Cisco EtherChannel: PAgP vs LACP Configuration

EtherChannel bundles up to eight physical links into one logical port-channel, multiplying bandwidth between switches while STP treats the bundle as a single port. Cisco gives you three ways to build it: PAgP (Cisco-proprietary negotiation), LACP (IEEE 802.3ad, multi-vendor) and static mode on. This guide configures each on a Catalyst switch, explains the mode combinations that actually work, and reads the show output that confirms a healthy bundle.

Choosing the Protocol

• PAgP — Cisco-only. Modes: desirable (actively negotiates) and auto (waits passively).
• LACP — open standard. Modes: active (sends PDUs) and passive (responds only). Use it when the far end is not Cisco.
• Mode on — no negotiation at all; both ends must be configured on manually.

Compatibility rules: active↔active, active↔passive and desirable↔auto/desirable work; passive↔passive and auto↔auto never bundle. Before bundling, make the member ports identical — duplex, speed, native/access VLAN, allowed VLANs and switchport mode.

Configuring LACP EtherChannel

SW1(config)# interface range GigabitEthernet0/1-2
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

The channel-group command creates Port-channel1 automatically; the logical interface then carries the VLAN configuration. PAgP uses mode desirable and static uses mode on — everything else is identical.

Verifying the EtherChannel

SW1# show etherchannel summary
SW1# show etherchannel 1 port-channel
SW1# show interfaces GigabitEthernet0/1 etherchannel

Read the flags in show etherchannel summary: P means bundled in the port-channel, I stand-alone, s suspended, H hot-standby (LACP only). Healthy output shows Po1(SU) with both members (P); the per-interface command confirms the negotiated mode, e.g. Protocol = LACP.

Load Balancing

The hashing algorithm decides how frames spread across members — choose it to match your traffic profile. L3-heavy traffic wants IP hashing; L2 forwarding environments may prefer MAC-based:

SW1(config)# port-channel load-balance src-dst-ip
SW1# show etherchannel load-balance

Options include src-ip, dst-mac, src-dst-mixed-ip-port (default on many platforms) and VLAN-aware variants. Same-vendor bundles can also use PAgP's pagp port-priority to influence member selection; LACP equivalents exist under lacp port-priority.

Common Mistakes

• Members with different speeds/duplex or VLAN config — the bundle refuses to form.
• Mode mismatch — one side active, the other on never negotiates.
• Putting member ports in different channel-groups.

EtherChannel on Cisco mirrors what other vendors call link aggregation — compare with Arista EOS port-channels and Dell OS10 VLT when you operate multi-vendor fabrics.

原文链接：https://networklessons.com/switching/etherchannel-cisco-ios-catalyst-switch