EtherChannel

 

What is EtherChannel?

EtherChannel combines multiple physical Ethernet links into one logical Port Channel link.

                                     Core Switches  
                                   ============
                                      || || || || || || || ||  
                                    EtherChannel  16  links ( 8 links active & 8 links standby )
                                      || || || || || || || ||  
                                Distribution Switches 
                            ====================
                                     || || || || || || || ||  
                                    EtherChannel   16  links ( 8 links active & 8 links standby )
                                     || || || || || || || ||  
                                Access Switches
                                ==============

Supported Interface Types

Interface Type    EtherChannel Supported?
FastEthernet (100 Mbps)    Yes
GigabitEthernet (1 Gbps)    Yes
10 Gigabit Ethernet    Yes

Benefits

  • ✅ Increased bandwidth
  • ✅ Redundancy (if one cable fails, traffic continues)
  • ✅ Prevents STP from blocking redundant links
  • ✅ Load balancing across member links

Lab Topology

              Switch 1  

           fa0/1 ===== fa0/1
           fa0/2 ===== fa0/2
           fa0/3 ===== fa0/3
           fa0/4 ===== fa0/4
          
               
              Switch 2
  • 1 Logical Port-Channel

IP Addressing (Loopback ip)

Management VLAN:

DeviceIP
SW1192.168.1.2
SW2192.168.1.3

EtherChannel Protocols

ProtocolStandard or ProprietaryVendor Support
LACP (802.3ad / 802.1AX)Open StandardCisco, other vendors, .
PAgPCisco ProprietaryCisco devices only

EtherChannel Modes

ProtocolModeMeaning


Link Aggregation Control Protocol (LACP)active        Actively negotiates
LACPpassive        Waits for negotiation

LACP is the recommended standard because it works with equipment from different vendors.

LACP Modes

ModeDescription
active        Actively sends LACP packets
passive        Waits for LACP packets

Formation rules:

active + active   = ✅ Forms EtherChannel ( but unnecessary traffic ) 
active + passive  = ✅ Forms EtherChannel ( recommended ) 
passive + passive = ❌ Does not form EtherChannel ( mismatch - Config error )

Core Switch = Active
Access Switch = Passive
only the core initiates LACP, and the access switch simply responds.


❌ Does not form EtherChannel
1. Speed mismatch : 
Gi0/1 = 1000 Mbps
Gi0/2 = 100 Mbps

2. Trunk Mismatch 
Switch A = trunk port 
Switch B = access port


Lab 1: Configure LACP

Switch 1

enable
configure terminal

interface loopback0
ip address 192.168.1.2 255.255.255.255

interface range fa0/1 - 4

channel-group 1 mode active

exit

interface port-channel 1

switchport mode trunk

no shutdown

Switch 2

enable
configure terminal

interface loopback0
ip address 192.168.1.3 255.255.255.255

interface range fa0/1 - 4

channel-group 1 mode passive

exit

interface port-channel 1

switchport mode trunk

no shutdown

Verify EtherChannel

show etherchannel summary

Example:

Group  Port-channel  Protocol  Ports

1      Po1(SU)       LACP      Gi0/1(P)
                                Gi0/2(P)

Meaning

SymbolMeaning
S        Layer 2
U        In Use
P        Port bundled successfully

View Port-Channel

show interfaces port-channel 1

Check LACP Neighbor

show lacp neighbor

Displays the neighboring switch participating in the LACP negotiation.

Lab 2: Static EtherChannel

On both switches:

interface range fa0/1 - 4

channel-group 1 mode on

No negotiation occurs—both sides must be configured identically.

Test Redundancy

  1. Start a continuous ping between devices. 
    ping 192.168.1.3 -t
  1. Disconnect one of the four cables.

    Expected result:

  • EtherChannel remains up.
  • Traffic continues with reduced bandwidth.
  • Little or no packet loss.

Common Problems

1. Speed Mismatch

SW1 → 1 Gbps
SW2 → 100 Mbps

EtherChannel will not form.

2. Duplex Mismatch

Full Duplex
Half Duplex

Fails to bundle.

3. Different VLAN Configuration

SW1 → Trunk
SW2 → Access

Configuration mismatch prevents formation.

4. Different Channel Groups

SW1 → Group 1
SW2 → Group 2

Must use the same channel group number on both ends.

5. Different Protocols

SW1 → LACP
SW2 → PAgP

Protocols must match.

Troubleshooting Commands

show etherchannel summary

show etherchannel port-channel

show lacp neighbor

show pagp neighbor

show interfaces trunk

show interfaces status

show running-config

show spanning-tree

Interview Questions

1. What is EtherChannel?

Combines multiple physical links into one logical interface to increase bandwidth and provide redundancy.

2. How many links can be bundled?

Typically up to 8 active interfaces in one EtherChannel (platform dependent; some platforms support additional 8 standby links with LACP).


Teaching Tip 

National Highway with 8 roads.. 

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