannel summary | begin Group
Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------------------------
1      Po1(SU)         LACP      Gi1/0/47(P)     Gi1/0/48(s)

Output

%EC-5-CANNOT_BUNDLE2: Gi1/0/48 is not compatible with Gi1/0/47 and will be suspended (vlan mask is different)

Someone added a VLAN on Gi1/0/48 directly instead of on Po1.

Flags worth knowing

P bundled, s suspended, I stand-alone, H hot-standby, D down. On the port-channel, SU is layer 2 and in use, RU layer 3 and in use, SD layer 2 and down.

Partner checks

LACP also checks that every member reaches the same partner system ID. A member cabled to a different switch cannot join the bundle. On Catalyst 9000, a member whose far end does not run LACP logs %ETC-5-L3DONTBNDL2: Gi1/0/47 suspended: LACP currently not enabled on the remote port.

Changing a live bundle

Cisco IOS

interface Port-channel1
 switchport trunk allowed vlan add 40

Done on the port-channel, the change reaches both members at once and nothing gets suspended.

They will ask next

How do you build a bundle without hitting mismatches?
Default the member ports, configure them identically with interface range, add channel-group, then make all later changes on the port-channel interface only.

Traps

Editing a member interface instead of the port-channel. The member drifts from the bundle and gets suspended.

How did you do?