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Difficulty: HardLayer 2 and Layer 3 EtherChannel (LACP)

Two Cisco switches, Switch-A and Switch-B, are linked using physical interfaces GigabitEthernet0/1 and GigabitEthernet0/2. The network team intends to aggregate these links using dynamic trunk negotiation. A technician executes `channel-group 5 mode active` on Switch-A's physical interfaces, but configures `channel-group 5 mode desirable` on Switch-B's physical interfaces.

After applying these configurations, the output of `show etherchannel summary` on Switch-A indicates a status of `Po5(SD)` with member ports showing `(D)`. Which explanation correctly accounts for this operational failure?

  1. LACP and PAgP are distinct negotiation protocols, causing channel negotiation to fail when combining `active` (LACP) with `desirable` (PAgP).Answer
  2. B
    Both switches are configured in passive listening states, preventing either side from sending the initial negotiation frames required to bring up the port-channel.
  3. C
    Switch-A is configured for unconditional static aggregation, which drops incoming dynamic link aggregation control frames sent by Switch-B.
  4. D
    An 802.1Q native VLAN mismatch between the physical interfaces causes the switch to suspend LACP control traffic across the member links.

Answer

LACP and PAgP are distinct negotiation protocols, causing channel negotiation to fail when combining active (LACP) with desirable (PAgP).
EtherChannel links require matching protocols on both ends. Mode `active` triggers dynamic negotiation using LACP (IEEE 802.3ad), while mode `desirable` triggers dynamic negotiation using PAgP (Cisco proprietary). Because LACP and PAgP packets are incompatible, the switches cannot complete handshaking, causing the port-channel to remain down `Po5(SD)` with physical interfaces down `(D)` or standalone.

Step-by-Step Solution

1
Identify the protocol associated with each channel-group mode.
Mode `active` utilizes IEEE 802.3ad LACP, whereas mode `desirable` utilizes Cisco proprietary PAgP.
EtherChannel negotiation protocols must match on both ends of the link.
2
Analyze protocol compatibility between Switch-A and Switch-B.
Switch-A transmits LACP PDUs while Switch-B transmits PAgP packets. Neither switch understands the other's control frames.
LACP and PAgP cannot interoperate or establish a negotiated bundle.
3
Correlate protocol mismatch with CLI verification status `Po5(SD)`.
The Port-channel remains down (D) at Layer 2 (S) because dynamic negotiation fails completely.
Without successful negotiation, physical ports are kept in a down or unbundled state to prevent layer 2 loops.

Key Concept

EtherChannel Protocol and Mode Compatibility (LACP vs PAgP)
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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