EtherChannel, LACP and PAgP

How link aggregation works

Traffic is distributed across member links using a deterministic hash based on fields such as source/destination MAC address, IP address or Layer 4 ports. A single flow generally remains on one physical member to preserve packet order; aggregate throughput improves when many flows produce varied hash results.

Capacity point: Four 1-Gbps links can provide up to 4 Gbps of aggregate capacity across multiple flows, but one ordinary flow does not automatically become a 4-Gbps flow.

Method	Type	Common modes
LACP	IEEE link-aggregation control protocol.	Active initiates negotiation; passive responds.
PAgP	Cisco proprietary negotiation protocol.	Desirable initiates; auto responds.
Static	No negotiation.	On; both ends must be configured consistently.

Bundle-formation requirements

Members normally need compatible settings: speed, duplex, switchport mode, allowed VLANs, native VLAN, access VLAN, channel protocol and Layer 2/Layer 3 mode. Platform rules determine the supported number of active and standby links.

LACP active + active forms a channel.
LACP active + passive forms a channel.
LACP passive + passive does not initiate a channel.
PAgP desirable + auto can form a channel.
PAgP auto + auto does not initiate a channel.
Static on + on can form, but mismatches are not detected through negotiation.

Load-distribution hash

The hash chooses a member for each conversation. Changing the hash method can improve distribution when most traffic shares the same source or destination. It does not guarantee perfectly equal byte counts, because flows differ in size and duration.

Hash input	Useful scenario	Limitation
Source MAC	Many source hosts toward a small number of destinations.	Few sources can concentrate traffic.
Destination MAC	One source reaching many Layer 2 destinations.	One dominant destination can concentrate traffic.
Source/destination IP	Routed traffic with varied endpoint pairs.	Encrypted tunnels may hide inner flows.
Layer 4 ports	Many application sessions between the same IP endpoints.	Support varies by hardware.

LACP configuration example

Cisco IOS-style Layer 2 bundle

interface range GigabitEthernet1/0/1-2
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30
 channel-group 10 mode active
!
interface Port-channel10
 switchport mode trunk
 switchport trunk allowed vlan 10,20,30

Verification

show etherchannel summary
show lacp neighbor
show interfaces port-channel 10
show interfaces trunk
show spanning-tree interface port-channel 10
show etherchannel load-balance

Interaction with spanning tree and routing

Spanning tree sees a correctly formed Layer 2 port-channel as one logical link. If individual physical links are accidentally treated separately, spanning tree may block members or a loop can develop. On a Layer 3 bundle, an IP address is assigned to the port-channel rather than to each physical member.

Minimum-links and fast failure detection can be useful when the application requires a minimum amount of capacity. A bundle remaining logically up with only one member may be reachable but unable to carry the expected load.

Troubleshooting checklist

Confirm each member is physically up and supported for bundling.
Compare LACP/PAgP/static modes on both ends.
Check speed, duplex, trunk/access, VLAN and Layer 3 consistency.
Inspect suspended, individual or hot-standby member flags.
Verify the port-channel—not only the physical members—has the intended configuration.
Check the load-balancing hash and per-member counters.
Look for cross-stack, virtual-chassis or multi-chassis requirements when links terminate on different devices.

Interview focus

Use these points to practise concise, operations-focused answers. Explain the concept first, then give a verification or troubleshooting example.

Can one flow use all EtherChannel members at the same time? Normally one flow hashes to one member; many flows provide aggregate utilisation.
Which LACP combinations form a channel? Active-active and active-passive.
What happens with LACP passive on both ends? Neither side initiates, so the bundle does not form.
How does spanning tree view a valid EtherChannel? As one logical port-channel.
Why can a member be suspended? A negotiation or configuration inconsistency can make it ineligible for the bundle.

Quick self-check

1. Can one flow use all EtherChannel members at the same time?

Answer: Normally one flow hashes to one member; many flows provide aggregate utilisation.

2. Which LACP combinations form a channel?

Answer: Active-active and active-passive.

3. What happens with LACP passive on both ends?

Answer: Neither side initiates, so the bundle does not form.

4. How does spanning tree view a valid EtherChannel?

Answer: As one logical port-channel.

5. Why can a member be suspended?

Answer: A negotiation or configuration inconsistency can make it ineligible for the bundle.

Frequently asked questions

1. Is EtherChannel the same as LACP?

Answer: EtherChannel is a common name for the logical bundle; LACP is one protocol used to negotiate it.

2. Does adding links always double application speed?

Answer: No. Hashing, flow count, endpoint capacity and application behaviour determine real throughput.

3. Can links in one bundle connect to different switches?

Answer: Only when those switches operate as one supported logical or multi-chassis aggregation system.

Standards and further reading

Editorial note: Standards explain protocol behaviour, while commands and defaults can differ by vendor, platform and software release. Verify production changes against current vendor documentation.