Configuring EtherChannel results in ports being errdisabled, and fails to form channels due to spanning tree loops caused by VLAN misconfiguration

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Wed, 07/22/2009 - 19:24
Jun 18th, 2009
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Core issue

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.


Resolution

Error Messages

Notice that the ports on switch B are now disabled and have a status of errdisable.


Switch-B> (enable)

    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



Switch-B> 
    (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 
    ----- ---------- --------- ----------- -------- 


Switch-B> 
    (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.

Refer to Error-Disabled Because of EtherChannel Misconfiguration for more information.


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