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Difficulty: MediumLayer 2 and Layer 3 EtherChannel (LACP)

A network engineer is establishing an EtherChannel bundle between two Cisco Catalyst switches using the Link Aggregation Control Protocol (LACP). Which two mode combinations across the opposing switches will result in a successfully negotiated EtherChannel? (Select two.)

  1. Configuring 'channel-group 5 mode active' on the first switch and 'channel-group 5 mode passive' on the second switchAnswer
  2. Configuring 'channel-group 5 mode active' on both switchesAnswer
  3. C
    Configuring 'channel-group 5 mode passive' on both switches
  4. D
    Configuring 'channel-group 5 mode active' on the first switch and 'channel-group 5 mode desirable' on the second switch

Answer

The correct answers are configuring active mode on one switch and passive mode on the other switch, and configuring active mode on both switches.
LACP requires at least one switch interface to actively initiate LACP protocol negotiations. The combination of active mode on both switches and the combination of active mode on one switch with passive mode on the opposing switch both satisfy this requirement, allowing the LACP negotiation handshake to complete successfully.

Step-by-Step Solution

1
Identify the aggregation protocol requested in the scenario.
The scenario explicitly specifies Link Aggregation Control Protocol (LACP), an IEEE 802.3ad open standard.
LACP supports only 'active' and 'passive' negotiation modes (or 'on' for unconditional bundling without LACP packets).
2
Analyze how LACP active and passive modes interact.
Active mode transmits LACP control frames proactively. Passive mode listent silently and responds only upon receiving LACP control frames.
At least one side of the connection must be set to active mode to initiate the protocol handshake.
3
Evaluate the provided configuration options against negotiation rules.
Active-to-passive and active-to-active combinations succeed. Passive-to-passive fails due to lack of initiation. Active-to-desirable fails due to mixing LACP with Cisco proprietary PAgP.
Matching compatible protocol negotiation settings is mandatory for EtherChannel establishment.

Key Concept

LACP EtherChannel Mode Negotiation Rules
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Topics

Layer 2 and Layer 3 EtherChannel (LACP)

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