Cisco Etherchannel or Aggregate Cause of Forwarding Loops

The Etherchannel is an aggregated set of several physical interface as a single logical interface. The purpose of Etherchannel is to provide greater capacity than a single avaibility physical interface. The creation of danger to avoid is forwarding loops, due to two interface or more connect to same device. The Spanning Tree Protocol (STP) should be avoid forwarding loops, but some condition for example channeling ports aren't consistent, intermittent physical layer can be reason of forwarding loops.

But don't worry if you are configuring good etherchannel, the physical port will remain down untul port-channel is up.

%EC-5-STAYDOWN: Fa0/1 will remain down as its port-channel Po1 is admin-down

Here is the output of Etherchannel didn't worked :

STP will keep one of physical port which incorrect configuration remain block (BLK).
The condition will be danger if both of port Fa0/1 and Fa0/2 should be as one port-channel keep in forward and loops will be happen.

SW1#show spanning-tree vlan 200

VLAN0200

  Spanning tree enabled protocol ieee

skip

Interface           Role Sts Cost      Prio.Nbr Type

------------------- ---- --- --------- -------- --------------------------------

Fa0/1               Root LRN 19        128.1    P2p 

Fa0/2               Altn BLK 19        128.2    P2p 

Fa0/3               Desg FWD 19        128.3    P2p 

To avoid forwarding loops, this is best practices to config Etherchannel L2.