EtherChannel Member Port Speed/Duplex Inconsistency
Presenting Symptom
An EtherChannel interface is down or only partially operational, with some member ports showing as 'err-disabled' or 'suspended'.
Network Context
A small branch office network with two Cisco Catalyst 2960 switches connected via four Gigabit Ethernet links configured as a single Layer 2 EtherChannel. The switches run IOS 15.0(2) and are configured for dynamic LACP negotiation. The problem was noticed after a junior engineer replaced a faulty cable on one of the links.
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 N - not in use, no aggregation
f - failed to allocate aggregator
M - not in use, no aggregation due to minimum links not met
m - not in use, port not aggregated due to 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(s) Gi0/4(D)Look for flags like 's' (suspended) or 'D' (down) on member ports. In this output, Gi0/3 is suspended and Gi0/4 is down, indicating a problem with those links.
Check the port-channel interface status
show interfaces port-channel 1Port-channel1 is up, line protocol is up (connected)
Hardware is EtherChannel, address is aaaa.bbbb.cccc (bia aaaa.bbbb.cccc)
MTU 1500 bytes, BW 2000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Full-duplex, 1000Mb/s
input flow-control is off, output flow-control is unsupported
Members in this channel: Gi0/1, Gi0/2
...Note that only two members are listed (Gi0/1 and Gi0/2). The bandwidth is 2 Gbps (2000000 Kbit/sec) instead of 4 Gbps, confirming that the other two ports are not bundled.
Check the suspended port for speed/duplex mismatch
show interfaces gigabitEthernet 0/3GigabitEthernet0/3 is down, line protocol is down (err-disabled)
Hardware is Gigabit Ethernet, address is aaaa.bbbb.ccdd (bia aaaa.bbbb.ccdd)
MTU 1500 bytes, BW 1000000 Kbit/sec, DLY 10 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Auto-duplex, 1000Mb/s
input flow-control is off, output flow-control is unsupported
...The port is err-disabled. Check the reason with 'show interfaces status err-disabled' or check the logs. The speed/duplex may be mismatched with the peer.
Check the down port for speed/duplex mismatch
show interfaces gigabitEthernet 0/4GigabitEthernet0/4 is up, line protocol is up (connected)
Hardware is Gigabit Ethernet, address is aaaa.bbbb.ccee (bia aaaa.bbbb.ccee)
MTU 1500 bytes, BW 10000 Kbit/sec, DLY 1000 usec,
reliability 255/255, txload 1/255, rxload 1/255
Encapsulation ARPA, loopback not set
Keepalive set (10 sec)
Half-duplex, 10Mb/s
...This port shows half-duplex and 10 Mbps, which is inconsistent with the other ports (full-duplex, 1000 Mbps). This mismatch causes the port to be suspended or down in the EtherChannel.
Root Cause
One member port (Gi0/4) has been manually configured with speed 10 and half-duplex, while the other ports are auto-negotiating to 1000/full. EtherChannel requires all member ports to have identical speed and duplex settings. The mismatch causes the port to be placed in a suspended state or to be err-disabled, preventing it from being part of the bundle.
Resolution
Verification
Run 'show etherchannel summary' again to confirm all ports are bundled (flag 'P'): Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SU) LACP Gi0/1(P) Gi0/2(P) Gi0/3(P) Gi0/4(P) Also verify speed/duplex consistency with 'show interfaces status' or 'show interfaces | include GigabitEthernet|Duplex|Speed'.
Prevention
1. Always use auto-negotiation for speed and duplex on all ports that are part of an EtherChannel. 2. Use consistent interface configurations across all member ports (e.g., same switchport mode, VLAN, etc.). 3. Implement port-channel load-balancing and ensure all physical links are identical in terms of media type and capabilities.
CCNA Exam Relevance
On the CCNA 200-301 exam, this scenario may appear as a troubleshooting question where you must identify why an EtherChannel is not fully operational. The exam tests your ability to interpret 'show etherchannel summary' output and recognize that speed/duplex mismatches cause ports to be suspended. A common multiple-choice question might ask: 'Which condition would cause an EtherChannel member port to be in a suspended state?' Correct answer: 'Speed or duplex mismatch.'
Exam Tips
Memorize the flags in 'show etherchannel summary': 'P' = bundled, 's' = suspended, 'D' = down, 'I' = stand-alone.
Remember that all member ports must have identical speed and duplex; auto-negotiation is recommended.
Know that 'show interfaces' output shows the actual speed/duplex; compare across member ports to find mismatches.
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
Displays detailed status and statistics for all interfaces or a specific interface, used to verify interface operational state, errors, and performance.
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