channel-group [id] mode [mode]
Assigns an interface to an EtherChannel bundle with a specified channel-group number and mode (active, passive, or desirable) to aggregate multiple physical links into a single logical link for redundancy and increased bandwidth.
Definition: channel-group [id] mode [mode] is a Cisco IOS interface config command. Assigns an interface to an EtherChannel bundle with a specified channel-group number and mode (active, passive, or desirable) to aggregate multiple physical links into a single logical link for redundancy and increased bandwidth.
Overview
The `channel-group [id] mode [mode]` command is a fundamental interface configuration command on Cisco IOS devices used to assign a physical interface to an EtherChannel bundle. EtherChannel, also known as Link Aggregation, allows multiple physical Ethernet links to be combined into a single logical link, providing increased bandwidth, load balancing, and redundancy. This command is essential for network engineers designing resilient and high-capacity networks, particularly in campus and data center environments where link aggregation is a standard practice.
The command is executed in interface configuration mode and requires the channel-group number (1-65535) and the mode (active, passive, or desirable). The mode determines the protocol used for negotiation: active and passive use LACP (IEEE 802.3ad), while desirable uses PAgP (Cisco proprietary). The command is typically used when you need to aggregate multiple links between switches, routers, or between a switch and a server.
Alternatives include static EtherChannel (mode 'on'), which does not use a negotiation protocol, but this is less flexible and can cause loops if misconfigured. The command fits into the broader configuration workflow after creating the port-channel interface (using `interface port-channel [id]`) and before verifying the bundle with `show etherchannel summary`. Important IOS behavior: the command is immediately applied to the running configuration; there is no buffered output.
It requires privilege level 15 (enable mode) to configure. The command can impact the running config by adding the interface to the channel group, and if the port-channel interface does not exist, it is automatically created. Misconfiguration can lead to inconsistent states, such as half of the links being down or spanning-tree issues.
Understanding this command is critical for CCNA and CCNP candidates as it appears frequently in exam scenarios and real-world network designs.
channel-group [id] mode [mode]When to Use This Command
- Aggregating two or more trunk links between switches to increase uplink bandwidth and provide redundancy.
- Combining access ports connecting a server to a switch for higher throughput and fault tolerance.
- Configuring LACP (active/passive) to negotiate EtherChannel with a partner switch that supports LACP.
- Using PAgP (desirable/auto) to form EtherChannel with a Cisco switch that only supports PAgP.
Parameters
| Parameter | Syntax | Description |
|---|---|---|
| id | <1-65535> | The channel-group number that identifies the EtherChannel bundle. Valid values range from 1 to 65535. This number must match the port-channel interface number if one is manually created. A common mistake is using the same channel-group number for different bundles on the same switch, which causes all interfaces to be assigned to the same bundle. |
| mode | active | passive | desirable | auto | on | The negotiation mode for the EtherChannel. 'active' and 'passive' are LACP modes; 'active' initiates negotiation, 'passive' responds. 'desirable' and 'auto' are PAgP modes; 'desirable' initiates, 'auto' responds. 'on' forces the link to be part of the channel without negotiation. Common mistake: using 'auto' on both sides results in no channel formation because neither side initiates. |
Command Examples
Configure LACP Active on an Interface
interface GigabitEthernet0/1
channel-group 1 mode activeGigabitEthernet0/1 is now in port-channel 1
The interface is added to channel-group 1 with LACP active mode, meaning it will actively negotiate the EtherChannel with the neighbor.
Configure PAgP Desirable on an Interface
interface FastEthernet0/2
channel-group 2 mode desirableFastEthernet0/2 is now in port-channel 2
The interface is added to channel-group 2 with PAgP desirable mode, initiating negotiation with the neighbor.
Understanding the Output
The command produces minimal output, typically just confirming the interface is added to the port-channel. To verify the EtherChannel status, use 'show etherchannel summary' or 'show etherchannel port-channel'. In 'show etherchannel summary', look for the port-channel interface (e.g., Port-channel1) and its status (SU for layer2 up, SD for down).
The member ports are listed with flags: 'P' for bundled in port-channel, 'S' for suspended, 'D' for down. A good EtherChannel shows all member ports as 'P' and the port-channel status as 'SU'. If any port shows 'S' or 'D', check for mismatched configurations (e.g., speed, duplex, VLAN allowed).
Configuration Scenarios
Configure LACP EtherChannel between two switches
Two switches, SW1 and SW2, are connected via two Gigabit Ethernet links. The goal is to aggregate these links into a single logical EtherChannel for increased bandwidth and redundancy using LACP.
