Cisco LACP — EtherChannel configuration and practice labs

Link Aggregation Control Protocol (LACP) is an IEEE-standard protocol (originally 802.3ad, now part of IEEE 802.1AX) used to negotiate a link aggregation group. Cisco switches commonly implement it through EtherChannel.

EtherChannel combines multiple compatible physical connections between the same pair of switches into one logical Port-channel interface.

Why use it?

• More aggregate capacity: different traffic flows can use different member links.
• Redundancy: if one member fails, the Port-channel can continue through the remaining members.
• Simpler Layer 2 topology: STP treats a correctly formed Port-channel as one logical link, rather than blocking its individual parallel member links.

Important: LACP does not normally combine two 100 Mbps links into one 200 Mbps path for a single flow. Load balancing uses a hash, and one flow typically uses one physical member.

| Feature | STP | LACP |
| --- | --- | --- |
| Main purpose | Prevent Layer 2 loops | Negotiate a bundled link |
| Operates on | Redundant Layer 2 topology | Compatible parallel interfaces |
| Main decision | Which ports forward or discard | Which ports can join the bundle |
| Traffic handling | Blocks redundant paths as needed | Distributes flows over active members |
| Failover | Can unblock an alternate path | Can keep the bundle alive after a member fails |

They work together, not as replacements for one another.

• Port-channel: the logical interface representing bundled physical members.
• Member interfaces: physical ports participating in the EtherChannel.
• LACPDU: control frame exchanged to negotiate aggregation.
• Active mode: actively sends LACP negotiation packets.
• Passive mode: responds to LACP negotiation but does not initiate it.
• `on` mode: forces a static EtherChannel without LACP negotiation; incorrect peer configuration can create problems.
• Load-balancing hash: mechanism that selects a member link for a given traffic flow.

| SW1 mode | SW2 mode | LACP bundle? |
| --- | --- | --- |
| active | active | Yes |
| active | passive | Yes |
| passive | active | Yes |
| passive | passive | No — neither side initiates |

A mismatch between active/passive and a static on configuration is not a valid LACP pairing.

Without bundling, STP normally places one of the redundant parallel paths into a non-forwarding role to prevent a loop. When LACP forms correctly, both member links can be active within the same Port-channel.

Member ports must have compatible characteristics, including:

• Compatible speed and duplex settings.
• Consistent Layer 2 mode: all access or all trunk.
• Matching access VLANs if using access mode.
• Compatible trunk VLAN, native VLAN, and allowed-VLAN settings if using trunk mode.
• Supported hardware and matching EtherChannel configuration at both ends.

Configure the intended settings consistently before or while joining interfaces to the channel group. Mismatches may cause ports to be suspended or kept out of the bundle.

The channel-group 1 command associates the physical interfaces with Port-channel 1. The number is locally significant; it does not have to match the peer's Port-channel number, although using the same number is easier to document.

Packet Tracer note: exact interface and trunking commands depend on the switch model. If your switch supports additional trunk encapsulations, configure the appropriate encapsulation before switchport mode trunk.

Common flags:

• S: Layer 2 Port-channel.
• U: Port-channel is in use.
• P: physical port is bundled in the Port-channel.
• I: standalone port, not bundled.
• s: suspended port (depending on IOS output).

If the physical members do not show P, investigate the mismatch before testing failover.

In the STP output, expect the logical Po1 interface to represent the bundled connection, depending on the switch and software.

If another separate path creates a loop elsewhere in the topology, STP can still put a Port-channel as a whole into a non-forwarding role.

Consider two 100 Mbps links aggregated into a Port-channel:

```text
Physical member 1: 100 Mbps
Physical member 2: 100 Mbps
Theoretical total aggregate capacity: 200 Mbps
Typical single-flow maximum: 100 Mbps
`

Multiple independent flows may be distributed across members. Actual distribution depends on the configured hash and traffic patterns. It may not be an even 50/50 split.

Not every switch model or Packet Tracer image implements this command or every hash option. Inspect ? help and use the commands available on your device.

1. Confirm that show etherchannel summary reports both Fa0/1(P) and Fa0/2(P).
2. Connect PCs on access ports, using a working VLAN and IP setup, then verify ping connectivity.
3. Disconnect one of the EtherChannel member cables.
4. Run show etherchannel summary again.
5. Explain why Po1 can remain up with the surviving member.
6. Reconnect the cable and verify that both members return.

LACP failover does not guarantee that every packet survives the transition. Some traffic may be lost during failure detection and recovery.

Expected: passive/passive does not negotiate an LACP channel because neither switch initiates the exchange. Restore active on at least one side afterward.

Some Packet Tracer images may not implement all negotiation behaviors exactly like real hardware.

Configure an incompatible trunk/access mode or VLAN parameter on one member to see whether the switch rejects or suspends that member. Restore matching configuration and verify the bundle again.

Only run mismatch experiments in your isolated lab, and always inspect the actual show etherchannel summary result rather than assuming a specific simulator response.

Some Packet Tracer versions support only a subset of these commands. Use show running-config when a narrower command is unavailable.

Check that both ends use compatible LACP modes, the same number of intended links, matching VLAN and trunk configuration, compatible physical port parameters, and available member interfaces.

The Port-channel removal command may vary by platform and whether the logical interface still exists. Confirm with show etherchannel summary and show running-config.

• Believing two passive ports will initiate a channel.
• Confusing LACP with static EtherChannel mode on.
• Assuming every single transfer can use the sum of all physical member speeds.
• Applying different access VLANs or trunk settings to member ports.
• Forgetting that STP still protects loops outside the aggregated connection.
• Looking only at link LEDs rather than verifying member P flags.
• Assuming all Cisco Packet Tracer switch images support the same commands.

1. Why might STP block one of two direct links if LACP is not configured?
2. What is the difference between an individual physical interface and Po1?
3. Which LACP mode combinations negotiate successfully?
4. Does a 2 × 100 Mbps channel guarantee 200 Mbps for a single file transfer?
5. What happens if one of the physical member links fails?
6. Can STP block an entire Port-channel if the network has another redundant path?

Workshop recap: STP prevents Layer 2 loops. LACP aggregates compatible physical links. A well-designed switched network often uses both.