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Difficulty: Very hardLayer 2 and Layer 3 EtherChannel (LACP)

A network administrator is troubleshooting an inter-switch link failure between Switch-1 and Switch-2. Switch-1 is configured for a Layer 3 EtherChannel, but the bundle fails to form, and Switch-2 is unable to route traffic across the link.

On Switch-1, the relevant configuration is:
text
interface Port-channel 5
ip address 10.10.50.1 255.255.255.252
!
interface range GigabitEthernet0/1 - 2
no switchport
channel-group 5 mode active

On Switch-2, the administrator issues the `show etherchannel 5 summary` command, which displays the following output:
text Flags: D - down P - bundled in port-channel I - stand-alone s - suspended H - Hot-standby s - Layer2 U - in use R - Layer3 S - Layer2 F - failed to allocate aggregator Group Port-channel Protocol Ports ------+------------+----------+----------------------------------------------- 5 Po5(SD) LACP Gi0/1(I) Gi0/2(I)

Additionally, inspecting `show running-config interface GigabitEthernet0/1` on Switch-2 reveals:
text
interface GigabitEthernet0/1
channel-group 5 mode passive

Which condition is the primary root cause of Switch-2 placing GigabitEthernet0/1 and GigabitEthernet0/2 into stand-alone `(I)` state and Port-channel 5 into Switch-port Down `(SD)` state?

  1. Switch-2 member interfaces are operating as default Layer 2 ports because `no switchport` was omitted on the physical member interfaces prior to or after adding them to the port-channel.Answer
  2. B
    LACP mode dynamic negotiation failed because Switch-1 is configured in `active` mode while Switch-2 is configured in `passive` mode.
  3. C
    The native VLAN configured on Switch-1 physical member interfaces does not match the default native VLAN configuration on Switch-2.
  4. D
    The IP address assigned to Port-channel 5 on Switch-1 was not replicated onto the physical member interfaces GigabitEthernet0/1 and GigabitEthernet0/2.

Answer

Switch-2 member interfaces are operating as default Layer 2 ports because `no switchport` was omitted on the physical member interfaces prior to or after adding them to the port-channel.
For a Layer 3 EtherChannel to aggregate properly, both switches must configure physical member ports and logical port-channel interfaces with the `no switchport` command. Switch-1 configured member interfaces as routed ports, while Switch-2 retained default Layer 2 behavior (indicated by the `S` flag in `Po5(SD)`). Because Layer 2 ports cannot bundle with Layer 3 ports across LACP peers, Switch-2 falls back to placing its physical interfaces into stand-alone `(I)` mode.

Step-by-Step Solution

1
Analyze the Switch-1 configuration
Switch-1 explicitly executes `no switchport` on physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2 and sets `channel-group 5 mode active` to create a routed (Layer 3) LACP EtherChannel.
Layer 3 EtherChannels require physical interfaces to operate in unswitched (routed) mode.
2
Inspect the CLI output flags on Switch-2
Flag `S` indicates Layer 2 operational mode and flag `D` indicates Down for `Po5(SD)`. Flag `I` indicates Stand-alone state for `Gi0/1(I)` and `Gi0/2(I)`.
When physical interfaces remain in default Layer 2 mode (`switchport`) while the peer switch expects Layer 3 routed interfaces, parameter mismatches prevent LACP bundle aggregation, forcing member ports into stand-alone state.
3
Evaluate LACP operational compatibility
LACP negotiation requires consistent Layer 2/Layer 3 port characteristics between peer switches.
To resolve the issue, Switch-2 must execute `no switchport` on both physical interfaces and `interface Port-channel 5`.

Key Concept

Layer 3 EtherChannel Member Interface Requirements
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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