%EC-5-CANNOT_BUNDLE2: Gi1/0/25 is not compatible with Gi1/0/26 and will be suspended (dtp mode of Gi1/0/25 is on, Gi1/0/26 is off)
In plain English: Several physical links can be bundled into one logical link (an EtherChannel / port-channel) — but only if every port in the bundle is configured identically.
Platform: Cisco IOS / IOS XE. Values shown are the documented defaults — confirm them on your platform with the check commands below.
What it means
Several physical links can be bundled into one logical link (an EtherChannel / port-channel) — but only if every port in the bundle is configured identically. This port's settings differ from its bundle partners (the message names the exact setting), so the switch suspended it. The bundle keeps working on fewer links, quietly, at reduced capacity.
Likely causes, in order
One port was configured by hand and differs: trunk vs access, different allowed VLANs, different speed/duplex, or DTP left on for one port only.
A replacement switch was racked with default port settings and only some ports got the standard config.
One port of the bundle was repurposed temporarily and never restored.
Check, in order
show etherchannel summaryWhich ports are actually in the bundle (marked P) and which are suspended or standalone (marked s or I).
show running-config interface Gi1/0/25Compare the suspended port's config line by line against a working bundle member. The difference is the answer.
The fix
Make the suspended port's configuration identical to the working members — the cleanest way is to copy a known-good member's port config.
Shut and re-open the port so it renegotiates the bundle.
Confirm in show etherchannel summary that every member shows (P) bundled.
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