EtherChannel allows you to group multiple physical interfaces into a single logical interface, enabling them to behave as if they are one interface.
The core problem EtherChannel solves is that Spanning Tree Protocol (STP) blocks all redundant links between switches except one, wasting available bandwidth even when multiple physical connections exist.
STP treats an EtherChannel as a single interface, so all member interfaces can forward traffic simultaneously without creating Layer 2 loops.
Traffic is load-balanced across member interfaces using an algorithm based on "flows" - a flow being a communication between two nodes in the network.
Frames within the same flow always use the same physical interface to prevent out-of-order delivery issues at the destination.
Layer 2 EtherChannels group switch ports operating as a single interface for switching traffic.
Layer 3 EtherChannels group routed ports operating as a single interface with an IP address assigned to the virtual port-channel interface.
Category
Details
Maximum Interfaces
Up to 8 active interfaces per EtherChannel
LACP Special Case
Supports up to 16 interfaces (8 active + 8 standby)
Bandwidth
Combined bandwidth of all member interfaces
Member Interface Requirements
Must match: duplex, speed, switchport mode, allowed VLANs, native VLAN
Channel-group Number
Must match on same switch; does NOT need to match between switches
Virtual Interface Name
Port-channel (e.g., Po1)
Term
Definition
EtherChannel
A logical aggregation of multiple physical interfaces that operates as a single interface
Port Channel
Another name for EtherChannel; also the name of the virtual interface created
LAG (Link Aggregation Group)
Industry term for EtherChannel
PAgP (Port Aggregation Protocol)
Cisco proprietary protocol for dynamic EtherChannel negotiation
LACP (Link Aggregation Control Protocol)
IEEE 802.3ad industry standard protocol for dynamic EtherChannel negotiation
Flow
A communication stream between two nodes in the network
Oversubscription
When bandwidth of end-host interfaces exceeds bandwidth to distribution switches
Item
Value/Details
LACP IEEE Standard
802.3ad
Max Active Interfaces
8
Max LACP Interfaces (with standby)
16 (8 active + 8 standby)
PAgP Modes
desirable, auto
LACP Modes
active, passive
Static Mode
on
Mode Mixing Fails
on + desirable = ❌ No EtherChannel
desirable
on + active = ❌ No EtherChannel
active
active + desirable = ❌ No EtherChannel (mixing LACP with PAgP)
Passive/Auto Both Sides
passive + passive = ❌ No EtherChannel
passive
auto + auto = ❌ No EtherChannel
auto
Command Keyword Confusion
Configure: channel-group and port-channel load-balance
channel-group
port-channel load-balance
Verify: show etherchannel commands
show etherchannel
Virtual interface name: port-channel
port-channel
Channel-group Numbers
MUST match between member interfaces on the SAME switch
Do NOT need to match between different switches
IP Address Location for L3 EtherChannel
Configure on the port-channel interface, NOT member interfaces
Member Interface Mismatch
Mismatched settings cause interface to be "suspended" (lowercase 's' flag)
PAgP: desirable ←→ desirable ✓
desirable ←→ auto ✓
auto ←→ auto ✗
LACP: active ←→ active ✓
active ←→ passive ✓
passive ←→ passive ✗
Static: on ←→ on ✓
on ←→ anything else ✗
Desirable/Auto = PAgP (Cisco proprietary - Deliberate Auto)
Active/Passive = LACP (Industry standard - APproved by IEEE)
"Channel-group" to Configure
"Etherchannel" to Examine (show commands)
"Port-channel" for Port interface name and load-balance config
show etherchannel summary
S = Layer 2 (Switchport)
R = Layer 3 (Routed)
U = In Use
P = Bundled in Port-channel ✓
D = Down
s (lowercase) = Suspended
Switch(config)# interface range g0/0 - 3
Switch(config-if-range)# channel-group 1 mode active
Switch(config-if-range)# exit
Switch(config)# interface port-channel 1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20,30
Switch(config-if-range)# no switchport
Switch(config-if)# ip address 10.0.0.1 255.255.255.252
Switch(config)# port-channel load-balance src-dst-ip
Switch# show etherchannel load-balance
Switch# show etherchannel summary
Switch# show etherchannel port-channel
Switch# show interfaces trunk
Switch# show spanning-tree
What is EtherChannel?
A technology that groups multiple physical interfaces into a single logical interface, providing increased bandwidth and redundancy while being treated as one interface by STP.
What problem does EtherChannel solve?
STP blocking redundant links between switches. Without EtherChannel, STP would disable all but one link to prevent Layer 2 loops, wasting bandwidth.
What are the three names for EtherChannel?
1) EtherChannel 2) Port Channel 3) LAG (Link Aggregation Group)
What are the three methods to configure EtherChannel?
1) PAgP (Cisco proprietary) 2) LACP (IEEE 802.3ad - industry standard) 3) Static (manual configuration using "on" mode)
What is the IEEE standard for LACP?
What are the PAgP modes?
Desirable (actively negotiates) and Auto (passively waits)
What are the LACP modes?
Active (actively negotiates) and Passive (passively waits)
Which PAgP mode combinations form an EtherChannel?
