EtherChannel is a link aggregation technology that bundles several physical Ethernet links between two devices into one logical link. Two switches connected by four cables see a single connection with the combined bandwidth, and if one cable fails the rest carry on.
It exists for two reasons: more bandwidth than one link can provide, and redundancy against a link failure. The members are agreed statically or through a negotiation protocol, PAgP on Cisco or the vendor-neutral LACP, and traffic is spread across the members by a hash that keeps each conversation on one link. EtherChannel is Cisco's name; the IEEE standard version is 802.3ad, known as LACP.
- EtherChannel bundles several physical links into one logical link
- It exists for more bandwidth and for redundancy against a link failure
- Two to eight links are bundled between the same pair of devices
- LACP is the vendor-neutral standard; PAgP is Cisco's own protocol
- Traffic is load-balanced by a hash, so one conversation uses one link
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What it isWhat EtherChannel is
The idea is to make many links behave as one, and the payoff is both speed and safety.
It bundles physical links into one logical link. EtherChannel is a port link aggregation technology that groups several physical Ethernet links into one logical Ethernet link, used primarily on Cisco switches but with an equivalent on every serious platform. The devices at each end treat the bundle as a single network interface, the port channel.
It provides fault tolerance and higher speed. EtherChannel exists to provide fault tolerance and high-speed links between switches, routers and servers. Link aggregation increases the total bandwidth beyond what a single connection could sustain, and it does so while adding redundancy rather than trading one for the other.
It bundles two to eight links. An EtherChannel can be created from between two and eight active Fast, Gigabit or 10-Gigabit Ethernet ports, between the same pair of devices. The exact maximum depends on the hardware.
Why bundleWhy bundle links at all
Two everyday problems push people toward EtherChannel, and it solves both at once.
One link is not enough bandwidth. When a single link between two switches is saturated, EtherChannel adds capacity without replacing the hardware: four one-gigabit links carry more aggregate traffic than one. It is often cheaper than moving the whole network path to a faster link type.
A single link is a single point of failure. Without a bundle, if the one cable between two switches fails, that part of the network is down until someone fixes it. In an EtherChannel, all but one of the physical links may fail and the logical link stays up, so a failure degrades capacity instead of causing an outage.
Spanning tree would block a second link anyway. Run two separate links between switches and spanning tree blocks one to prevent a loop, wasting it. EtherChannel is seen by spanning tree as a single logical link, so every member carries traffic and none is blocked.
How it formsHow the links are bundled
The members have to be agreed, and there are three ways to do it.
LACP is the standard way. The Link Aggregation Control Protocol, defined in IEEE 802.3ad and since moved to IEEE 802.1AX, negotiates the bundle between the two ends and works across vendors. Both sides exchange LACP messages and only form the channel if the configuration matches, which catches mistakes before they cause a loop.
Other vendors call the result a link aggregation group, or LAG, where Cisco says EtherChannel or port channel. The protocols and the behavior are the same, so a Cisco port channel and another vendor's LAG interoperate over LACP.
PAgP is Cisco's own. The Port Aggregation Protocol is Cisco's proprietary equivalent to LACP, doing the same negotiation between two Cisco devices. EtherChannel and the IEEE 802.3ad standard are very similar and accomplish the same goal; LACP is the one to choose when either end is not Cisco.
Static is no negotiation at all. A channel can also be forced on with no protocol, where both sides simply assume the bundle. It works, but with nothing checking the two ends agree, a mismatch can create a loop, so a negotiation protocol is the safer default.
ConfigurationEtherChannel modes and configuration
Each negotiation protocol has two modes, and the mode on each port decides whether a channel forms. This is the part of EtherChannel that certification exams test and that real configuration mistakes come from.
| Protocol | Mode | What the port does |
|---|---|---|
| LACP | active | Sends LACP packets and tries to form a channel |
| LACP | passive | Answers LACP packets, never starts the negotiation |
| PAgP | desirable | Sends PAgP packets and tries to form a channel |
| PAgP | auto | Answers PAgP packets, never starts the negotiation |
| None | on | Forces the port into the channel with no protocol |
A channel forms with active on at least one end for LACP, and desirable on at least one end for PAgP. Passive to passive and auto to auto never form, because neither side starts. Mode on only works against on.
