EtherChannel (Link Aggregation)

Mental model

Two switches connected by one cable: STP sees one path, you get one link’s worth of bandwidth, and if that cable dies you’re disconnected. Connect them with four cables instead and STP — being STP — will block three of them to prevent a loop. You paid for four cables, you can only use one.

EtherChannel is the trick that lets STP see all four cables as one logical link (a Port-Channel). Now nothing gets blocked, all four links forward traffic in parallel, and if one cable fails you lose 25% of bandwidth but stay online.

Three ways to form an EtherChannel

Mode	Protocol	When to use
LACP	802.1AX (industry standard)	Always your first choice — works with non-Cisco gear
PAgP	Cisco-proprietary	Legacy Cisco-only environments
Static (on)	None — both sides forced on	When negotiation isn’t possible / for max performance

For LACP and PAgP, both ends negotiate before bringing the bundle up. For static, you tell both sides “you’re a Port-Channel, end of story” — risky if one side is misconfigured (creates a loop).

LACP modes (active / passive)

• active — actively sends LACP packets
• passive — answers if asked, doesn’t initiate
• At least one side must be active

PAgP modes (desirable / auto)

• desirable — actively negotiates
• auto — passive, answers only
• At least one side must be desirable

Commands

LACP — the typical case

! Both SW1 and SW2 — pick matching ports
SW1(config)# interface range GigabitEthernet0/1 - 4
SW1(config-if-range)# channel-protocol lacp
SW1(config-if-range)# channel-group 1 mode active
!
SW1(config)# interface Port-channel 1
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20,30

Configure the Port-channel interface (the logical one), not the individual physical interfaces. Settings on Po1 propagate to all members.

Static (no negotiation)

SW1(config-if-range)# channel-group 1 mode on

Make sure both ends are on — mismatched modes create a black hole.

Choose a load-balancing method

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

Default is src-dst-mac — good enough for switch-to-switch in a flat LAN. For router-to-switch or many-to-one flows, use src-dst-ip so different conversations actually hash to different physical links.

Verification

SW1# show etherchannel summary
SW1# show etherchannel 1 detail
SW1# show interfaces Port-channel 1
SW1# show lacp neighbor

The most useful one: show etherchannel summary. The bundle is healthy if you see Po1(SU) and (P) next to each member port — S = layer 2 / switch, U = in use, P = bundled.

Common mistakes

1. Mismatched settings on member ports. All ports in a bundle must have identical speed, duplex, native VLAN, allowed VLAN list, and switchport mode. One mismatch and the port falls out of the bundle.

2. One end LACP, other end PAgP. They don’t speak each other’s protocols. Bundle never forms. Both ends must use the same negotiation.

3. Both ends in passive / auto. Neither side initiates negotiation → bundle never forms. At least one side must be active (LACP) or desirable (PAgP).

4. Configuring physical interfaces individually after bundling. Once channel-group 1 is on a port, that port inherits everything from Port-channel 1. Configure on Po1, not on the members.

5. Static on on one end, LACP on the other. Static sends no LACP packets — the LACP side waits for negotiation that never comes. Both ends must agree on mode.

6. Default load-balancing on a router-to-switch link. Default src-dst-mac hashes everything from the router to the same physical link (router’s MAC never changes). Use src-dst-ip for variety.

Lab to try tonight

1. Two switches connected with four physical Ethernet links.
2. Verify STP behavior first: show spanning-tree should show three of the four links blocked.
3. Configure LACP EtherChannel on all four ports of both switches (one side active, other side passive).
4. Run show etherchannel summary — should show all four members bundled and in use.
5. Re-run show spanning-tree — now you should see ONE Port-channel interface, no blocked links.
6. Unplug one cable. Verify the Port-channel stays up with 3 members. Plug it back, watch it re-bundle.
7. Bonus: change load-balancing method and observe the change in show etherchannel load-balance.

Cheat strip

Concept	Plain English
EtherChannel / Port-Channel	2–8 physical links bundled into one logical interface
LACP	Industry-standard negotiation (use this)
PAgP	Cisco-only negotiation (avoid unless required)
Static on	No negotiation — both sides forced
Po1	Shorthand for Port-Channel 1, the logical interface
Load-balancing	How traffic is distributed across member links. src-dst-ip is the safe default.
Failure	One member dies → bundle stays up minus that bandwidth