Topology
SW1(Gi0/0)---Gi0/0 SW2
SW1(Gi0/1)---Gi0/1 SW2Steps
- 1.Step 1: Enter global configuration mode on SW1: SW1> enable, SW1# configure terminal
- 2.Step 2: Create the port-channel interface: SW1(config)# interface port-channel 1
- 3.Step 3: Configure the port-channel as a trunk (optional): SW1(config-if)# switchport mode trunk
- 4.Step 4: Exit and configure the physical interfaces: SW1(config-if)# interface range gigabitEthernet 0/0-1
- 5.Step 5: Assign them to channel-group 1 in active mode: SW1(config-if-range)# channel-group 1 mode active
- 6.Step 6: Repeat similar configuration on SW2, but use mode passive (or active): SW2(config)# interface port-channel 1, SW2(config-if)# switchport mode trunk, SW2(config-if)# interface range gigabitEthernet 0/0-1, SW2(config-if-range)# channel-group 1 mode passive
! SW1 Configuration interface Port-channel1 switchport mode trunk ! interface GigabitEthernet0/0 channel-group 1 mode active ! interface GigabitEthernet0/1 channel-group 1 mode active ! ! SW2 Configuration interface Port-channel1 switchport mode trunk ! interface GigabitEthernet0/0 channel-group 1 mode passive ! interface GigabitEthernet0/1 channel-group 1 mode passive
Verify: Use 'show etherchannel summary' on both switches. Expected output: Flags: D - down, P - 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, u - unsuitable for bundling, w - waiting to be aggregated, d - default port. Number of channel-groups in use: 1. Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(SU) LACP Gi0/0(P) Gi0/1(P)
Watch out: A common mistake is forgetting to create the port-channel interface before assigning physical interfaces. While IOS auto-creates it, the port-channel may inherit default settings (like switchport mode access) which can cause mismatches. Always explicitly configure the port-channel interface.
Configure PAgP EtherChannel between a switch and a router
A router R1 and switch SW1 are connected via two Fast Ethernet links. The router runs Cisco IOS and supports PAgP. The goal is to aggregate the links using PAgP for redundancy.
Topology
R1(Fa0/0)---Fa0/0 SW1
R1(Fa0/1)---Fa0/1 SW1Steps
- 1.Step 1: On R1, enter interface configuration for FastEthernet0/0: R1> enable, R1# configure terminal, R1(config)# interface fastEthernet 0/0
- 2.Step 2: Assign to channel-group 1 in desirable mode: R1(config-if)# channel-group 1 mode desirable
- 3.Step 3: Configure FastEthernet0/1 similarly: R1(config-if)# interface fastEthernet 0/1, R1(config-if)# channel-group 1 mode desirable
- 4.Step 4: On SW1, configure the port-channel interface: SW1(config)# interface port-channel 1, SW1(config-if)# no switchport (if Layer3) or switchport mode trunk (if Layer2)
- 5.Step 5: Assign physical interfaces on SW1: SW1(config-if)# interface range fastEthernet 0/0-1, SW1(config-if-range)# channel-group 1 mode desirable
! R1 Configuration interface FastEthernet0/0 channel-group 1 mode desirable ! interface FastEthernet0/1 channel-group 1 mode desirable ! ! SW1 Configuration interface Port-channel1 no switchport ip address 10.0.0.1 255.255.255.0 ! interface FastEthernet0/0 channel-group 1 mode desirable ! interface FastEthernet0/1 channel-group 1 mode desirable
Verify: On R1, use 'show etherchannel summary'. Expected output: Flags: D - down, P - 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, u - unsuitable for bundling, w - waiting to be aggregated, d - default port. Number of channel-groups in use: 1. Group Port-channel Protocol Ports ------+-------------+-----------+----------------------------------------------- 1 Po1(RU) PAgP Fa0/0(P) Fa0/1(P). Also verify connectivity with ping.
Watch out: PAgP is Cisco proprietary; ensure both devices support it. If one side uses LACP and the other PAgP, the channel will not form. Also, on routers, the port-channel interface is automatically created as a Layer3 interface; you must assign an IP address to it.
Troubleshooting with This Command
When troubleshooting EtherChannel issues, the `channel-group` command itself is not used for verification, but its configuration is the starting point. The primary verification command is `show etherchannel summary`, which provides a quick overview of all channel groups, their protocol, and the status of member ports. Healthy output shows all member ports in the 'P' (in port-channel) state and the port-channel interface in 'SU' (Layer2 in use) or 'RU' (Layer3 in use).
Problem indicators include ports in 'I' (stand-alone) or 'D' (down) state. If ports are stand-alone, it indicates a negotiation failure. Focus on the 'Protocol' column: mismatched protocols (e.g., one side LACP, the other PAgP) will prevent formation.
