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.
Week 3 · Lesson 6 · Active ~32 min · Video ~6 min (segment TBD) · Lab ~10 min · Domain: Network Access
This lesson picks up exactly where STP left off: STP blocks redundant links to break loops. EtherChannel is how you keep those redundant links all forwarding. Trunk and VLAN terms are used as defined earlier.
Prerequisites: stp-rapid-pvst-basics
Paths: Start lesson · Test out (jump to the quick check) · Review sheet
Baseline check
You cable two 1 Gbps trunks between SW1 and SW2 hoping for 2 Gbps of bandwidth. STP is running with defaults. How much of that 2 Gbps is actually usable for forwarding, and why? Commit before you continue.
The answer — only 1 Gbps, because STP blocks the second link to prevent a loop — is the exact problem EtherChannel removes.
The problem EtherChannel solves. From the STP lesson: two parallel links between switches form a loop, so STP blocks one of them. You paid for two links and can use one. EtherChannel bundles the physical links into a single logical interface — a Port-channel (also called a LAG, link bundle, or, on servers, NIC teaming/bonding). STP sees the bundle as one interface, so it blocks none of the members. All links forward, giving you aggregated bandwidth and instant resilience: if one member fails, the bundle keeps running on the survivors with no STP reconvergence.
Logical vs physical. After bundling, you configure the interface Port-channel 1 — set 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 carry the traffic, load-balanced by a hash (of MACs/IPs), 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 members decides how the two switches agree to bundle:
- LACP (IEEE 802.3ad, the open standard): modes active and passive.
activeinitiates negotiation;passiveonly responds. At least one end must be active. - PAgP (Cisco proprietary): modes desirable and auto.
desirableinitiates;autoonly responds. At least one end must be desirable. - Static
on: no negotiation protocol at all. Both ends must be set on — the switch bundles blindly, which is risky if the other side is misconfigured.
LACP vs PAgP — the comparison to memorize.
| Feature | 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) |
| 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 single most-tested fact: two passive (LACP) or two auto (PAgP) ends never form a bundle, because neither side initiates — the exact mirror of the two-dynamic auto trunk fact from the trunk lesson. And you cannot mix protocols: an LACP end will not bundle with a PAgP end.
Consistency requirements. Even with compatible modes, every member must agree on its Layer 1/Layer 2 settings or it is left out of the bundle: speed, duplex, access-vs-trunk mode, native VLAN, and allowed-VLAN list must match across all members (and match the far side). A single member with a stray allowed-VLAN list or left in access mode silently drops out.
EtherChannel rules, mode matrix, and misconfiguration traps (topic guide)
Free CCNA | EtherChannel | Day 23 | CCNA 200-301 Complete Course
~6 min
Watch that STP treats the bundle as one interface, and how channel-group mode determines whether the bundle negotiates via LACP, PAgP, or forms statically.
The negotiation-mode table and the show etherchannel summary read are the core; the bandwidth-motivation intro can go faster.
Watch on YouTubeConfigure and verify an LACP EtherChannel
! Put both member links into an LACP channel group (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; members inherit it
Switch(config)# interface Port-channel 1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20
! Verify the bundle
Switch# show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
R - Layer3 S - Layer2
Number of channel-groups in use: 1
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------
1 Po1(SU) LACP Gi0/1(P) Gi0/2(P)
Read the flags: Po1(SU) = a Layer 2 port-channel that is up; each member Gi0/x(P) is bundled. If a member shows (D) it is down/not bundled, (I) means it is standing alone (negotiation failed), and (s) means suspended (a consistency mismatch). After bundling, show spanning-tree shows a single Po1 interface instead of two members with one blocked.
Predict each outcome before revealing.
Pause and predict
- SW1 members:
channel-group 1 mode passive. SW2 members:channel-group 1 mode passive. Does the bundle form? - SW1:
mode active(LACP). SW2:mode desirable(PAgP). Does the bundle form? - SW1:
mode on. SW2:mode active. Does the bundle form? - Both ends use
mode active, but on SW2 one member port still hasswitchport access vlan 99while the others are trunks. What doesshow etherchannel summaryshow for that port?
Reveal answer
- No. Two LACP passive ends both wait to be asked — neither initiates, so no bundle. One side must be active.
- No. LACP and PAgP are different protocols and do not interoperate;
activespeaks LACP,desirablespeaks PAgP. - No. Static on does no negotiation and expects the far end to also be on. An LACP
activeend sends LACP frames theonend ignores, so it will not form a clean bundle — modeonmust be on on both ends. - That member is left out of the bundle — it shows a non-
(P)flag such as(s)suspended or(D)down, because its access-vs-trunk and VLAN settings do not match the other members. Aligning it to a trunk with the same allowed-VLAN list lets it re-bundle.
Return to the two redundant trunk links from the STP stage — the ones STP was blocking. This EtherChannel stage of the progressive lab-week3-vlan-trunk lab bundles them so both forward.
Lab checkpoint · lab-week3-vlan-trunk · ~6 min · packet_tracer
Skill: Bundle the two redundant trunk links into one LACP EtherChannel
Starting: SW1 and SW2 have two parallel trunk links; STP currently blocks one of them.
Action: On both switches put the two member ports in channel-group 1 mode active, then configure interface Port-channel 1 as a trunk.
Verify: show etherchannel summary lists Po1 with both members flagged (P) bundled; show spanning-tree now shows one logical Po1 forwarding instead of a blocked member.
Variant: Set one side mode active and the other mode on and observe the bundle fail to form or behave inconsistently.
Lab checkpoint · lab-week3-vlan-trunk · ~4 min · troubleshooting
Skill: Diagnose a consistency mismatch that keeps members out of the bundle
Starting: One member port has a different allowed-VLAN list (or access vs trunk) than its peers.
Action: Read show etherchannel summary for a member flagged down, compare the member port configs, and align them.
Verify: After aligning speed/duplex/trunk/allowed-VLAN settings, the member re-bundles and shows (P) in the summary.
Variant: Distinguish a negotiation failure (mode incompatibility) from a consistency failure (mismatched member config) by the flag pattern.
Quick check (3 items) · test
Answer from memory:
- Why does STP block a member of two separate links but not a member of an EtherChannel?
- Name the LACP modes and the PAgP modes, and give the one mode combination in each that never bundles.
- List three member settings that must match or the port stays out of the bundle.
Review sheet
- EtherChannel = one logical Port-channel over many links; STP sees one interface and blocks no member.
- LACP = open 802.3ad (active/passive); PAgP = Cisco (desirable/auto); on = static, no negotiation.
- Never bundles: passive/passive (LACP), auto/auto (PAgP); protocols do not mix.
- Members must match speed, duplex, access/trunk, native VLAN, allowed-VLAN list.
- Verify with
show etherchannel summary— (P) bundled, (D) down, (s) suspended.
Troubleshooting scenario
Symptom. A new port-channel is up but only one of its two members is forwarding; throughput never exceeds a single link.
Diagnosis. show etherchannel summary shows one member (P) and the other (s) suspended — the suspended member has a different allowed-VLAN list from the bundle, a consistency mismatch rather than a mode problem.
Fix. Align the suspended member's trunk allowed-VLAN list (and native VLAN / mode) to match the other member and the far end; it re-bundles and both members forward.
Drill the LACP/PAgP mode combinations, the static-on rule, and the consistency checklist until recall is automatic.
Open flashcardsYou have built and bundled links across switches; the last switching skill is confirming what is connected to what — using CDP and LLDP to map neighbors and catch mismatches.
Next: CDP and LLDP Neighbor Discovery