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

An enterprise network team is troubleshooting an LACP-based routed EtherChannel between Switch-1 and Switch-2.

Switch-1 configuration snippet:
interface range GigabitEthernet0/1 - 2
no switchport
channel-group 12 mode active
!
interface Port-channel 12
no switchport
ip address 10.1.12.1 255.255.255.252

Switch-2 command output:
# show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby S - Layer2
R - Layer3 U - in use
Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
12 Po12(SD) LACP Gi0/1(I) Gi0/2(I)

Which configuration change on Switch-2 will successfully establish the Layer 3 EtherChannel and bring Port-channel 12 to the operational Layer 3 state (RU)?

  1. Configure 'no switchport' and 'channel-group 12 mode passive' on physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2, then assign the IP address under interface Port-channel 12.Answer
  2. B
    Assign the IP address 10.1.12.2 255.255.255.252 directly to physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2 and set 'channel-group 12 mode passive' under both interfaces.
  3. C
    Execute 'channel-group 12 mode auto' on physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2 on Switch-2 to initiate dynamic channel negotiation with Switch-1.
  4. D
    Modify the native VLAN setting on GigabitEthernet0/1 and GigabitEthernet0/2 to match Switch-1, then execute 'channel-group 12 mode active' on both physical interfaces.

Answer

The correct action is to configure 'no switchport' and 'channel-group 12 mode passive' on physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2, and assign the IP address under interface Port-channel 12.
To transition Port-channel 12 on Switch-2 from (SD) Layer 2 Down to (RU) Layer 3 In-Use, the physical member interfaces (Gi0/1 and Gi0/2) must have switchport functionality disabled via 'no switchport'. LACP dynamic negotiation requires compatible modes across both switches; since Switch-1 is configured in 'active' mode, Switch-2 can use either 'active' or 'passive' mode. Lastly, IP addressing in a Layer 3 EtherChannel must be assigned directly to the Port-channel 12 logical interface rather than to the individual member interfaces.

Step-by-Step Solution

1
Analyze the output of 'show etherchannel summary' on Switch-2.
Port-channel 12 shows flag 'S' (Layer 2) and 'D' (Down), while physical interfaces show 'I' (Stand-alone). Switch-1 is configured as Layer 3 ('R').
The flags indicate a Layer mismatch (Layer 2 on Switch-2 vs Layer 3 on Switch-1) and independent member states due to incomplete Layer 3/LACP setup.
2
Determine Layer 3 EtherChannel requirements for member interfaces.
Member interfaces must be routed ports using the 'no switchport' command prior to or matching port-channel settings.
EtherChannel member interfaces must match Layer 2 or Layer 3 properties to form an operational bundle.
3
Verify LACP mode compatibility.
Switch-1 is set to 'active'. Switch-2 must be set to 'active' or 'passive'.
LACP requires at least one side to be in active mode to initiate link aggregation control protocol negotiation.
4
Verify Layer 3 IP address placement.
The IP address must be assigned to interface Port-channel 12, not to individual physical interfaces.
In a routed EtherChannel, the port-channel interface acts as the single logical Layer 3 routing point.

Key Concept

Layer 3 EtherChannel LACP Negotiation and Troubleshooting
Estimated Time:3m 0s
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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