Information Technology Reference
In-Depth Information
Both the auto and desirable modes allow ports to negotiate with connected ports to determine whether
they can form a channel, based on criteria such as port speed, trunking state, native VLAN, and so on.
Ports can form an EtherChannel when they are in different channel modes, as long as the modes are
compatible. For example:
•
A port in desirable mode can form an EtherChannel successfully with another port that is in
desirable or auto mode.
•
A port in auto mode can form an EtherChannel with another port in desirable mode.
•
A port in auto mode cannot form an EtherChannel with another port that is also in auto mode
because neither port will initiate negotiation.
•
A port in on mode can form a channel only with a port in on mode because ports in on mode do not
exchange PAgP packets.
•
A port in off mode will not form a channel with any port.
When using EtherChannel, if a “SPANTREE-2: Channel misconfig—
x/x-x
will be disabled” or similar
syslog message is displayed, it indicates a mismatch of EtherChannel modes on the connected ports. We
recommend that you correct the configuration and re-enable the ports by entering the
set port enable
command. Valid EtherChannel configurations include these:
Port Channel Mode
Valid Neighbor Port Channel Mode(s)
Desirable
Desirable or auto
Auto (default)
1
Desirable or auto
On
On
Off
Off
1.
If both the local and neighbor ports are in auto mode, an EtherChannel bundle will not form.
Table 23-4 is a summary of all the possible channeling mode scenarios. Some of these combinations may
cause the Spanning-Tree Protocol to put the ports on the channeling side into errdisable state (that is,
shut them down).
Table23-4 Summary of All Possible Channeling Mode Scenarios
Switch A Channel Mode
Switch B Channel Mode
Channel State
On
On
Channel
On
Off
Not Channel (errdisable)
On
Auto
Not Channel (errdisable)
On
Desirable
Not Channel (errdisable)
Off
On
Not Channel (errdisable)
Off
Off
Not Channel
Off
Auto
Not Channel
Off
Desirable
Not Channel
Auto
On
Not Channel (errdisable)
Auto
Off
Not Channel
Auto
Auto
Not Channel
