EtherChannel bonds up to 8 physical links into one logical interface. It provides bandwidth aggregation and link-level redundancy without STP blocking — all member ports share the same STP state.

EtherChannel Protocols

Use LACP in multi-vendor networks. Use PAgP for Cisco-only networks. Use on for static configurations where negotiation is undesirable.

LACP Configuration

LACP Modes

At least one side must be active.

Basic LACP EtherChannel

interface range Gi0/23-24
 channel-group 1 mode active

The switch creates Port-Channel 1 automatically.

Named Port-Channel

interface Port-channel1
 description CORE-SW-Uplink
 switchport mode trunk
 switchport trunk native vlan 99
 switchport trunk allowed vlan 10,20,30,99

LACP with Standby Links

LACP supports up to 16 links in a group: 8 active, 8 standby.

interface range Gi0/23-24
 channel-group 1 mode active
 
interface range Gi0/21-22
 channel-group 1 mode active
 lacp port-priority 65535

Lower lacp port-priority determines which links are active. Standby links activate if an active link fails.

PAgP Configuration

PAgP Modes

Basic PAgP EtherChannel

interface range Gi0/23-24
 channel-group 1 mode desirable

Static EtherChannel (On Mode)

No negotiation — both sides must be manually configured identically:

! Switch A
interface range Gi0/23-24
 channel-group 1 mode on
 
! Switch B
interface range Gi0/23-24
 channel-group 1 mode on

Use this only when connecting devices that do not support LACP or PAgP (some servers, firewalls in transparent mode).

Load Balancing

EtherChannel does not round-robin packets. It hashes flows to member links.

Hash Algorithm

port-channel load-balance src-dst-ip

Available algorithms:

For switch-to-switch trunks carrying many hosts, src-dst-mac or src-dst-ip gives the best distribution.

! Verify current algorithm
show etherchannel load-balance
 
! Set algorithm
port-channel load-balance src-dst-mac

Check Flow Distribution

show etherchannel port-channel
show etherchannel load-balance hash

To see which link a specific flow uses:

test etherchannel load-balance interface Port-channel1 192.168.1.10 192.168.2.20

Verification Commands

show etherchannel summary
show etherchannel port-channel
show etherchannel detail
show etherchannel protocol
show interfaces port-channel 1
show lacp neighbor
show lacp counters
show pagp neighbor
show pagp counters

EtherChannel Summary

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      N - not in use, no aggregation
        f - failed to allocate aggregator

Number of channel-groups in use: 1
Group  Port-channel  Protocol    Ports
------+-------------+-----------+----------------------------------------------
1      Po1(SU)      LACP        Gi0/23(P) Gi0/24(P)

All member ports should show (P) — bundled in port-channel. (s) indicates suspended. (I) indicates stand-alone (protocol mismatch).

Troubleshooting EtherChannel

Common Failure Modes

Golden Rules for EtherChannel

All member interfaces must be identical in:

1. Speed — Same speed on every link
2. Duplex — All full duplex
3. Mode — All trunk or all access (mix breaks the bundle)
4. Native VLAN — Identical across members
5. Allowed VLAN list — Identical across members
6. STP configuration — Same portfast/bpduguard settings

If any of these differ, the port is suspended or the channel fails to form.

VLAN Mismatch Example

! Interface Gi0/23
interface Gi0/23
 switchport trunk allowed vlan 10,20,30
 
! Interface Gi0/24
interface Gi0/24
 switchport trunk allowed vlan 10,20,30,40
 
! Result: Gi0/24 will be suspended or the bundle rejects it

Fix: Match the allowed VLAN lists:

interface range Gi0/23-24
 switchport trunk allowed vlan 10,20,30

Real-World Scenarios

Scenario 1: “EtherChannel not coming up”

show etherchannel summary
# Gi0/23(P) Gi0/24(s)
 
show interfaces Gi0/24
# Gi0/24 is down, line protocol is down

The physical link is down. Check the cable, SFP, and far-end interface.

Scenario 2: “One port in stand-alone mode”

show etherchannel summary
# Gi0/23(P) Gi0/24(I)
 
show lacp neighbor
# Gi0/24 has no LACP neighbor

LACP is not reaching the far end. Check the protocol mode — one side may be set to on or desirable while the other is passive.

Scenario 3: “Uneven traffic distribution”

show etherchannel load-balance
# src-mac
 
show interfaces Gi0/23 | include rate
# 900 Mbps
show interfaces Gi0/24 | include rate
# 100 Mbps

src-mac hashing is unbalanced when most traffic comes from a single MAC (e.g., a firewall). Switch to src-dst-ip for better distribution.

Best Practices

• Use LACP, not PAgP — Multi-vendor compatibility and more features (standby links)
• Standardize member ports — Same media type, speed, and config across all members
• Distribute member ports across ASICs — On modular switches, spread links across different line cards for redundancy
• Set load-balance to src-dst-mac or src-dst-ip — Best distribution for most network designs
• Monitor utilization per member — If one link saturates before others, change hash algorithm
• Add one link at a time — When expanding an EtherChannel, verify each link bundles correctly before adding the next

Quick Reference