Also check the 'Flags' column for 's' (suspended) which often indicates a speed/duplex mismatch or VLAN inconsistency. Common symptoms: no connectivity across the bundle, only one link active, or spanning-tree blocking some ports. The diagnostic flow: 1) Verify physical layer: cables, speed, duplex.
Use `show interfaces [interface]` to check for errors. 2) Check channel-group configuration on each interface: `show running-config interface [interface]` to ensure mode is correct. 3) Verify port-channel interface exists and has correct configuration (e.g., VLAN trunk allowed, IP address). 4) Use `show etherchannel summary` to see overall status. 5) For LACP, use `show lacp neighbor` to see if the peer is detected. For PAgP, use `show pagp neighbor`. 6) Check for configuration mismatches: all ports in a channel must have same speed, duplex, VLAN membership, trunk mode, and STP port cost. Use `show interfaces trunk` to verify trunking consistency. 7) If using Layer3 EtherChannel, ensure the port-channel interface has an IP address and is not shutdown. 8) Correlate with `show spanning-tree` to see if the port-channel is blocking due to STP.
A common mistake is mixing access and trunk ports in the same channel; all ports must have the same switchport mode. Also, ensure that the allowed VLAN list on trunk ports matches. The `debug etherchannel` command can provide real-time negotiation details but should be used cautiously in production.
In summary, methodical verification of physical, configuration, and protocol layers resolves most EtherChannel issues.
CCNA Exam Tips
CCNA exam tip 1: LACP modes are 'active' and 'passive'; PAgP modes are 'desirable' and 'auto'. At least one side must be active/desirable to form the channel.
CCNA exam tip 2: All interfaces in a channel group must have the same speed, duplex, and VLAN configuration (or trunk allowed VLANs).
CCNA exam tip 3: The maximum number of interfaces in an EtherChannel is 8 (up to 16 with LACP standby).
CCNA exam tip 4: On the CCNA exam, remember that 'channel-group' is configured under interface configuration mode, not global config.
Common Mistakes
Mistake 1: Mixing LACP and PAgP modes on the same channel group — they are incompatible and the channel will not form.
Mistake 2: Forgetting to configure the same allowed VLANs on all member trunk ports, causing some ports to be suspended.
Mistake 3: Using 'channel-group' without first creating the port-channel interface (though IOS auto-creates it, but best practice is to create it manually).
channel-group [id] mode [mode] vs show etherchannel summary
These commands are frequently paired because while 'channel-group' configures an interface into an EtherChannel bundle, 'show etherchannel summary' verifies the operational state and membership of those bundles. They serve complementary roles in the configuration-to-verification workflow.
| Aspect | channel-group [id] mode [mode] | show etherchannel summary |
|---|---|---|
| Configuration mode | Interface configuration mode | Privileged EXEC mode |
| Scope | Single interface assignment | All EtherChannels system-wide |
| Persistence | Persists in running-config | Display only, no config change |
| Timing | Applied during config, takes effect immediately | Real-time status read |
| Error detection | Pre-checks compatibility with channel group | Shows protocol state and bundle health |
Use channel-group [id] mode [mode] when adding a physical port to an EtherChannel bundle during initial configuration or expansion.
Use show etherchannel summary when verifying that bundles are up, all ports are active, and the correct protocol (LACP or PAgP) is functioning.
Platform Notes
In IOS-XE (e.g., Catalyst 3650/3850, ISR 4000), the `channel-group` command syntax is identical to classic IOS. However, the output of `show etherchannel summary` may include additional flags like 'H' for Hot-standby (LACP) and 'b' for bundle in backup. In NX-OS (e.g., Nexus 9000), the equivalent command is `channel-group [id] mode [active|passive|on]` under interface configuration, but the port-channel interface is created automatically.
NX-OS also supports `lacp mode` and `lacp rate` commands. The `show port-channel summary` command is used instead of `show etherchannel summary`. In ASA firewalls, EtherChannel is called 'Port Channel' and configured with `channel-group [id] mode [active|passive|on]` under interface config, but the ASA does not support PAgP.
In IOS-XR (e.g., ASR 9000), the command is `bundle id [id] mode [active|passive|on]` under interface configuration, and the bundle interface is created with `interface Bundle-ether [id]`. There is no `channel-group` command. Differences between IOS versions: In 12.x, the command is the same but the maximum number of channel groups may be lower (e.g., 64).
In 15.x and 16.x, the limit is higher (up to 65535). Also, in older IOS, the port-channel interface might not be automatically created; you had to create it manually. Always check the specific platform documentation for exact syntax and capabilities.
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