Etherchannel

🧠 EtherChannel – The Ultimate Technical Guide

Learn. Revise. Interview. Repeat.

📌 A complete EtherChannel reference built for learners, job seekers, interviewers, and network professionals. 💡 Covers every essential technical detail. No fluff. No missing pieces.

1. What is EtherChannel?

▪ Definition:

EtherChannel (also called Link Aggregation or Port-Channel) is a technology that combines multiple physical Ethernet links into a single logical link to increase bandwidth, improve fault tolerance, and optimize Layer 2/3 network design.

▪ Core Benefits:

• Maximized Bandwidth: Aggregates capacity across multiple links.

• Redundancy: If one link fails, others keep traffic flowing.

• STP Efficiency: Spanning Tree Protocol treats it as a single link—avoiding blocked redundant paths.

• Simplified Management: Easier to configure and monitor as one logical interface.

2. EtherChannel in Cisco IOS

• Logical interface: Port-channel

• Configuration uses: channel-group, etherchannel, port-channel keywords

• STP sees EtherChannel as one link, reducing blocked paths.

• BUM (Broadcast, Unknown Unicast, Multicast) traffic is never sent back through the EtherChannel, avoiding loops.

3. Bandwidth vs Speed – Know the Difference

• Bandwidth = Total Capacity (e.g., 4x1Gbps = 4Gbps EtherChannel)

• Speed = Per Flow (One flow still maxes out at 1Gbps)

EtherChannel increases capacity, not the speed of a single communication flow.

4. EtherChannel Types & Modes

🛠 A. Dynamic EtherChannel

1. PAgP (Cisco Proprietary)

• Modes:

  • desirable – Actively negotiates

  • auto – Passive; waits to form EtherChannel

  • on – Forces EtherChannel statically (no negotiation)

Formation Table:

Mode 1	Mode 2	Forms EtherChannel?
desirable	desirable	✅ Yes
desirable	auto	✅ Yes
auto	auto	❌ No
on	any other	❌ No

1. LACP (IEEE 802.3ad – Open Standard)

• Modes:

  • active – Actively negotiates

  • passive – Waits passively

  • on – Static (no negotiation)

• Supports: 16 total ports (8 active + 8 standby)

• Formation Table:

Mode 1	Mode 2	Forms EtherChannel?
active	active	✅ Yes
active	passive	✅ Yes
passive	passive	❌ No
on	any other	❌ No

🛠 2. Static EtherChannel (Mode on)

• No negotiation; both sides must match.

• Not recommended due to risk of loops unless you’re connecting to devices like WLCs.

5. Port Configuration Requirements

All physical ports in a bundle must match:

• Speed
• Duplex
• Mode (Access/Trunk)
• Native VLAN (Trunk)
• Allowed VLANs (Trunk)
• Access VLAN (Access mode)

⚠️ Mismatch = Suspended Ports

6. Managing EtherChannel – Best Practices

• Assign physical interfaces using:

interface range g0/1 - 4  
channel-group 1 mode active

• Logical interface auto-creates: interface Port-channel1

• Always configure trunk/access mode, VLANs, etc. on the Port-channel, not physical interfaces.

7. Load Balancing – How It Works

Frames are sent through one physical link in the EtherChannel using a hashing algorithm.

• Common Hash Parameters:

  • src-mac, dst-mac, src-dst-mac

  • src-ip, dst-ip, src-dst-ip

💡 Default varies by device, often src-dst-ip

• Real-world Optimization (RFC 7424):

• Recognize large flows and redistribute across links:

  • Tools: sFlow, NetFlow, PSAMP, inline monitoring

  • Actions:

    • Move large flows using Policy-Based Routing (PBR)

    • Rebalance small flows for congestion relief

• ⚠️ Avoid frequent changes to maintain packet order (especially TCP)

8. Layer 3 EtherChannel (Routed Port Channels)

Uses routed ports (no switchport), ideal for inter-VLAN routing or Layer 3 links.

🔧 Configuration Example:

interface range g0/2 - 3
 no switchport
 channel-group 2 mode active
!
interface Port-channel2
 ip address 192.168.1.1 255.255.255.252

✅ Use show etherchannel summary → R flag = Routed Port Channel

9. Verification & Troubleshooting Commands

Command	Description
show etherchannel summary	Main status view
show interfaces trunk	Shows trunk info (logical port-channel only)
show ip interface brief	Check interface status
show running-config	Review current config
show pagp neighbor	PAgP-specific info
show lacp neighbor	LACP-specific info
show etherchannel load-balance	View/load balancing config
show etherchannel [group] port-channel	Detailed view of a specific port-channel
show interfaces [intf] etherchannel	EtherChannel status on individual interface

🏁 Summary – Key Interview & Revision Highlights

• EtherChannel = Logical bundling of Ethernet links.

• PAgP (Cisco) vs LACP (IEEE) – use LACP for multi-vendor.

• Static mode (on) = risky, use only when necessary.

• Configuration mismatches = Suspended ports.

• Load balancing is hash-based – optimize with correct flow parameters.

• Layer 3 EtherChannels simplify routing between devices.

• Always verify with show etherchannel summary and others.

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