Port Channels

A port channel (EtherChannel) bundles up to eight physical links into one logical interface. STP sees a single link, so all members forward instead of all but one being blocked. Traffic is spread per flow, not per packet: a single connection never gets more than one member’s bandwidth.

flowchart LR
  subgraph po1 [Port-channel 1]
    direction TB
    g1[Gi0/1 ⇄ Gi0/1]
    g2[Gi0/2 ⇄ Gi0/2]
  end
  sw1[SW1] --- po1 --- sw2[SW2]

Configuration

Configure the members; the Port-channel interface is created automatically. Settings applied to the port channel afterwards are copied to all members.

SW1

SW1# configure terminal
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)# description uplink to SW2
SW1(config-if)# switchport mode trunk
SW1(config-if)# end
SW1# write memory

Caution

All members need the same speed, duplex, switchport mode and VLANs. A mismatched port is suspended (s in show etherchannel summary) instead of joining the bundle.

Modes

Mode	Protocol	Behavior
active	LACP (802.3ad)	Sends LACP packets and negotiates.
passive	LACP	Only answers LACP packets.
desirable	PAgP (Cisco)	Sends PAgP packets and negotiates.
auto	PAgP	Only answers PAgP packets.
on	none	Forces the bundle without any negotiation.

Which combinations form a channel:

	active	passive	desirable	auto	on
active	✅	✅
passive	✅
desirable			✅	✅
auto			✅
on					✅

Tip

Prefer active on both sides. LACP is a standard (works with servers and other vendors) and detects a cabling mistake, whereas on silently creates a loop or black hole when the far side isn’t bundled.

Verification

Command	Shows
show etherchannel summary	Each bundle with its members and flags (P bundled, s suspended).
show interfaces Port-channel 1	Bandwidth (sum of members) and counters of the logical interface.
show lacp neighbor	The LACP partner of every member port.
show etherchannel load-balance	Which header fields choose the member link.
port-channel load-balance src-dst-ip	(global config) Hash on source and destination IP instead.