Exam ScenariosCCNA 200-301

EtherChannel LACP/PAgP Modes That Will vs Will Not Bundle

Presenting Symptom

An EtherChannel between two switches fails to come up; the port-channel interface shows down/down or up/down, and member ports show err-disabled or not-negotiating.

Network Context

Two Cisco Catalyst 2960 switches (IOS 15.x) are connected via four GigabitEthernet links in a small branch office. The engineer configured one switch with LACP active mode and the other with PAgP desirable mode, expecting automatic negotiation. The EtherChannel is intended to aggregate the four links for increased bandwidth and redundancy.

Diagnostic Steps

Check EtherChannel status and member ports

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

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

Verify EtherChannel protocol and mode on each side

show running-config interface port-channel 1

interface Port-channel1
 no ip address
 switchport mode trunk
!

The port-channel interface is configured as a trunk. Note: The protocol is not explicitly set on the port-channel; it is inherited from member ports.

Check member port configuration for channel-group mode

show running-config interface gigabitethernet 0/1

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

This switch uses LACP active mode. The other switch must use LACP passive or active to form a bundle. If the other switch uses PAgP, negotiation will fail.

Check the other switch's configuration

show running-config interface gigabitethernet 0/1

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

The other switch uses PAgP desirable. LACP and PAgP are incompatible; they cannot negotiate an EtherChannel together. This is the root cause.

Root Cause

The two switches are configured with incompatible EtherChannel negotiation protocols: one uses LACP (mode active) and the other uses PAgP (mode desirable). LACP and PAgP cannot interoperate, so the member ports remain in stand-alone state and the EtherChannel does not form.

Resolution

Configure both switches to use the same protocol. For CCNA, LACP is preferred. On the switch currently using PAgP, change the channel-group mode to LACP active or passive. Commands on the PAgP switch: ``` interface range gigabitethernet 0/1-4 no channel-group 1 mode desirable channel-group 1 mode active ``` Alternatively, if both switches support PAgP, change the LACP switch to PAgP: ``` interface range gigabitethernet 0/1-4 no channel-group 1 mode active channel-group 1 mode desirable ``` After changing, the EtherChannel should negotiate and come up.

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) for bundled. Also verify with 'show interfaces port-channel 1' to see the interface is up/up.

Prevention

1. Standardize on one EtherChannel protocol across the network (preferably LACP for standards compliance). 2. Use consistent channel-group modes: either both active or one active and one passive for LACP; both desirable or one desirable and one auto for PAgP. 3. Document the protocol and mode used in the network design to avoid mismatches during troubleshooting or changes.

CCNA Exam Relevance

On the CCNA 200-301 exam, this scenario tests knowledge of EtherChannel negotiation modes and protocol compatibility. Expect multiple-choice questions asking which mode combinations will form an EtherChannel (e.g., LACP active/passive, PAgP desirable/auto) and which will not (e.g., LACP active with PAgP desirable). Also, troubleshooting questions may present a 'show etherchannel summary' output with (I) flags and ask for the root cause.

Exam Tips

Memorize the compatible mode pairs: LACP active/passive and active/active; PAgP desirable/auto and desirable/desirable. Any cross-protocol combination fails.

Remember that (I) in 'show etherchannel summary' means stand-alone (not bundled) due to negotiation failure; (P) means bundled.

Know that 'channel-group <number> mode <mode>' is configured on each physical interface, not on the port-channel interface.

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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