In the previous STP tutorial, we learned that a loop between switches is dangerous and one path gets blocked. But what if we actually want to use multiple physical cables at once to increase bandwidth? That's where EtherChannel comes in — a technique that combines multiple physical ports into a single logical link, without STP blocking any of them.
What exactly does EtherChannel do?
Say two switches are connected by 2 cables. Instead of STP putting one of them in Blocking state, EtherChannel combines both cables into a single logical "bundle" — from STP's and the rest of the network's perspective, this bundle counts as just one link, but its actual bandwidth is the sum of both cables (and if one fails, traffic flows uninterrupted through the other).
Step 1: Select the target ports
Let's say we want to bundle ports fastEthernet 0/1 and fastEthernet 0/2 together (both connected to the second switch):
enable
configure terminal
interface range fastEthernet 0/1-2
Step 2: Create the EtherChannel using LACP
Cisco supports two protocols for automatic EtherChannel negotiation: PAgP (Cisco-proprietary) and LACP (industry standard). LACP is recommended since it's also compatible with non-Cisco equipment:
channel-group 1 mode active
exit
The command above makes both selected ports members of EtherChannel number 1, and active mode means this switch actively initiates LACP negotiation with the other side.
Step 3: Repeat the configuration on the second switch
On the second switch, run the exact same two commands (on the corresponding ports):
interface range fastEthernet 0/1-2
channel-group 1 mode active
exit
Step 4: Configure the logical Port-Channel interface
After creating the EtherChannel, a new logical interface named Port-channel1 is automatically created. If this link needs to be a trunk (to carry multiple VLANs), apply the settings to this logical interface, not to the individual physical ports:
interface port-channel 1
switchport trunk encapsulation dot1q
switchport mode trunk
exit
Step 5: Verify
show etherchannel summary
This command shows which ports are in which bundle and whether the bundle formed successfully (an SU status means Layer 2 EtherChannel is active and complete).
Why is EtherChannel critical in real networks?
The link between two core switches (or between a switch and a server) is usually the busiest point in the network. EtherChannel both multiplies that critical point's bandwidth and adds an automatic layer of redundancy — exactly the combination every real enterprise network design needs, and part of my CCNA courses.
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