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Question

Difficulty: MediumLayer 2 and Layer 3 EtherChannel (LACP)

A network administrator executes the `show etherchannel summary` command on Switch-1 to inspect an EtherChannel connection to Switch-2:

text
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby
SD - Layer2&Down SU - Layer2&In-use

Group Port-channel Protocol Ports
------+-------------+-----------+-----------------------------------------------
1 Po1(SD) LACP Gi0/1(I) Gi0/2(I)

Which condition on Switch-2 is the most likely cause of the `(I)` flag on the Switch-1 member interfaces?

  1. Switch-2 member interfaces are configured with the `channel-group 1 mode desirable` command.Answer
  2. B
    Switch-2 member interfaces are configured with the `channel-group 1 mode passive` command while Switch-1 uses `active` mode.
  3. C
    Switch-2 member interfaces are configured with the `channel-group 1 mode active` command while Switch-1 uses `active` mode.
  4. D
    Switch-2 member interfaces are configured with a native VLAN mismatch relative to Switch-1.

Answer

Switch-2 member interfaces are configured with the `channel-group 1 mode desirable` command.
The `(I)` flag in `show etherchannel summary` output stands for stand-alone, meaning the physical interfaces are operating as separate single links rather than a bundled channel. Switch-1 is configured for LACP (Link Aggregation Control Protocol). If Switch-2 is configured with `channel-group 1 mode desirable`, it attempts to use PAgP (Port Aggregation Protocol). Because LACP and PAgP are incompatible aggregation protocols, negotiation fails completely, placing Switch-1's ports into stand-alone state.

Step-by-Step Solution

1
Analyze the CLI output flags from `show etherchannel summary`.
The port-channel Po1 is in Layer 2 Down state `(SD)`, and the physical member interfaces GigabitEthernet 0/1 and 0/2 display the `(I)` flag, indicating stand-alone mode.
The `(I)` flag signifies that the physical interfaces are operating independently as individual links because EtherChannel negotiation failed.
2
Evaluate the configured protocol and negotiation modes.
Switch-1 is attempting to form an EtherChannel using IEEE 802.3ad LACP.
LACP and Cisco proprietary PAgP protocols are mutually incompatible and cannot negotiate an aggregation bundle together.
3
Determine the configuration on Switch-2 that caused the negotiation failure.
Setting Switch-2 to `desirable` mode forces it to use PAgP. PAgP packets are ignored by Switch-1 running LACP, preventing bundle aggregation.
To form an LACP EtherChannel bundle, Switch-2 must use LACP modes (`active` or `passive`), with at least one side set to `active`.

Key Concept

LACP vs PAgP Protocol Mismatch and EtherChannel Verification
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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