EtherChannel bundles multiple physical links into one logical Port-Channel: more bandwidth, redundancy — and one STP link instead of several.

Hardware rules

• All member links same speed; up to 16 configured, max 8 active per port-channel (rest standby).

Protocols & negotiation

Mode pair	PAgP (Cisco)	LACP (802.3ad)
desirable/desirable · desirable/auto · active/active · active/passive	✅ forms	✅ forms
auto/auto · passive/passive	❌ down	❌ down
on (no negotiation)	⚠️ forced up — not STP-detectable, loop risk; Cisco discourages

Rule of thumb: PAgP needs desirable on at least one side; LACP needs active on at least one side — best practice active/active.

What must match

Layer (L2 vs L3), speed, duplex, switchport mode, access VLAN or allowed-VLAN list + native VLAN, channel-group number, protocol.

Load balancing is hashing, never per-packet

port-channel load-balance src-dst-ip   # Cisco default best practice
show etherchannel load-balance

src-dst-port distributes best but is platform-dependent (DC designs). Per-packet would reorder TCP.

Config (best practice)

interface range gi1/0/1-2
 switchport mode trunk
 channel-group 1 mode active
!
interface port-channel 1        ! configure everything on the master
 switchport trunk allowed vlan 10,20,30

CCNP extras: lacp system-priority / lacp port-priority decide which links go active; lacp max-bundle 4 caps active links. Verify with show etherchannel summary (flags P=active, S=standby, s=suspended, I=individual).

Common faults

Fault	Result
VLAN mismatch	interface suspended
speed/duplex mismatch	no bundle
passive + passive	no adjacency
one side “on”	loop risk