EtherChannels

September 30, 2025 - Reading time: 4 minutes

EtherChannel, also known as Port-Channel, is a technology that combines multiple physical switch ports into one logical interface. This bundling increases bandwidth while providing redundancy if one physical link fails. Configuration options include modes that automatically negotiate channels (such as LACP) or statically configure them. It is widely used between switches to ensure higher throughput and fault tolerance.

In this lab, we will configure an EtherChannel between Switch1 and Switch2 using LACP in active mode. The focus is on bundling interfaces Fa0/1 and Fa0/2 on each switch into Port-channel1, then verifying that the logical link is formed and operational.

1. Basic Device Setup

Switch1

Switch1> enable
Switch1# configure terminal
Switch1(config)# hostname Switch1
Switch1(config)# interface range FastEthernet0/1 - 2
Switch1(config-if-range)# channel-group 1 mode active
Switch1(config-if-range)# no shutdown
Switch1(config-if-range)# end
Switch1#

• This configures FastEthernet0/1 and FastEthernet0/2 on Switch1 adding them to EtherChannel (group 1) in active mode (LACP).

Switch2

Switch2> enable
Switch2# configure terminal
Switch2(config)# hostname Switch2
Switch2(config)# interface range FastEthernet0/1 - 2
Switch2(config-if-range)# channel-group 1 mode active
Switch2(config-if-range)# no shutdown
Switch2(config-if-range)# end
Switch2#

• This configures matching FastEthernet0/1 and FastEthernet0/2 on Switch2 with the same EtherChannel group and LACP mode.

2. Port-Channel Interface Configuration

Note: By default, creating a channel-group automatically generates the Port-Channel interface (e.g., Port-channel1). You can fine-tune it if needed.

On Switch1 and Switch2, after creating the channel group, you can configure additional parameters on the logical interface if desired (for example, forcing trunk mode). Below is a simple example on Switch1 (and similarly on Switch2, if needed):

Switch1# configure terminal
Switch1(config)# interface port-channel1
Switch1(config-if)# switchport mode trunk
Switch1(config-if)# end
Switch1#

3. Expected Outcomes

• A new logical interface Port-channel1 should appear on both switches.
• The physical interfaces FastEthernet0/1 and FastEthernet0/2 will show as bundled into EtherChannel group 1.
• Trunking will be operational across the port-channel.

Verification Commands

Below are the key verification commands with explanation and actual console output one might see. (Outputs are typical and can vary slightly depending on IOS version.)

1. show etherchannel summary

   • Displays an overview of all configured EtherChannel groups. It lists the Port-channel number, the protocol in use (LACP, PAgP, or a dash if static on), and the member interfaces.
     
     
   • Each Port-channel line includes flags in parentheses that indicate its status (for example, SU = Layer2 and in use, SD = Layer2 and down).
     
     
   • Member interfaces are also marked with flags such as P (in port-channel), I (stand-alone), and s (suspended).

   Example Output:

   Switch1# show etherchannel summary
   Flags:  D - down        P - in port-channel
           I - stand-alone s - suspended
           H - Hot-standby (LACP only)
           R - Layer3      S - Layer2
           U - in use      f - failed to allocate aggregator
   
   Group  Port-channel  Protocol    Ports
   ------+------------+-----------+------------------------
   1      Po1(SU)        LACP      Fa0/1(P) Fa0/2(P)
   

2. show interfaces port-channel 1

   • Shows detailed status and configuration of the Port-Channel interface.
     
     

   Example Output:

   Switch1# show interfaces port-channel 1
   Port-channel1 is up, line protocol is up
   Hardware is EtherChannel, address is 0001.6432.ab00 (bia 0001.6432.ab00)
   MTU 1500 bytes, BW 2000000 Kbit/sec, DLY 10 usec
   Full-duplex, 2000Mb/s, media type is SX
   Member interfaces: Fa0/1, Fa0/2
   

   Note: Bandwidth shown is aggregated logically, but depends on actual link speeds.

3. show run interface port-channel1

   • Confirms the configured settings for the logical port-channel interface.
     
     

   Example Output:

   Switch1# show run interface port-channel1
   Building configuration...
   
   Current configuration : 58 bytes
   interface Port-channel1
    switchport mode trunk
   end
   

4. show run interface FastEthernet0/1

   • Verifies that the physical interface references the channel-group.
     
     

   Example Output:

   Switch1# show run interface FastEthernet0/1
   Building configuration...
   
   Current configuration : 90 bytes
   interface FastEthernet0/1
    switchport mode trunk
    channel-group 1 mode active
   end
   

Common Mistakes

1. Mode/Protocol Mismatch

   • Symptom: Bundle doesn’t form; ports remain in stand-alone or suspended state.

   • Solution: Use compatible modes on both ends (e.g., LACP active↔active or active↔passive, or mode on↔mode on).

2. Interface Parameter Mismatches

   • Symptom: One or more member ports show as suspended in show etherchannel summary.

   • Causes:

     • Speed mismatch

     • Duplex mismatch

     • Switchport mode mismatch (access vs trunk)

     • VLAN mismatch (different access VLANs, or different trunk allowed VLAN lists)

     • Different native VLAN on trunk ports

     • STP setting mismatches (portfast, cost, priority, BPDU guard/filter)

     • Layer type mismatch (one side Layer 2, other Layer 3)

   • Solution: Ensure all member interfaces in the channel have identical Layer 1 and Layer 2 settings on both ends before adding them.

User Challenge

1. Add an Additional Interface to the Existing EtherChannel

   • Task: Add FastEthernet0/3 on both switches to the same EtherChannel (group 1) in active mode.

2. Convert the Existing LACP Configuration to Static “On”

   • Task: Reconfigure both sides to remove LACP and set the channel group to mode on.