EtherChannel and LACP
Also known as: port channel, link aggregation
An EtherChannel bundles multiple physical Ethernet links between two switches into a single logical link, combining their bandwidth and providing redundancy. LACP is the standards-based protocol that negotiates the bundle automatically.
An EtherChannel (also called a port channel) aggregates several physical Ethernet links between the same pair of devices into one logical link. Instead of Spanning Tree Protocol blocking the redundant links to prevent loops, the switch treats the whole bundle as a single interface — so all member links forward traffic simultaneously, multiplying available bandwidth and surviving the failure of any individual cable or port.
Traffic is distributed across the member links by a load-balancing hash: the switch examines fields such as source and destination MAC or IP addresses and consistently sends each conversation down one member link. For the bundle to form, member ports must match on speed, duplex, allowed VLANs, and switchport mode — a mismatch causes the port to be excluded or the channel to fail.
Bundles can be negotiated or forced. LACP (Link Aggregation Control Protocol), defined in IEEE 802.3ad and since moved to IEEE 802.1AX, is the vendor-neutral negotiation protocol: ports in active mode initiate negotiation, while passive ports respond but never initiate, so at least one side must be active. Cisco's proprietary alternative is PAgP, and static "on" mode forces the bundle with no negotiation at all — both ends must use compatible methods or the channel will not come up.
The CCNA exam tests EtherChannel within Ethernet LAN administration: recognizing why aggregation is used, the LACP active/passive mode combinations that succeed or fail, the configuration requirements for member ports, and how EtherChannel interacts with Rapid PVST+ and Spanning Tree Protocol.
Key takeaways
- EtherChannel combines multiple physical links into one logical link for added bandwidth and redundancy.
- STP sees the bundle as a single interface, so no member link is blocked.
- A hash on MAC or IP address fields load-balances conversations across member links.
- LACP (IEEE 802.3ad, now 802.1AX) negotiates bundles with active and passive modes; PAgP is Cisco-proprietary, and mode "on" forces the channel statically.
- The CCNA exam tests LACP mode combinations, member-port requirements, and EtherChannel's interaction with Spanning Tree.