Lesson 6 — EtherChannel and LACP Basics

Week 3Lesson 6Required32 min estimated0% progress

EtherChannel and LACP Basics

Bundle redundant links into one logical EtherChannel, choose between LACP, PAgP and static on mode, and diagnose why a bundle fails to form from show etherchannel summary.

What you'll learn (5 objectives)~32 min: video → lesson → check → apply → lab prep

Learning objectives

Explain why bundling links gives more bandwidth and resilience without STP blocking a member
Distinguish the logical Port-channel interface from its physical member interfaces
Compare LACP, PAgP and static on mode, and predict which mode combinations form a bundle
List the consistency requirements members must share and the mismatches that break a bundle
Read show etherchannel summary and interpret the member flags

Terms you will see

EtherChannelPort-channelLACPPAgPStatic onchannel-groupMember portSuspended member

Time breakdown

Read the notes16 min
Mode and mismatch practice6 min
Lab stage — bundle the redundant links10 min

Opens on YouTube

Free CCNA | EtherChannel | Day 23 | CCNA 200-301 Complete Course

By Jeremy's IT Lab · Opens externally on YouTube

Assigned watch: ~6 min

Watch on YouTube

The link opens in a new browser tab. Return here when you finish watching.

Watch for these concepts

STP treating the bundle as a single interface rather than two links
How channel-group mode decides whether the bundle negotiates or forms statically
Reading show etherchannel summary and its member flags

Go beyond the video

The assigned video introduces the idea. CCNA Practice Labs completes the learning path: clarify core concepts, explore how each device behaves, visualize the communication path, check your understanding, and prepare for hands-on lab work.

Watch the concept → understand it → visualize it → practice it → apply it.

The problem EtherChannel solves

From the last lesson: two parallel links between switches form a loop, so STP blocks one of them. You paid for two 1 Gbps links and can forward on one. That is the exact waste EtherChannel removes.

One logical interface

EtherChannel bundles the physical links into a single logical interface called a Port-channel — also known as a LAG, a link bundle, or on servers NIC teaming or bonding. STP sees the bundle as one interface, so it blocks none of the members. All links forward, giving aggregated bandwidth and instant resilience: if one member fails, the bundle keeps running on the survivors with no STP reconvergence at all.

After bundling, you configure interface Port-channel 1 — its trunk mode, allowed VLANs, and so on — once, on the logical interface, and the members inherit it. You do not configure trunking on each member separately and hope they agree; the Port-channel is the single point of configuration. The physical members still carry the traffic, load-balanced by a hash of addresses, but they behave as one link to STP and to Layer 2.

Three ways to form a bundle

The channel-group <n> mode <mode> command on the member ports decides how the two switches agree to bundle.

LACP, PAgP, and static on

	LACP	PAgP	Static on
Standard	IEEE 802.3ad — open, multi-vendor	Cisco proprietary	None
Modes	active / passive	desirable / auto	on
Actively initiates	active	desirable	No negotiation at all
Only responds	passive	auto	—
Never bundles when both ends are	passive / passive	auto / auto	n/a
Preferred for CCNA	Yes — open standard	Cisco-only environments	Avoid unless required

The most-tested fact in the lesson

Two LACP passive ends, or two PAgP auto ends, never form a bundle — neither side initiates. This is the exact mirror of the two-dynamic-auto trunk rule from Lesson 3, and the two are worth learning as a pair. You also cannot mix protocols: an LACP end will not bundle with a PAgP end, because active speaks LACP and desirable speaks PAgP.

Even with compatible modes, every member must agree on its Layer 1 and Layer 2 settings or it is left out of the bundle. Speed, duplex, access-versus-trunk mode, native VLAN and allowed-VLAN list must all match across the members and match the far side. A single member with a stray allowed-VLAN list, or one left in access mode, silently drops out while the rest of the bundle comes up — which is why a port-channel can be up and still deliver only one link's throughput.

