EtherChannelCCNA 200-301

EtherChannel Mode Mismatch — LACP vs PAgP vs Static

Presenting Symptom

EtherChannel does not come up; interfaces show as 'down/down' or 'err-disabled' in the channel-group, and no traffic is load-balanced across the bundle.

Network Context

A small branch office with two Cisco Catalyst 2960 switches connected via four Gigabit Ethernet links configured as an EtherChannel. The switches run IOS 15.x. One switch is configured with LACP (mode active), the other with PAgP (mode desirable), causing a mode mismatch. The network uses VLAN 10 for user traffic and VLAN 20 for management.

Diagnostic Steps

Check EtherChannel status

show etherchannel summary

Flags:  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(I)   Gi0/2(I)   Gi0/3(I)   Gi0/4(I)

The port-channel is up (SU) but all member ports show 'I' (stand-alone), meaning they are not bundled. This indicates a protocol mismatch or negotiation failure.

Verify EtherChannel configuration on both switches

show running-config interface port-channel 1

interface Port-channel1
 switchport mode trunk
 switchport trunk allowed vlan 10,20
!

The port-channel interface is configured correctly. The issue is likely on the member interfaces.

Check member interface configuration

show running-config interface gigabitEthernet 0/1

interface GigabitEthernet0/1
 switchport mode trunk
 channel-group 1 mode active
!

On this switch, the mode is 'active' (LACP). If the other switch is configured with 'desirable' (PAgP), this mismatch prevents the channel from forming.

Check the neighbor switch's interface configuration

show running-config interface gigabitEthernet 0/1 | include channel-group

 channel-group 1 mode desirable

The neighbor uses PAgP (mode desirable) while this switch uses LACP (mode active). LACP and PAgP are incompatible; both ends must use the same protocol.

Root Cause

EtherChannel mode mismatch: one switch is configured with LACP (channel-group mode active) and the other with PAgP (channel-group mode desirable). LACP and PAgP are incompatible protocols; both ends must use the same protocol (LACP or PAgP) or be set to 'on' (static) for the channel to form.

Resolution

Configure both switches to use the same EtherChannel protocol. For example, change the neighbor switch to LACP active: On the neighbor switch: interface range gigabitEthernet 0/1-4 no channel-group 1 mode desirable channel-group 1 mode active end Alternatively, change both to PAgP or static 'on'. The key is consistency.

Verification

Run 'show etherchannel summary' on both switches. Expected output: Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SU) LACP Gi0/1(P) Gi0/2(P) Gi0/3(P) Gi0/4(P) All member ports should show 'P' (bundled). Also verify with 'show interfaces port-channel 1' to confirm the interface is up/up.

Prevention

1. Standardize on one EtherChannel protocol (preferably LACP) across the network. 2. Use configuration templates or automation to ensure consistent channel-group mode on both ends. 3. Document the EtherChannel configuration and include protocol choice in the design.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario appears as a troubleshooting question where you must identify why an EtherChannel is not forming. The exam tests your knowledge of LACP and PAgP modes and their compatibility. Key fact: LACP modes are active and passive; PAgP modes are desirable and auto. Only active/active, active/passive, desirable/desirable, and desirable/auto combinations work. LACP and PAgP cannot interoperate.

Exam Tips

Memorize the compatible mode pairs: LACP active/active, active/passive; PAgP desirable/desirable, desirable/auto.

Remember that 'on' mode (static) does not use a protocol and must be configured on both ends; it will not negotiate with LACP or PAgP.

Use 'show etherchannel summary' to quickly see if ports are bundled (P) or stand-alone (I).

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.

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