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When the network grows and traffic increases, the trunks between switches start to become the weak point of the system.

If this single cable breaks, all communication is lost, and all traffic shares the bandwidth of only one port:

Simply adding another cable does not fully solve the problem:

STP will automatically block one of the ports to prevent loops, so traffic is still limited to one port, with the second serving only as a spare in case the primary fails.

EtherChannel creates one virtual interface made up of up to 8 ports. The resulting connection speed equals the sum of each port’s bandwidth.

enable 
configure terminal 
no ip domain lookup
line con 0
logging synchronous
hostname Switch1

enable 
configure terminal 
no ip domain lookup
line con 0
logging synchronous
hostname Switch2

Set EtherChannel Static:

interface range g0/1-2
channel-group 1 mode on

interface range g0/1-2
channel-group 1 mode on

Set EtherChannel to negotiate via PAgP protocol (Cisco):

interface range g0/1-2
channel-group 1 mode desirable

interface range g0/1-2
channel-group 1 mode desirable

Set EtherChannel to negotiate via LACP protocol:

interface range g0/1-2
channel-group 1 mode active

interface range g0/1-2
channel-group 1 mode active

interface range g0/1-2
channel-group 1 mode passive

No EtherChannel:
– Single port speed
– 1 spare port

EtherChannel:
– Combined port speed
– No dedicated spare port, but if one link goes down the other carries the traffic.

Mode Definitions:

• ON – ON
  • Forces link aggregation without negotiation.
• Desirable – Desirable
  • Both sides initiate negotiation and form the link.
• Desirable – Auto
  • One side initiates negotiation and the other responds to form the link.
• Auto – Auto
  • Both sides wait and will NOT initiate negotiation, so the link will NOT form.
• Active – Active
  • Both sides initiate negotiation and form the link.
• Active – Passive
  • One side initiates negotiation and the other responds to form the link.
• Passive – Passive
  • Both sides wait and will NOT initiate negotiation, so the link will NOT form.

To verify the configuration on each switch, use the following commands:

show spanning-tree
show etherchannel summary
show interfaces port-channel 1

EtherChannel can also be used on access ports, but it is most commonly applied to trunk ports since they are the natural bottleneck for traffic.