Configure and verify an LACP bundle

Bundle two links with LACP — do this on BOTH switches

Switch(config)# interface range GigabitEthernet0/1 - 2
Switch(config-if-range)# channel-group 1 mode active

! Configure the LOGICAL interface once; the members inherit it
Switch(config)# interface Port-channel 1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20

Switch# show etherchannel summary

show etherchannel summary

Flags:  D - down        P - bundled in port-channel
        I - stand-alone  s - suspended
        R - Layer3       S - Layer2
        U - in use
Number of channel-groups in use: 1

Group  Port-channel  Protocol    Ports
------+-------------+-----------+-----------------------------
1      Po1(SU)         LACP       Gi0/1(P)   Gi0/2(P)

Reading the flags

Flag	Meaning	What it tells you
Po1(SU)	Layer 2 port-channel, in use	The bundle itself is up
(P)	Bundled in the port-channel	This member is forwarding as part of the bundle
(D)	Down	The member link itself is down
(I)	Stand-alone	Negotiation failed — the member is up but not bundled
(s)	Suspended	A consistency mismatch kept this member out

The flag distinction is the diagnostic. A stand-alone member points at a mode problem — the two ends did not negotiate. A suspended member points at a consistency problem — the modes were fine but the member's own settings differ from its peers. After bundling, show spanning-tree shows a single Po1 interface rather than two members with one of them blocked, which is the visible proof that STP now treats the pair as one link.

When only half the bundle forwards

Troubleshooting — port-channel up, throughput stuck at one link

Symptom: a new port-channel is up but only one of its two members forwards, and throughput never exceeds a single link. Diagnosis: show etherchannel summary shows one member (P) and the other (s) suspended. Suspended means a consistency mismatch, not a mode problem — most often the member carries a different allowed-VLAN list from the bundle. Fix: align the suspended member's trunk mode, native VLAN and allowed-VLAN list to match the other member and the far end, and it re-bundles. If the flag had been (I) instead, you would be looking at mode incompatibility rather than configuration drift.

What you should retain

An EtherChannel is one logical Port-channel over many links; STP sees one interface and blocks no member.
LACP is the open IEEE 802.3ad standard with modes active and passive; PAgP is Cisco with desirable and auto; on is static with no negotiation.
Passive/passive and auto/auto never bundle, and LACP will not interoperate with PAgP.
Members must match speed, duplex, access-versus-trunk mode, native VLAN, and allowed-VLAN list.
Configure the logical Port-channel once; members inherit it.
In show etherchannel summary: (P) bundled, (D) down, (I) stand-alone means negotiation failed, (s) suspended means a consistency mismatch.

Pause and predictNot scored — nothing is recorded

Before you read on

Predict each outcome. (1) Both SW1 and SW2 members use mode passive — does the bundle form? (2) SW1 uses mode active, SW2 uses mode desirable — does it form? (3) SW1 uses mode on, SW2 uses mode active — does it form? (4) Both ends use mode active, but one SW2 member still has switchport access vlan 99 while the others are trunks — what does the summary show for that port?

Section quiz

Check this section before moving on

Required8 questions~10 min

Redundancy: STP & EtherChannel

The quiz opens on its own page so you can focus on it. It is a Week 1 milestone, tracked separately from this lesson's own completion.

Topics covered

• Loops, broadcast storms, and why Ethernet has no TTL
• Root election, port roles, and path cost
• Rapid PVST+ states and per-VLAN trees
• LACP and PAgP modes, and the consistency requirements

Start the quiz

Lab preparation

Prepare for the lab

Available nowWeek 3

Week 3 Lab — VLAN and Trunk (stage 4 of 4)

Return to the two redundant trunk links that STP was blocking in stage 3, and bundle them so both forward. This closes the progressive lab: VLANs, trunk, spanning tree, and now link aggregation over one topology you built yourself.

This lesson prepares you to:

• Putting both member ports into channel-group 1 mode active on both switches
• Configuring interface Port-channel 1 as a trunk once, and letting members inherit it
• Confirming both members show (P) and that show spanning-tree now lists one Po1
• Diagnosing a consistency mismatch that keeps a member suspended

Final stage — after this the topology has served all four stages of the lab.

Open the lab

Study deeper

Topic guides extend this lesson — they do not replace the first-party walkthrough above.

STP & EtherChannel

For the full EtherChannel rule set, the complete mode matrix, and the misconfiguration traps beyond the four practised here

Open guide

Lesson orientation