EtherChannel

EtherChannel


  


EtherChannel – Table of Contents

  1. Introduction.
  2. Why do we need EtherChannel.
  3. EtherChannel Modes.
  4. PAGP & LACP Behavior Diagram.
  5. Show Commands. 
  6. Guidelines for using EtherChannel . 
  7. Configuration Example.
  8. LAB (EtherChannel Between Switches).
  9. Differences Between PAGP & LACP.
  10. What is Cross-Stack Aggregation.
  11. Interview Questions asked from Network Engineers from Etherchannel.



·    Introduction:-


  1. EtherChannel → It is used to aggregate bandwidth b/w multiple L2/L3 interfaces. (backup)
  2. EtherChannel increases bandwidth & provides redundancy by aggregating individual links b/w switches.
  3. EtherChannel load balances traffic over all the links in the bundle.
  4. Up to 8 links can be used to combine into one logical link.
  5. EtherChannel can be configured as L2 / L3.
  6. Port Channel is a logical instance of the physical interfaces.

 

·    Why do we need Etherchannel:-

 

EtherChannel is used when a network needs both higher speed and eradicate downtime through backup connectivity between switches.

For example, imagine an office where hundreds of employees are connected to an access switch and constantly using applications like video meetings, file sharing, and internet services. If there is only one cable (Lets say of 10 Mbps) connecting the access switch to the core switch, that single link becomes congested and slows down the network. As a network engineer the client asks you for 30 Mbps speed link to eradicate the issue-

 

As a solution if you add 3 cables (Each giving the speed of 10 Mbps) between the switches normally, STP (Spanning Tree Protocol) would block the extra links to prevent loops, meaning those additional cables would remain unused. That’s where EtherChannel solves the problem by combining multiple (3 in this case) physical cables or links into one logical connection (Link that is Port-Channel). This allows all links to actively carry traffic together, increasing bandwidth and improving network performance. At the same time, if one cable fails, the other links continue working, so users do not lose connectivity. In simple terms, EtherChannel helps make the network faster, more reliable providing High Availability, and more efficient.

 

 

·    EtherChannel Modes:-

It can be dynamically configured into switches using two protocols:

  1. PAgP (Port Aggregation Protocol)
  2. LACP (Link Aggregation Control Protocol)

- Mode vs Result:

 

MODE

RESULT

ON

PAgP & LACP disabled

AUTO

Passively listen for PAgP

DESIRABLE

Actively negotiate PAgP

PASSIVE

Passively listen for LACP

ACTIVE

Actively negotiate LACP

 

·    PAgP & LACP Behavior Diagram:-


- PAgP combinations:


Auto  <-->  Auto  ❌ (No formation- that is Etherchannel won’t work)

Auto  <-->  Desirable ✅

Desirable <--> Desirable ✅


- LACP combinations:


Active <--> Active   ✅

Active <--> Passive  ✅

Passive <--> Passive ❌ 


·    Show Commands:-


show etherchannel 

show etherchannel summary

show interface port-channel 'number' 

 

 

 

·                 ·    Guidelines for using EtherChannel:-


  1. Interfaces in the EtherChannel don’t have to be physically next to each other or on the same module.
  2. All ports must be of same speed & duplex.
  3. All ports in the bundle should be enabled.
  4. None of the bundle ports can be a span port.
  5. Put all bundle ports in the same VLAN or make them all trunk.
  6. Configuration applied on port-channel interface affects entire EtherChannel.

·                 ·    Configuration Example:-


# int range f0/1-4

# switchport mode trunk

# channel-protocol pagp

# channel-group 1 mode auto   (1 to 6)


** To create EtherChannel → Will need to run on both switches.

 

·                ·    LAB (EtherChannel Between Switches):-

 



 

 





- Step 1: EtherChannel Configuration


On both Switch0 and Switch1, bundle interfaces e1–e4 using PAgP protocol.

Switch0(config)# interface range fastEthernet 0/1-4

Switch0(config-if-range)# switchport mode trunk

Switch0(config-if-range)# channel-protocol pagp

Switch0(config-if-range)# channel-group 1 mode auto


- Repeat the same on Switch1:

Switch1(config)# interface range fastEthernet 0/1-4

Switch1(config-if-range)# switchport mode trunk

Switch1(config-if-range)# channel-protocol pagp

Switch1(config-if-range)# channel-group 1 mode auto


** This creates a logical Port-Channel 1 between the two switches.

- Step 2: VLAN Creation


Define VLANs for HR and IT on both switches.