A basic Cisco configuration
On a Cisco switch the member ports join a channel group, and the switch creates a matching logical interface called a port channel. This example bundles two ports with LACP and makes the port channel a trunk.
interface range GigabitEthernet0/1 - 2
channel-group 1 mode active
!
interface Port-channel1
switchport mode trunkConfigure the other switch the same way, with active or passive. After that, make VLAN and trunk changes on the port channel interface, not on the member ports, so the members stay identical. The channel group number is local to each switch and does not have to match.
Checking the channel
The command show etherchannel summary lists each group, its protocol and its ports. A healthy Layer 2 bundle shows the port channel flagged SU, in use, and every member port flagged P, bundled. A port flagged I, stand alone, or s, suspended, points to a configuration mismatch on that port.
The hash is set globally with port-channel load-balance, for example src-dst-ip, and show etherchannel load-balance confirms which method is active. To read back the logical interface itself, show running-config interface Port-channel1 prints its configuration.
Layer 2, Layer 3 and more than one switch
A port channel can be a Layer 2 interface, an access or trunk port, or a Layer 3 interface with an IP address on the port channel itself. LACP also allows up to sixteen ports in one group on Cisco switches, eight active and eight in standby that take over when an active port fails.
Standard EtherChannel runs between one pair of network devices. Multi-chassis designs such as Cisco vPC and StackWise Virtual make two switches present one link aggregation group (LAG) to a neighbor, so the bundle survives the loss of a whole switch.
Load balancingHow traffic is spread across the links
The one thing that surprises people is that a bundle is not a single fat pipe.
A hash picks the link per conversation. Traffic is distributed across the member links by a hash of fields such as the source and destination address or port. All the packets of one conversation hash to the same member, which keeps them in order.
One conversation uses one link. Because a flow stays on a single member, a single conversation cannot go faster than one member link. A bundle of four one-gigabit links gives four gigabits of aggregate capacity, but any one transfer is still capped at one gigabit.
Balance depends on variety. The bundle spreads load well when there are many conversations to hash across the members, and poorly when a few heavy flows all hash to the same link. Choosing a hash that matches the traffic, by port as well as address, is how the balance is improved.
Where it is usedWhere EtherChannel is used
Link aggregation is a basic networking technique rather than a Cisco invention, and it shows up in a few standard places on a network.
Between two switches. The uplink between two switches is often the busiest link on a building network, and bundling several Ethernet ports into one aggregation raises that uplink bandwidth and removes it as a single point of failure. This switch-to-switch case is the classic use.
Between a switch and a server. A server with multiple network ports can bundle them into one link to its switch, commonly called NIC teaming or bonding, for the same bandwidth and redundancy on the host side. The switch treats the server ports as one aggregation just as it would another switch.
Between a switch and a router. The link from a distribution switch to the router carries all the traffic leaving the segment, so aggregating multiple Ethernet ports there keeps a single failure from cutting the network off and adds capacity where it is needed most.
Across the data center. Wherever two devices exchange more traffic than one link can carry, EtherChannel, or the IEEE 802.3ad equivalent on non-Cisco gear, turns multiple ports into one, and modern switches load-balance across the active members automatically.
Side effectsWhat the rest of the network sees
A bundle changes what every other mechanism on the network is looking at, and most of the operational value of EtherChannel is in that simplification.
Spanning tree sees one interface. The bundle counts as a single link in the topology calculation, so no member is blocked and a member failure does not trigger a recalculation. Four separate cables between the same two switches would give spanning tree three links to block.
The MAC address table sees one interface. Addresses learned across the bundle are recorded against the port channel, so a flow moving from one member to another is not a MAC move and the network never relearns anything.
A routed bundle gives a routing protocol one neighbor. A Layer 3 port channel carries a single IP address, so OSPF or any other routing protocol forms one adjacency across all the members instead of one per cable. Losing a member costs capacity and does not cost an adjacency, so nothing reconverges.
Monitoring sees the aggregate. Counters exist per member and per port channel, and a network monitoring system polling only the logical interface reports the total. An unevenly loaded member hides inside an average that looks comfortable, which is why the per member counters are the ones to graph.
