EtherChannel Member Ports with Different VLAN Configs
Presenting Symptom
Pings between hosts on the same VLAN across an EtherChannel fail intermittently, and the EtherChannel shows some member ports are down or not bundled.
Network Context
A small branch office has two Cisco Catalyst 2960 switches connected via a 4-port EtherChannel (Port-channel 1) trunking multiple VLANs. The switches run IOS 15.0. The EtherChannel is configured as a Layer 2 trunk. One switch (SW1) has member ports Gi0/1-4, the other (SW2) has Gi0/1-4. The problem occurs after a junior engineer reconfigured some ports to add a new VLAN, but inadvertently changed the VLAN configuration on some member ports.
Diagnostic Steps
Check EtherChannel status and member ports
show etherchannel summaryFlags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use f - failed to allocate aggregator
M - not in use, minimum links not met
u - unsuitable for bundling
w - waiting to be aggregated
d - default port
Number of channel-groups in use: 1
Number of aggregators: 1
Group Port-channel Protocol Ports
------+-------------+-----------+----------------------------------------------
1 Po1(SU) LACP Gi0/1(P) Gi0/2(P) Gi0/3(D) Gi0/4(D)The output shows that Gi0/3 and Gi0/4 are down (D) or not bundled. This indicates a problem with those ports. Normal output would show all ports as (P) for bundled.
Check the VLAN configuration on the problematic member ports
show interfaces gigabitEthernet 0/3 switchportName: Gi0/3 Switchport: Enabled Administrative Mode: trunk Operational Mode: trunk Administrative Trunking Encapsulation: dot1q Operational Trunking Encapsulation: dot1q Negotiation of Trunking: On Access Mode VLAN: 1 (default) Trunking Native Mode VLAN: 1 (default) Administrative Native VLAN tagging: enabled Voice VLAN: none Administrative private-vlan host-association: none Administrative private-vlan mapping: none Administrative private-vlan trunk native VLAN: none Administrative private-vlan trunk Native VLAN tagging: enabled Administrative private-vlan trunk encapsulation: dot1q Administrative private-vlan trunk normal VLANs: none Administrative private-vlan trunk private VLANs: none Operational private-vlan: none Trunking VLANs Enabled: ALL Pruning VLANs Enabled: 2-1001 Capture Mode Disabled Capture VLANs Allowed: ALL Protected: false Unknown unicast blocked: disabled Unknown multicast blocked: disabled Appliance trust: none
The output shows the port is in trunk mode with all VLANs allowed. However, we need to compare with a working port. The issue may be that the allowed VLAN list differs between member ports.
Compare allowed VLANs on all member ports
show interfaces trunkPort Mode Encapsulation Status Native vlan Gi0/1 on 802.1q trunking 1 Gi0/2 on 802.1q trunking 1 Gi0/3 on 802.1q trunking 1 Gi0/4 on 802.1q trunking 1 Port Vlans allowed on trunk Gi0/1 1-1005 Gi0/2 1-1005 Gi0/3 1-1005 Gi0/4 1-1005 Port Vlans allowed and active in management domain Gi0/1 1,10,20,30 Gi0/2 1,10,20,30 Gi0/3 1,10,20 Gi0/4 1,10,20 Port Vlans in spanning tree forwarding state and not pruned Gi0/1 1,10,20,30 Gi0/2 1,10,20,30 Gi0/3 1,10,20 Gi0/4 1,10,20
Notice that Gi0/3 and Gi0/4 are missing VLAN 30 in the 'Vlans allowed and active' list. This mismatch causes the EtherChannel to suspend those ports because all member ports must have identical VLAN configurations.
Check EtherChannel port-channel configuration
show running-config interface port-channel 1interface Port-channel1 switchport mode trunk switchport trunk allowed vlan 1,10,20,30 !
The port-channel interface has the correct allowed VLAN list. The problem is that the individual member ports Gi0/3 and Gi0/4 have a different allowed VLAN list (missing VLAN 30).
Root Cause
The EtherChannel member ports Gi0/3 and Gi0/4 have a different VLAN allowed list than the port-channel interface and the other member ports. Specifically, they are missing VLAN 30. Cisco EtherChannel requires all member ports to have identical VLAN configurations (allowed VLANs, native VLAN, etc.) for the bundle to form. The mismatch causes those ports to be suspended or not bundled.
Resolution
Verification
After applying the fix, verify the EtherChannel status: show etherchannel summary Expected output: Group Port-channel Protocol Ports ------+-------------+-----------+---------------------------------------------- 1 Po1(SU) LACP Gi0/1(P) Gi0/2(P) Gi0/3(P) Gi0/4(P) Also verify trunk allowed VLANs: show interfaces trunk Expected output shows all ports have VLAN 30 in the allowed list.
Prevention
1. Always configure VLAN settings on the port-channel interface rather than on individual member ports. The port-channel configuration is automatically propagated to member ports. 2. Use consistent configuration templates for all member ports to avoid manual mismatches. 3. Before adding a new VLAN, ensure it is added to all member ports or to the port-channel interface.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario tests understanding of EtherChannel configuration requirements. Questions may present a troubleshooting scenario where an EtherChannel is not forming, and you must identify that VLAN mismatch is the cause. Expect multiple-choice questions asking 'What is the most likely cause?' or drag-and-drop steps to resolve. Key fact: All member ports must have identical VLAN configuration (allowed VLANs, native VLAN) for the bundle to form.
Exam Tips
Remember that the 'show etherchannel summary' command quickly shows which ports are bundled (P) or down (D).
The 'show interfaces trunk' command reveals VLAN mismatches between member ports.
On the exam, if an EtherChannel is not forming, always check VLAN consistency first.
Commands Used in This Scenario
show etherchannel summary
Displays the status and configuration of all EtherChannel interfaces on the switch, used to verify channel bundling, port membership, and protocol state.
show interfaces trunk
Displays trunk interface status, allowed VLANs, and pruning information for all trunk ports on a Cisco switch, used to verify trunking configuration and VLAN membership.
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