Cisco EtherChannel

In EtherChannel, you can combine multiple physical links between switches into a single logical link
to increase the bandwidth size.

You can combine maximum eight physical links into one etherchannel link for the purpose of
high speed communication and load balancing between switches.

Etherchannel can be configured using one of these modes :

Port Aggregation Protocol (PAgP) and Link Aggregation Control Protocol (LACP)

LACP allows a network device to negotiate an automatic bundling of links by sending LACP packets
to the peer (directly connected device that also implements LACP).

While STP is helping us by preventing switching loops, STP is also hurting us by preventing us
from using a perfectly valid path between SW1 and SW2.
We could literally double the bandwidth available between the two switches if we could use that
path that is currently being blocked.

The secret to using the currently blocked path is configuring an Etherchannel.
An Etherchannel is simply a logical bundling of 2 – 8 physical connections between two Cisco switches.

Configuring an Etherchannel is actually quite simple. Use the command “channel-group 1 mode on“ on
every port you want to be placed into the Etherchannel. Of course, this must be done on both switches
if you configure an Etherchannel on one switch and don’t do so on the correct ports on the other switch,
the line protocol will go down and stay there.

The beauty of an Etherchannel is that STP sees the Etherchannel as one connection.
If any of the physical connections inside the Etherchannel go down, STP does not see this,
and STP will not recalculate. While traffic flow between the two switches will obviously be slowed,
the delay in transmission caused by an STP recalculation is avoided.
An Etherchannel also allows us to use multiple physical connections at one time.

Here’s how to put these ports into an Etherchannel:

SW1 :

Configuration in the Cisco 3560 Catalyst Switch 1 :

config t
interface port-channel 1
no switchport
no shutdown
ip address 192.168.1.1 255.255.255.0

Note: This example shows how to configure Gigabit Ethernet ports 0/45- 0/48 into port channel 1
with PAgP mode desirable