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EtherChannel and Link Aggregation for CompTIA Network+ N10-009

EtherChannel (Link Aggregation Group / LAG) bundles multiple physical Ethernet links between switches into a single logical link, increasing bandwidth and providing redundancy. CompTIA Network+ N10-009 tests link aggregation concepts, negotiation protocols (LACP vs PAgP), and how EtherChannel interacts with spanning tree. This technology is common in enterprise switch uplinks and server connections where bandwidth and redundancy are critical.

7 min

3 sections · 7 exam key points

1 practice questions

EtherChannel Fundamentals

EtherChannel combines 2–8 physical links into a single logical bundle (port channel). The logical interface has the combined bandwidth — four 1 Gbps links create a 4 Gbps logical channel. Traffic is distributed across member links using a load-balancing algorithm (based on source/destination MAC, IP, or port). If one member link fails, traffic automatically moves to the remaining links without reconvergence.

From STP's perspective, an EtherChannel appears as a single link — STP runs on the logical port channel, not the individual physical links. This prevents STP from blocking the redundant physical links within the EtherChannel. Without EtherChannel, adding redundant links between switches would cause STP to block all but one.

Negotiation Protocols: LACP and PAgP

LACP (Link Aggregation Control Protocol): IEEE 802.3ad standard — vendor-neutral. Modes: Active (initiates LACP negotiation) and Passive (responds to LACP, does not initiate). An Active-Active or Active-Passive combination forms an EtherChannel. Passive-Passive does not — neither side initiates.

PAgP (Port Aggregation Protocol): Cisco proprietary. Modes: Desirable (initiates PAgP negotiation) and Auto (responds but doesn't initiate). Desirable-Desirable or Desirable-Auto forms a channel. Auto-Auto does not.

On mode: forces EtherChannel without negotiation — no LACP or PAgP exchanged. Both sides must be set to On. Useful for connecting to devices that don't support negotiation protocols. Risk: no negotiation means misconfiguration isn't detected automatically.

Load Balancing and Requirements

EtherChannel load balancing distributes frames across member links, not packets within a flow. A single TCP flow always uses the same physical link — load balancing is per-flow (session-based). The algorithm hashes source and/or destination addresses: src-mac, dst-mac, src-dst-mac, src-ip, dst-ip, src-dst-ip. For switch-to-switch uplinks, source-destination IP is usually most effective for distributing diverse traffic.

Requirements: all member ports must have identical speed, duplex, VLAN configuration, and trunk settings. Mixing different speed ports in one EtherChannel is not supported. Mismatched configuration causes the EtherChannel to fail and individual ports to enter an error-disabled state.

EtherChannel Negotiation Protocol Comparison

Protocol	Standard	Modes	Forms Channel When
LACP	IEEE 802.3ad (open)	Active / Passive	Active-Active or Active-Passive
PAgP	Cisco proprietary	Desirable / Auto	Desirable-Desirable or Desirable-Auto
On (static)	No protocol	On	Both sides set to On

Key exam facts — Network+

EtherChannel combines multiple links into one logical channel — increases bandwidth and redundancy
STP treats an EtherChannel as a single link — no STP blocking within the bundle
LACP = IEEE standard (Active/Passive); PAgP = Cisco proprietary (Desirable/Auto)
Active-Passive (LACP) and Desirable-Auto (PAgP) form EtherChannel
Passive-Passive and Auto-Auto do NOT form EtherChannel — neither initiates
All member ports must match speed, duplex, VLAN settings
Load balancing is per-flow (session), not per-packet

Common exam traps

EtherChannel doubles throughput for individual connections

EtherChannel increases aggregate bandwidth across multiple flows. A single TCP session still uses only one member link — the bandwidth boost is realized across multiple concurrent sessions

LACP Passive-Passive forms an EtherChannel

Both Passive (LACP) and Auto (PAgP) wait for the other side to initiate — two passive/auto endpoints never form an EtherChannel. At least one side must be Active or Desirable

Practice questions — EtherChannel

These questions are representative of what you will see on Network+ exams. The correct answer and explanation are shown immediately below each question.

Q1.A network administrator needs to bundle four 10 Gbps links between two switches using an IEEE standards-based protocol. Which EtherChannel negotiation protocol should be used?

A.PAgP in Desirable mode

B.LACP in Active mode

C.PAgP in Auto mode

D.On mode without protocol

Explanation: LACP (Link Aggregation Control Protocol) is the IEEE 802.3ad standard for link aggregation — it is vendor-neutral and works between any manufacturer's equipment. PAgP is Cisco proprietary. Active mode initiates LACP negotiation. On mode works but provides no negotiation or error detection.

Frequently asked questions — EtherChannel

What happens if EtherChannel member ports have mismatched configurations?

Mismatched ports (different speed, duplex, or VLAN settings) cause the EtherChannel to fail. Individual ports may enter an error-disabled (err-disabled) state, or the EtherChannel may form with degraded behavior. Always verify that all member interfaces have identical Layer 2 configurations before forming an EtherChannel.

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