Switch0(config)# vlan 10

Switch0(config-vlan)# name HR

Switch0(config)# vlan 20

Switch0(config-vlan)# name IT


Do the same on Switch1.


- Step 3: Assign VLANs to Access Ports


Connect PCs to their VLANs:


Switch0:


Switch0(config)# interface fastEthernet 0/6

Switch0(config-if)# switchport mode access

Switch0(config-if)# switchport access vlan 10

 

Switch0(config)# interface fastEthernet 0/5

Switch0(config-if)# switchport mode access

Switch0(config-if)# switchport access vlan 20


Switch1:


Switch1(config)# interface fastEthernet 0/5

Switch1(config-if)# switchport mode access

Switch1(config-if)# switchport access vlan 10

 

Switch1(config)# interface fastEthernet 0/6

Switch1(config-if)# switchport mode access

Switch1(config-if)# switchport access vlan 20


- Step 4: Verification


Use these commands to confirm everything is working:


show etherchannel summary

show vlan brief

show interfaces trunk


You should see Port-Channel 1 active and VLANs properly assigned.

- Step 5: Connectivity Test


  • PCs in VLAN 10 (HR) should ping each other (PC2 ↔ PC3).
  • PCs in VLAN 20 (IT) should ping each other (PC1 ↔ PC4).
  • Cross-VLAN communication will require a router or Layer 3 switch. I have discussed the theory and LAB of inter-vlan communication along with VTP in the other Blog. Below is the Link-

   https://networkengineerl1.blogspot.com/2026/02/ccna-lab-vlan-trunking-protocol-vtp.html


 

·               ·    PortFast:-


- It saves the 30 sec time as the loop can be formed in switches & not on PCs.


Configuration:


int f0/1   (individually on every access interface)

spanning-tree portfast


** Can only be run on access interfaces

 

 

·              ·      Differences between PAGP and LACP:-

 

Features

PAGP

LACP

Standardization

Cisco proprietary

IEEE 802.3ad / 802.1AX (open standard)

Modes

Auto (passive) and Desirable (active)

Active (initiates negotiation) and Passive (responds only)

Interoperability

Works only between Cisco devices

Works across multi‑vendor devices

Link Support

Up to 8 active links

Up to 8 active + 8 standby links

Cross‑Stack Aggregation

Not supported

Supported (links can span multiple switches)

Failure Handling

Detects partner capability but limited redundancy

Automatically removes failed links, keeps traffic flowing

Configuration

Requires manual setup, less flexible

Auto‑detects and groups links, reducing errors

Use Case

Cisco‑only networks

Mixed vendor environments needing resilience

 

 

 

·            ·    What is Cross-stack Aggregation:-

 

- Normally, EtherChannel groups interfaces within one switch.

- With cross‑stack aggregation, you can connect a device (like a server or another switch) to two different physical switches and still treat those links as one logical channel.

- This provides redundancy (if one switch fails, traffic continues through the other) and load balancing across the stack.



l Interview questions for Network Engineer  asked from Etherchannel:-


## DISCLAIMER:- The questions asked in the interview are all scenario based and indirect, So you should try to understand the concept instead of cramming. You will be fortunate if you receive a direct question from the interviewer.



1. What is Etherchannel?

2. Lets consider a scenario- A client sees congestion on a LAN link of where each link is of 10 Gig and asks you to get the speed to 30 Gig. What would you suggest?

Ans- Etherchannel is the answer to this, binding 3 10 Gig links resolves the issues. 

3. Difference between PAGP & LACP?

4. Maximum how many links could be bound in a Port-channel?

5. Is it possible to Bind interfaces into a etherchannel that are not connected in sequence or not consecutive? Yes

6. If you have to create Etherchannel between Fortigate Firewall and a cisco switch, which etherchannel protocol would you use?

7. What is Port-Channel?

8. How does config applied on Port-Channel impact the etherchannel?

9. If there is a mismatch in speed or duplex of interfaces in a etherchannel, would the etherchannel form?

10. What are the steps to configure etherchannel?

11. Commands to check etherchannel?

12. What is cross-stack aggregation and which protocol of etherchannel supports it?


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