EtherChannel and LACP: bundle links on a switch
What an EtherChannel is, and the mode rule that decides most questions
An EtherChannel (also called a port channel or link aggregation group) bundles two or more physical switch links between the same two devices into one logical link. You get more bandwidth and the bundle survives the loss of a member link. LACP (Link Aggregation Control Protocol, IEEE 802.3ad, now maintained as 802.1AX) is the open protocol that negotiates the bundle. PAgP is the Cisco-proprietary alternative, and “on” skips negotiation entirely.
The decision rule: at least one side must actively start the negotiation, and both sides must use the same protocol. With LACP, active+active and active+passive form a channel; passive+passive never does. With PAgP, desirable+desirable and desirable+auto form one; auto+auto does not. Mixing protocols, or “on” against a negotiating mode, does not form a working channel. Cisco’s Catalyst configuration guide states these pairings, and also warns against running PAgP and LACP in the same channel group.
Why not just plug in a second cable
Without a bundle, a second link between two switches is a loop, and spanning tree blocks one of them. Two cables, one usable. After bundling, spanning tree treats the whole EtherChannel as a single logical port, so no member is blocked and traffic uses all of them. The same logic applies to a trunk: the VLANs and trunking settings are configured on the port channel interface and must be consistent on the members.
Mode compatibility at a glance
| Side A | Side B | Result |
|---|---|---|
| LACP active | LACP active | Channel forms |
| LACP active | LACP passive | Channel forms |
| LACP passive | LACP passive | No channel, nobody initiates |
| PAgP desirable | PAgP desirable or auto | Channel forms |
| PAgP auto | PAgP auto | No channel |
| on | on | Forms, without negotiation |
| on | active, passive, desirable or auto | Does not form a working channel |
“On” is the dangerous one. No packets are exchanged, so a cabling or configuration mistake on one side is never detected by the protocol.
A worked example: two switches, two links
Two access switches SW1 and SW2 are connected by Gi1/0/1 and Gi1/0/2 and should carry VLANs 10 and 20 as one trunk. On both switches:
interface range GigabitEthernet1/0/1 - 2
switchport mode trunk
switchport trunk allowed vlan 10,20
channel-group 1 mode active
!
interface Port-channel1
switchport mode trunk
switchport trunk allowed vlan 10,20
Using mode active on both ends is the safe habit: either side can start negotiation. Configure the member interfaces first and the logical Port-channel1 interface is created automatically when you assign the channel-group. Then verify:
SW1# show etherchannel summary
Group Port-channel Protocol Ports
1 Po1(SU) LACP Gi1/0/1(P) Gi1/0/2(P)
Read the flags: S means Layer 2, U means in use, P means the port is bundled in the port channel. The Cisco guide also lists D (down), I (stand-alone), s (suspended) and H (hot-standby, LACP only). A member shown as I or s is the clue that something on its settings does not match.
Layer 3 EtherChannel
A Layer 3 EtherChannel has an IP address on the port channel itself and no VLANs. Remove the switchport behavior on the logical interface and on the members, then address the logical interface:
interface range GigabitEthernet1/0/3 - 4
no switchport
channel-group 2 mode active
!
interface Port-channel2
no switchport
ip address 10.0.0.1 255.255.255.252
This is the form used for a routed link between a distribution switch and a core switch. The member ports carry no IP address of their own. Both the v1.1 and v2.0 CCNA outlines ask for Layer 2 and Layer 3 EtherChannel.
Why a bundle does not double the speed of one flow
Frames are spread over the members by a hash of header fields (source or destination MAC, IP, or ports, depending on the platform and configuration). All frames of one conversation hash to the same link, so a single file transfer between two hosts never exceeds one member’s speed. The gain shows up with many conversations. The default hash differs by platform: Cisco’s Catalyst 9300 guide gives source MAC as its default, and show etherchannel load-balance shows what a switch is using. If one link carries nearly everything, the hash input is the problem, for example a hash on source MAC when one router is the only sender.
What members must have in common
Cisco documents that the ports in a bundle must match in speed, duplex, native VLAN, allowed VLAN range and trunking status. A port that does not match is not bundled, and in “on” mode mismatched ports are suspended. A port can belong to only one group. LACP supports up to 16 ports in a bundle, of which up to 8 are active and the rest hot-standby, according to the same guide, but check the platform for the exact figure because limits vary.
Which exams test this
| Exam | How it appears | Where on this site |
|---|---|---|
| 200-301 CCNA (v1.1, until Feb 2, 2027) | Network Access (20%), objective 2.4: configure and verify Layer 2 and Layer 3 EtherChannel using LACP | Switching and network access explained |
| 200-301 CCNA (v2.0, from Feb 3, 2027) | Switching and Network Access (25%), 2.1.c: Layer 2/Layer 3 LACP port channel between switches and routers; 2.2: port channel and LACP on edge-host ports | Switching and network access explained |
| N10-009 Network+ | Network Implementation (20%), 2.2: link aggregation listed among interface configuration items; LACP appears in the acronym list. Concept level only | Network implementation explained (brief mention) |
The CCNA expects commands and verification. Network+ expects you to know that link aggregation combines links for bandwidth and redundancy, and to recognise LACP.
How the questions are built
The trap is nearly always a mismatch hidden in the scenario. Typical versions: one side passive and the other passive; one side on and the other active; PAgP on one switch and LACP on the other; one member port set to access mode while the other is a trunk (the CCNA practice test on this site uses this shape); different speeds or duplex; different native VLANs. Constraint words that decide the answer are “open standard” or “vendor-neutral” (LACP, not PAgP), “without negotiation” (on), and “Layer 3” (needs no switchport and an IP on the port channel). Another favourite is “why is one link carrying most traffic?”, which is a hash question, not a failure.
Question 1. SW1 has Gi1/0/1 and Gi1/0/2 in channel-group 1 with mode passive. SW2 is configured with channel-group 1 mode passive on its two matching ports. The cables are correct and all four ports are up. What is the result?
- A. The channel forms and uses both links
- B. The channel forms but only one link is active
- C. No channel forms because neither side starts negotiation
- D. The channel forms because both ends use the same protocol
Show answer
Answer: C
With LACP, a channel forms only if at least one side is active, because passive ports only respond to LACP packets and never initiate. Two passive ends never negotiate, so no bundle forms. Mode on is a different, non-negotiating mode and is not what is configured. Matching speed and duplex are required but are not the problem here. Spanning tree would treat the two links as a loop only if no bundle formed, which is the symptom, not the cause.
Question 2. A network team must bundle two links between a Cisco switch and a switch from another vendor. Which configuration on the Cisco side is the correct choice?
- A. channel-group 1 mode desirable
- B. channel-group 1 mode active
- C. channel-group 1 mode auto
- D. A second link needs no configuration; spanning tree will load-balance it
Show answer
Answer: B
LACP is the IEEE standard and works between vendors; active mode lets the Cisco switch initiate negotiation. PAgP is Cisco-proprietary, so desirable or auto will not negotiate with a third-party switch. Static on mode does bundle without a protocol, but it gives no detection of mismatches and is not what the question asks for, since a negotiated, standards-based bundle is the safer answer.
Go deeper
- Cisco: Configuring EtherChannels (Catalyst 9300), the source for the modes, matching rules and flags above.
- Cisco CCNA 200-301 v2.0 exam topics (PDF), the outline that lists EtherChannel and LACP from February 3, 2027.
- Related guides: spanning tree protocol, VLANs and trunking, and HSRP and VRRP, which covers redundancy at the gateway rather than the link. The CCNA cheat sheet has the mode table in short form.