Etherchannel also known as Port Bonding is a common way to provide redundancy in your switched and routed environment. It allows you to combine between 2 and 8 ports into one single logical channel. This way if a single port or cable fails you have no loss of network connectivity. Etherchannel also overcomes an issue with Spanning Tree Protocol where if you have multiple links between switches that are not port bonded some can be blocked to prevent a switching loop.
You can create both layer 2 and layer 3 Etherchannels which have slightly different configurations.
In this example we will connect two Cisco 2960's together between ports fa 0/1-4
int range fa 0/1-4
Selects all four ports
shutdown
Shuts the ports down
switchport mode trunk
Enable trunking on the ports
channel-group 1 mode on
Creates the EtherChannel 1 with mode on
no shut
Brings the ports back up
We have decided to use the option "on" but there are 5 different options that you can choose from as seen below.
channel-group 1 mode on
Enable Etherchannel only
channel-group 1 mode active
Enable LACP unconditionally
channel-group 1 mode passive
Enable LACP only if a LACP device is detected
channel-group 1 mode desirable
Enable PAgP unconditionally
channel-group 1 mode auto
Enable PAgP only if a PAgP device is detected
In this example we will connect two Cisco 3560's together between ports fa 0/1-4
int range fa 0/1-4
Selects all four ports
shutdown
Shuts the ports down
no switchport
Makes the ports layer 3
int port-channel 1
To config the port-channel
no switchport
Makes the Etherchannel layer 3
ip add 10.1.1.1 255.255.255.0
Assign an IP address to the EtherChannel
ip add 10.1.1.2 255.255.255.0
On the other switch
int range fa 0/1-4
Puts us back on interface config
channel-group 1 mode on
Create the Etherchannel on the ports
no shut
Bring the ports back up
Here are some commands to verify the EtherChannel
show etherchannel summary
Gives a good overview of all EtherChannels. R for Layer 3 and S for Layer2, P indicates that the ports are in the port-channel.
show etherchannel port-channel
This gives more detailed information on the port-channels.