The network administrator wants to enable an EtherChannel between two switches in “on” mode.
The administrator connects the cables and enables the interfaces, but while configuring the
EtherChannel in the first switch, a spanning-tree loop was detected. Which two of these
procedures can avoid this problem? (Choose two.)

A.
Configure the EtherChannel as “desirable” first.

B.
Assign all interfaces to the same VLAN.

C.
Disable PortFast on the interfaces in the EtherChannels.

D.
Disable all interfaces first.

E.
Fast Ethernet and Gigabit Ethernet ports cannot be assigned to the same EtherChannel.

F.
Fix cabling problems.

Explanation:
If a workstation or a server is connected with a single Network Interface Card (NIC) to a switch
port, this connection cannot create a physical loop. These connections are considered leaf nodes.
There is no reason to make the workstation wait 30 seconds while the switch checks for loops
when the workstation cannot cause a loop. With the addition of the PortFast or fast-start feature,
the STP for this port assumes that the port is not part of a loop. In this case, the port immediately
moves to the forwarding state, and skips the blocking, listening, or learning states. This command
does not turn STP off. This command makes STP skip a few steps in the beginning on the
selected port, although unnecessary in this circumstance.
Note:
The PortFast feature must never be used on switch ports that connect to other switches, hubs, or
routers. These connections can cause physical loops, and it is very important that Spanning Tree
go through the full initialization procedure in these situations. A Spanning Tree loop can bring the
network down. If the PortFast feature is turned on for a port that is part of a physical loop, it can
cause packets to be continuously forwarded and even multiply in such a way that the network
cannot recover.

https://supportforums.cisco.com/docs/DOC-4897