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Difficulty: HardLayer 2 and Layer 3 EtherChannel (LACP)

An engineer is troubleshooting a routed (Layer 3) EtherChannel between Switch-A and Switch-B. The output of `show etherchannel summary` on Switch-A displays the following status:

text
Group Port-channel Protocol Ports
------+------------+----------+-----------------------------------
10 Po10(SD) LACP Gi0/1(I) Gi0/2(I)

Flags: S - Layer3, D - Down, I - Stand-alone

Switch-A's physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2 are configured with `no switchport` and `channel-group 10 mode active` under interface configuration mode.

Which remote configuration on Switch-B explains why the physical member interfaces on Switch-A remain in a stand-alone (I) state and the port-channel is down (SD)?

  1. Switch-B has configured member interfaces with `channel-group 10 mode auto`.Answer
  2. B
    Switch-B has configured member interfaces with `channel-group 10 mode passive`.
  3. C
    Switch-B has a native VLAN mismatch configured on interface GigabitEthernet0/1.
  4. D
    Switch-B has assigned an IP address directly to member interface GigabitEthernet0/1 while leaving interface Port-channel10 unnumbered.

Answer

Switch-B has configured member interfaces with `channel-group 10 mode auto`.
The option stating that Switch-B has configured member interfaces with `channel-group 10 mode auto` is correct because `mode auto` enables PAgP negotiation. PAgP and LACP are mutually incompatible protocols; an LACP active interface will never form an EtherChannel with a PAgP auto interface, leaving local ports in the stand-alone (I) state.

Step-by-Step Solution

1
Analyze the `show etherchannel summary` output and flags on Switch-A.
The port-channel Po10 is flagged as `SD` (Layer 3, Down) and member ports Gi0/1 and Gi0/2 are marked with flag `I` (Stand-alone).
Flag `I` indicates the switch is operating physical ports individually because dynamic aggregation negotiation has failed.
2
Examine the local channel-group configuration and protocol on Switch-A.
Switch-A uses LACP (`channel-group 10 mode active`).
An LACP active port sends LACP Data Units (LACPDUs) to solicit aggregation from the remote peer.
3
Evaluate peer negotiation protocol compatibility.
PAgP modes (`auto` and `desirable`) are incompatible with LACP modes (`active` and `passive`).
If Switch-B is configured with `mode auto`, it uses Cisco proprietary PAgP, which does not process or respond to IEEE 802.3ad LACP packets, preventing bundle formation.

Key Concept

LACP vs PAgP Protocol Compatibility and Operational State Verification
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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