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

Two Cisco switches, SW1 and SW2, are interconnected using three physical links: GigabitEthernet1/0/1, GigabitEthernet1/0/2, and GigabitEthernet1/0/3. An engineer attempts to aggregate all three links into a routed Layer 3 EtherChannel (Port-channel 12).

On SW1, the engineer configures `no switchport` on `interface Port-channel 12` along with IPv4 addressing, and assigns all three physical interfaces using `channel-group 12 mode active`.

On SW2, `no switchport` and IP addressing are configured on `interface Port-channel 12`. Interfaces GigabitEthernet1/0/1 and GigabitEthernet1/0/2 are configured with `channel-group 12 mode passive`, while interface GigabitEthernet1/0/3 is configured with `channel-group 12 mode auto`.

Upon checking operational status, Port-channel 12 is up with IPv4 connectivity, but only GigabitEthernet1/0/1 and GigabitEthernet1/0/2 are bundled (`P` flag in `show etherchannel summary`), whereas GigabitEthernet1/0/3 remains unbundled (`I` flag on SW1 and `D` flag on SW2).

Which condition correctly explains why interface GigabitEthernet1/0/3 failed to join the EtherChannel bundle?

  1. Interface GigabitEthernet1/0/3 on SW2 is configured with PAgP mode auto, which cannot negotiate an EtherChannel bundle with SW1's LACP mode active.Answer
  2. B
    LACP passive mode on SW2 limits channel-group bundles to a maximum of two active member links when interacting with LACP active mode on SW1.
  3. C
    Interface GigabitEthernet1/0/3 on SW2 is missing the ip routing command required for individual member ports of a routed Layer 3 EtherChannel.
  4. D
    SW1 requires channel-group 12 mode desirable to successfully form an EtherChannel with a member port set to mode auto on SW2.

Answer

Interface GigabitEthernet1/0/3 on SW2 is configured with PAgP mode auto, which cannot negotiate an EtherChannel bundle with SW1's LACP mode active.
The root cause of the bundling failure on GigabitEthernet1/0/3 is a protocol mismatch. SW1 is configured with LACP (`channel-group 12 mode active`), while SW2's GigabitEthernet1/0/3 is configured with PAgP (`channel-group 12 mode auto`). Because IEEE 802.3ad LACP and Cisco proprietary PAgP speak completely different protocol languages, negotiation fails and the link drops out of the bundle, showing standalone (`I`) or down (`D`) flags.

Step-by-Step Solution

1
Analyze negotiation protocol assignment on SW1.
SW1 uses `channel-group 12 mode active` across all three physical interfaces, designating LACP (IEEE 802.3ad) as the aggregation control protocol.
The `active` keyword explicitly enables LACP and actively sends LACP PDUs.
2
Analyze negotiation protocol assignment on SW2 member interfaces.
Interfaces Gig1/0/1 and Gig1/0/2 use `passive` (LACP), whereas Gig1/0/3 uses `auto` (PAgP).
The `auto` keyword enables Cisco proprietary PAgP, which expects PAgP packets (`desirable` or `auto`), not LACP PDUs.
3
Evaluate protocol compatibility for GigabitEthernet1/0/3.
SW1 sends LACP PDUs on Gig1/0/3, while SW2 listens for PAgP packets on Gig1/0/3. Neither switch receives compatible aggregation negotiation frames.
LACP and PAgP are incompatible protocols and cannot form an EtherChannel across opposing endpoints.

Key Concept

LACP and PAgP Mode Incompatibility in EtherChannel Aggregation
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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