If ports (2/1-4) forming an EtherChannel have inconsistent VLAN configuration, on the one side (A) set to AUTO mode, and the other side (B) is correctly configured but set to ON, Spanning-Tree Protocol (STP) errors can be generated during EtherChannel configuration. This can cause the port to be shut down because of a spanning loop detection.
This is because switch B ports 2/1-4 are channelling and all acting as one big port, while switch A ports 2/1-4 are not channeling and are acting as totally independent ports. A broadcast sent from switch B to switch A on port 2/1 will be sent back to switch B on ports 2/2, 2/3, and 2/4 because switch A treats these ports as independent ports. This is why switch B thinks there is a spanning tree loop.
Notice that the ports on switch B are now disabled and have a status of errdisable.
2000 Jan 17 22:55:48 %SPANTREE-2-CHNMISCFG: STP loop - channel 2/1-4 is disabled in vlan 1.
2000 Jan 17 22:55:49 %PAGP-5-PORTFROMSTP:Port 2/1 left bridge port 2/1-4
2000 Jan 17 22:56:01 %PAGP-5-PORTFROMSTP:Port 2/2 left bridge port 2/1-4
2000 Jan 17 22:56:13 %PAGP-5-PORTFROMSTP:Port 2/3 left bridge port 2/1-4
2000 Jan 17 22:56:36 %PAGP-5-PORTFROMSTP:Port 2/4 left bridge port 2/1-4
(enable) show port channel
Port Status Channel Channel Neighbor Neighbor
mode status device port
----- ---------- --------- ----------- -------
2/1 errdisable on channel
2/2 errdisable on channel
2/3 errdisable on channel
2/4 errdisable on channel
----- ---------- --------- ----------- --------
(enable) show port
Port Name Status Vlan Level Duplex Speed Type
----- ------------------ ---------- ----------
2/1 errdisable 1 normal auto auto 10/100BaseTX
2/2 errdisable 1 normal auto auto 10/100BaseTX
2/3 errdisable 1 normal auto auto 10/100BaseTX
2/4 errdisable 1 normal auto auto 10/100BaseTX
To resolve these issues, perform the following steps:
Verify that all channel ports belong to the same VLAN.
Configure both sides simulteanously if possible.
Set the EtherChannel mode to on/on or on/desirable.
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