Desirable + Desirable ✓
Desirable + Auto ✓
Auto + Auto ✗
Which LACP mode combinations form an EtherChannel?
Active + Active ✓
Active + Passive ✓
Passive + Passive ✗
Can "on" mode form an EtherChannel with "active" or "desirable"?
No. "On" mode only works with "on" mode on the other side.
What is the maximum number of active interfaces in an EtherChannel?
8 active interfaces
How many interfaces can LACP support in total?
16 interfaces (8 active + 8 standby)
What is a "flow" in EtherChannel load balancing?
A communication between two nodes in the network. All frames in the same flow use the same physical interface.
Why do frames in the same flow use the same physical interface?
To prevent frames from arriving out of order at the destination, which can cause problems for some applications.
What inputs can be used for EtherChannel load balancing?
Source MAC address
Destination MAC address
Source and Destination MAC
Source IP address
Destination IP address
Source and Destination IP
(Some switches: Layer 4 port numbers)
What command configures EtherChannel load balancing?
port-channel load-balance [method] (global config mode)
port-channel load-balance [method]
What command shows the current load-balancing method?
show etherchannel load-balance
What command adds an interface to an EtherChannel?
channel-group [number] mode [mode] (interface config mode)
channel-group [number] mode [mode]
Does the channel-group number need to match between switches?
No. It must match between member interfaces on the SAME switch, but can differ between switches.
What is the most useful show command for EtherChannel?
In show etherchannel summary, what does the flag "P" mean?
The port is properly bundled in the port-channel (desired state)
In show etherchannel summary, what does the flag "S" mean?
Layer 2 EtherChannel (Switchport)
In show etherchannel summary, what does the flag "R" mean?
Layer 3 EtherChannel (Routed port)
In show etherchannel summary, what does lowercase "s" mean?
Suspended - interface excluded due to configuration mismatch
What settings must match on all EtherChannel member interfaces?
Duplex
Speed
Switchport mode (access/trunk)
Allowed VLANs (if trunk)
Native VLAN (if trunk)
How do you configure a Layer 3 EtherChannel?
Use no switchport on member interfaces
no switchport
Use channel-group [#] mode [mode]
channel-group [#] mode [mode]
Configure IP address on the port-channel interface
Where do you configure the IP address for a Layer 3 EtherChannel?
On the port-channel interface, NOT on the member interfaces
What happens if you configure trunk settings on the port-channel interface?
The settings are automatically applied to all member physical interfaces as well.
Why is LACP preferred over PAgP?
LACP is an industry standard (IEEE 802.3ad) that works with switches from any vendor, while PAgP is Cisco proprietary.
What does the channel-protocol command do?
channel-protocol
Manually configures the EtherChannel negotiation protocol (LACP or PAgP). Not typically needed as the mode command automatically sets the protocol.
Will "active" and "desirable" form an EtherChannel?
No. Active is LACP and desirable is PAgP - they are incompatible protocols.
How does STP view an EtherChannel?
As a single interface. Physical member interfaces don't appear in spanning-tree output.
What is oversubscription in network design?
When the combined bandwidth of interfaces connected to end hosts exceeds the bandwidth of the uplink to distribution switches.
Why might Layer 3 connections between switches be preferred in modern network design?
Because routed ports don't forward Layer 2 broadcasts, eliminating the need for STP and allowing all links to be active.
What command shows the channel-group mode (active, passive, etc.)?
show etherchannel port-channel
If an EtherChannel uses 4 gigabit interfaces, what is the effective bandwidth?
4 Gbps (the bandwidth of member interfaces is combined)
What happens when a broadcast frame is received on an EtherChannel?
Only ONE copy is received, regardless of the number of physical interfaces, because the EtherChannel behaves as a single interface.
Complete the command: Switch(config-if)# channel-group 1 mode ____ for LACP active negotiation
Switch(config-if)# channel-group 1 mode ____
Complete the command: Switch(config-if)# channel-group 1 mode ____ for PAgP active negotiation
What is the virtual interface called when you create channel-group 1?
port-channel 1 (or Po1)
Side A
Side B
Result
✓ Static EtherChannel
✗ No EtherChannel
✓ PAgP EtherChannel
✓ LACP EtherChannel
Which protocol is used for industry-standard EtherChannel negotiation?
A. DTP
B. VTP
C. PAgP
D. LACP
Answer: D
Which TWO of the following EtherChannel mode combinations will
form a working EtherChannel? (Choose 2)
A. active - active
B. passive - passive
C. auto - auto
D. desirable - auto
E. on - active
Answers: A, D
Drag the EtherChannel mode to the correct protocol:
MODES: PROTOCOLS:
┌─────────┐ ┌─────────┐
│ active │ ──────► │ LACP │
├─────────┤ ├─────────┤
│ passive │ ──────► │ LACP │
│desirable│ ──────► │ PAgP │
│ auto │ ──────► │ PAgP │
└─────────┘ └─────────┘
Fill in the blank to complete the command that adds an interface
to EtherChannel group 1 using LACP active mode:
Switch(config-if)# _____________ 1 mode active
Answer: channel-group
SCENARIO:
You are configuring EtherChannel between SW1 and SW2.