Security features are the exception to all of this. Controls that act on one physical port, such as 802.1X authentication, belong on user facing access ports rather than on the switch to switch and switch to server links where a bundle normally lives.
PitfallsWhere people go wrong
Expecting one transfer to use the whole bundle. It cannot. A single flow rides one member link, so a lone large transfer sees one link's speed, not the sum. The aggregate bandwidth only appears across many flows.
Mismatching the member configuration. Every member link must share the same speed, duplex and VLAN settings. A mismatch stops the channel forming or, worse with a static channel, creates a loop.
Using static mode carelessly. Forcing a channel on with no negotiation removes the safety check that both ends agree. Where a mistake could bridge the wrong ports, LACP or PAgP is the safer choice.
Mixing LACP and PAgP. The two negotiation protocols do not interoperate. Both ends must run the same one, and LACP is the choice whenever the link is not purely Cisco to Cisco.
Forgetting the hash when balancing is uneven. If traffic piles onto one member, the default hash may not suit the traffic. Balancing is a function of the hash and the variety of flows, not something the bundle guarantees on its own.
ComparisonLACP, PAgP and static compared
| Criterion | LACP | PAgP | Static (on) |
|---|---|---|---|
| Origin | IEEE 802.3ad standard | Cisco proprietary | No protocol |
| Works across vendors | Yes | No | Yes |
| Negotiates the bundle | Yes | Yes | No |
| Catches a mismatch | Yes | Yes | No |
| When to use | Any mixed environment | All-Cisco, by preference | Rarely, only with care |
LACP is the sensible default; PAgP is fine between Cisco devices, and static mode should be reserved for the few cases where negotiation genuinely cannot be used.
FAQFrequently asked questions
What is EtherChannel?
A link aggregation technology that bundles several physical Ethernet links between two devices into one logical link, giving the combined bandwidth of the members and staying up if one member fails. It is used primarily on Cisco switches.
Why use EtherChannel?
For two reasons at once: more bandwidth than a single link can provide, and redundancy, since one member can fail without bringing the logical link down. It also avoids spanning tree blocking a second parallel link.
How many links can an EtherChannel have?
Between two and eight active Ethernet ports, between the same pair of devices. The exact maximum depends on the hardware.
What is the difference between EtherChannel and LACP?
EtherChannel is Cisco's name for the link aggregation feature; LACP is the IEEE 802.3ad protocol that negotiates the bundle. EtherChannel can be formed with LACP, with Cisco's PAgP, or statically with no protocol.
What is the difference between LACP and PAgP?
LACP is the vendor-neutral IEEE standard and works across different vendors. PAgP is Cisco's proprietary equivalent and only between Cisco devices. They do not interoperate, so both ends must use the same one.
Does EtherChannel double my speed for one transfer?
No. Traffic is balanced per conversation, and a single conversation stays on one member link. A bundle raises the aggregate bandwidth across many flows, but one transfer is still capped at a single link's speed.
How does EtherChannel balance traffic?
By hashing fields such as source and destination address or port to pick a member link, keeping each conversation on one member so packets stay in order. The balance depends on the hash and how varied the traffic is.
Does EtherChannel work with spanning tree?
Yes, and that is part of the point. Spanning tree sees the bundle as one logical link, so it does not block any member. Without a bundle, spanning tree would block a redundant parallel link to prevent a loop.
What is a port channel?
Another name for the logical interface an EtherChannel creates. The bundle of physical links is configured and treated as a single port-channel interface.
What settings must match on the member links?
All members must share the same speed, duplex and VLAN configuration. A mismatch stops the channel forming, and in a static channel a mismatch can create a loop.
Is EtherChannel the same as link aggregation?
EtherChannel is Cisco's implementation of link aggregation. The general concept, combining multiple links in parallel for bandwidth and redundancy, is link aggregation, standardized as IEEE 802.3ad and negotiated by LACP.
What is static EtherChannel?
A bundle forced on with no negotiation protocol, where both ends simply assume the channel. It works but removes the check that the two ends agree, so it is riskier than using LACP or PAgP.
Is Ethernet bonding the same as EtherChannel?
In effect, yes. EtherChannel is Cisco's name for combining several physical links into one logical link. