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Difficulty: MediumLayer 2 and Layer 3 EtherChannel (LACP)

A network administrator is connecting two Cisco Catalyst switches using two physical Gigabit Ethernet links. On Switch-1, the physical interfaces are assigned using the command `channel-group 12 mode active`. On Switch-2, the corresponding physical interfaces are assigned using `channel-group 21 mode passive`. All physical interface speed, duplex, and VLAN settings match on both ends. What is the operational status of this EtherChannel bundle?

  1. A
    The EtherChannel link fails to form because channel group numbers must match on both switches for LACP negotiation packets to be accepted.
  2. The EtherChannel bundle forms successfully because channel group numbers are locally significant, and active-to-passive LACP modes successfully negotiate.Answer
  3. C
    The EtherChannel interfaces enter an err-disabled state because passive mode on Switch-2 rejects connections from a different group identifier.
  4. D
    The EtherChannel falls back to static manual mode because mismatched channel group IDs force the switches to disable LACP frame processing.

Answer

The EtherChannel bundle forms successfully because channel group numbers are locally significant, and active-to-passive LACP modes successfully negotiate.
The channel group number specified in the `channel-group <number> mode <mode>` command is strictly locally significant to the local Cisco switch. It identifies which logical Port-channel interface (e.g., Port-channel 12 vs Port-channel 21) member ports belong to locally. Furthermore, LACP negotiation succeeds when one switch is in `active` mode (initiating LACP packets) and the peer switch is in `passive` mode (responding to LACP packets).

Step-by-Step Solution

1
Evaluate channel group number significance across switch boundaries.
Channel group numbers (e.g., 12 on Switch-1 and 21 on Switch-2) define the local Port-channel interface ID on each individual switch and are not exchanged in LACP Data Units (LACPDUs).
Channel group numbers are strictly locally significant.
2
Evaluate LACP negotiation modes between Switch-1 and Switch-2.
Switch-1 is set to 'active' (actively initiates LACP negotiation) and Switch-2 is set to 'passive' (listens and responds to incoming LACP packets).
An active-to-passive combination satisfies LACP negotiation requirements.
3
Determine final EtherChannel bundle state.
Because physical parameters match and LACP negotiation succeeds, the Port-channel forms and transitions to an operational bundled state.
No configuration parameters block EtherChannel formation.

Key Concept

LACP Mode Negotiation and Channel Group Local Significance
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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