SW1 interfaces G0/1-2 are configured with "channel-group 1 mode active"
SW2 interfaces G0/1-2 are configured with "channel-group 2 mode passive"
[Exhibit shows network diagram and configurations]
Question 1 of 3:
Will the EtherChannel form successfully?
A. Yes
B. No
Answer: A (Yes - active + passive works with LACP)
Question 2 of 3:
What protocol is being used?
A. PAgP
B. LACP
C. Static
D. DTP
Answer: B
You have console access to Switch1. Use show commands to
answer the following questions.
[You can type commands like:]
Switch1# show etherchannel summary
Switch1# show etherchannel port-channel
Question: How many interfaces are bundled in Port-channel 1?
A. 2
B. 3
C. 4
D. 5
TASK:
Configure an EtherChannel between SW1 and SW2 using the
following requirements:
- Use interfaces GigabitEthernet 0/1 and 0/2
- Use LACP with SW1 actively negotiating
- Use port-channel number 1
- Configure as a trunk allowing VLANs 10, 20, and 30
[You have full CLI access to configure the switch]
SW1(config)# interface range g0/1 - 2
SW1(config-if-range)# channel-group 1 mode active
SW1(config-if-range)# exit
SW1(config)# interface port-channel 1
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20,30
A. 802.1Q
B. 802.1D
C. 802.3ad ✓
D. 802.1w
Which are valid load-balancing methods? (Choose 2)
A. src-mac ✓
B. src-dst-ip ✓
C. vlan-id
D. port-channel-id
Match flag to meaning:
P → Bundled in port-channel
S → Layer 2 EtherChannel
R → Layer 3 EtherChannel
D → Down
Switch# show __________ summary
Answer: etherchannel
etherchannel
A network administrator configured the following on SW1:
The administrator configured the following on SW2:
SW2(config)# interface range g0/1 - 2
SW2(config-if-range)# channel-group 1 mode desirable
What will be the result?
A. An EtherChannel will form using LACP
B. An EtherChannel will form using PAgP
C. An EtherChannel will not form
D. An EtherChannel will form using static mode
Answer: C
Explanation: Active is LACP, desirable is PAgP.
Mixing protocols prevents EtherChannel formation.
Refer to the exhibit:
SW1# show etherchannel summary
Flags: D - down P - bundled in port-channel
I - stand-alone s - suspended
H - Hot-standby (LACP only)
R - Layer3 S - Layer2
U - in use f - failed to allocate aggregator
Group Port-channel Protocol Ports
------+-------------+-----------+------------------------
1 Po1(SD) LACP Gi0/1(D) Gi0/2(D)
What is the current state of the EtherChannel?
A. The EtherChannel is operational and forwarding traffic
B. The EtherChannel is down
C. The EtherChannel is in standby mode
D. The EtherChannel is suspended due to a configuration mismatch
Explanation: The flags show SD (Layer2, Down) and member
interfaces show D (down).
Which THREE statements about EtherChannel are correct?
(Choose 3)
A. LACP is defined in IEEE 802.3ad
B. PAgP is an industry standard protocol
C. Up to 8 active interfaces can be in an EtherChannel
D. LACP supports up to 16 interfaces with 8 in standby
E. The channel-group number must match on both switches
F. Member interfaces must have matching speed and duplex
Answers: A, C, D, F (Choose any 3 of these 4 correct ones)
Note: B is wrong (PAgP is Cisco proprietary)
E is wrong (doesn't need to match between switches)
Match the EtherChannel mode with its behavior:
MODES BEHAVIORS
─────────────────────────────────────────────────
active → Actively negotiates LACP
passive → Waits for LACP negotiation
desirable → Actively negotiates PAgP
auto → Waits for PAgP negotiation
on → Forces EtherChannel without negotiation
You are a network administrator for a company. You need
to configure an EtherChannel between SW1 and SW2.
Requirements:
• Use interfaces GigabitEthernet0/1 and GigabitEthernet0/2
• Use LACP protocol
• SW1 should actively negotiate
• Use channel-group 5
• Configure the port-channel as a trunk
• Allow only VLANs 100 and 200
Configure SW1 to meet these requirements.
SOLUTION:
SW1(config-if-range)# channel-group 5 mode active
SW1(config)# interface port-channel 5
SW1(config-if)# switchport trunk allowed vlan 100,200
Users report connectivity issues between buildings.
The EtherChannel between SW1 and SW2 is not working.
You have console access to both switches.
Use show commands to diagnose the issue.
SW1# show run | section interface GigabitEthernet0/1
interface GigabitEthernet0/1
switchport mode trunk
switchport trunk allowed vlan 10,20,30
channel-group 1 mode active
SW2# show run | section interface GigabitEthernet0/1
switchport trunk allowed vlan 10,20
channel-group 1 mode passive
Question: Why is the EtherChannel not working properly?
A. Mode mismatch between switches
B. VLAN mismatch on trunk interfaces
C. Channel-group number mismatch
D. Protocol mismatch
Explanation: SW1 allows VLANs 10,20,30 but SW2 only
allows VLANs 10,20. This mismatch causes issues.
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