Troubleshoot

3.1.b Troubleshoot static and dynamic EtherChannels

3 min read ENCOR 350-401 v1.2

Aligned to Cisco's 350-401 ENCOR v1.2 exam topics.

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EtherChannel combines multiple physical links into one logical link. Spanning tree sees the bundle as one port-channel, so you get more bandwidth without blocking every extra link.

The important word is logical. The member links must agree on the things that make a Layer 2 link behave like the same link: speed, duplex, trunk/access mode, allowed VLANs, native VLAN, and channel protocol.

SW1 · Gi1/0/1 + Gi1/0/2
Port-channel1
Port-channel1
SW2 · Gi1/0/1 + Gi1/0/2
One STP port · one trunk · one hash bundle

Spanning tree and routing should reference Port-channel1, not the individual member interfaces.

Protocols and modes

Method	Modes	Notes
Static	`on`	No negotiation. Both sides must be `on`. Risky if miswired.
LACP	`active`, `passive`	IEEE standard. `active` starts negotiation.
PAgP	`desirable`, `auto`	Cisco proprietary. `desirable` starts negotiation.

Best default for modern Cisco switching: LACP active/active.

Basic LACP trunk EtherChannel

Topology:

SW1
Gi1/0/1 ↔ Gi1/0/1
SW2
Gi1/0/2 ↔ Gi1/0/2

On both switches:

vlan 10
 name USERS
vlan 20
 name VOICE
vlan 999
 name NATIVE_UNUSED
!
interface range gi1/0/1 - 2
 description LACP bundle to peer switch
 switchport mode trunk
 switchport trunk native vlan 999
 switchport trunk allowed vlan 10,20,999
 channel-group 1 mode active
 no shutdown
!
interface Port-channel1
 description LACP bundle to peer switch
 switchport mode trunk
 switchport trunk native vlan 999
 switchport trunk allowed vlan 10,20,999

Apply the Layer 2 configuration to the physical ports before or with the channel group. Then confirm the port-channel interface has the same intent.

Verification commands

show etherchannel summary
show etherchannel port-channel
show interfaces port-channel 1 switchport
show interfaces trunk
show lacp neighbor
show spanning-tree interface port-channel 1

In show etherchannel summary, common flags matter:

Flag	Meaning
`P`	Member is bundled in the port-channel. Good.
`I`	Standalone. Not bundled.
`s`	Suspended. Often a mismatch.
`D`	Down. Physical or protocol problem.
`SU`	Layer 2 port-channel in use. Good.
`RU`	Layer 3 port-channel in use. Good for routed EtherChannel.

Common failures

Mode mismatch

One side is LACP active. The other side is static on. They do not negotiate the same thing.

Check:

show etherchannel summary
show run interface gi1/0/1
show lacp neighbor

Fix both sides to LACP:

interface range gi1/0/1 - 2
 no channel-group
 channel-group 1 mode active

Then reapply or verify the Port-channel configuration.

VLAN mismatch

One member link allows VLAN 10, another allows VLAN 20. EtherChannel cannot safely bundle links that forward different VLAN sets.

Check:

show interfaces trunk
show etherchannel summary

Fix the physical members and the port-channel:

interface range gi1/0/1 - 2
 switchport trunk allowed vlan 10,20,999
 switchport trunk native vlan 999
!
interface port-channel1
 switchport trunk allowed vlan 10,20,999
 switchport trunk native vlan 999

Access vs trunk mismatch

A member configured as access cannot join a trunk port-channel.

Check:

show interfaces gi1/0/1 switchport
show interfaces port-channel 1 switchport

Fix:

interface range gi1/0/1 - 2
 switchport mode trunk

Physical inconsistency

Speed, duplex, or media issues can prevent bundling.

Check:

show interfaces status
show interfaces gi1/0/1
show interfaces gi1/0/2

Fix the physical issue before chasing the protocol.

Layer 3 EtherChannel

A routed EtherChannel is one logical Layer 3 interface. The member links are not switchports.

interface range gi1/0/1 - 2
 no switchport
 channel-group 10 mode active
 no shutdown
!
interface Port-channel10
 no switchport
 ip address 10.12.0.1 255.255.255.252

Verify:

show etherchannel summary
show ip interface brief | include Port-channel10
show lacp neighbor

Lab: Fix a suspended member

Topology

Two switches connected by two links. Build a trunking LACP EtherChannel with VLANs 10 and 20.

Task 1: Configure the bundle

Use the LACP config from the earlier example.

Task 2: Break one member

On SW2:

interface gi1/0/2
 switchport trunk allowed vlan 10

Task 3: Diagnose

On SW2:

show etherchannel summary
show interfaces trunk
show run interface gi1/0/2

You should see the member fail to bundle or become suspended.

Task 4: Repair

interface gi1/0/2
 switchport trunk allowed vlan 10,20,999

Then verify both members show P.

Exam traps

Static on does not negotiate. It can hide mismatches until traffic breaks.
LACP active/passive works. Passive/passive does not start.
PAgP desirable/auto works. Auto/auto does not start.
EtherChannel member links must match. Treat them like copies of the same link.
Configure and verify the Port-channel interface, not only the physical interfaces.

Quick checklist

Are both ends using compatible channel mode/protocol?
Are speed, duplex, trunk/access mode, native VLAN, and allowed VLANs consistent?
Are member interfaces bundled, suspended, standalone, or down?
Does the port-channel interface have the intended configuration?
Is STP treating the port-channel as one logical link?
Is load balancing understood as per-flow, not per-packet?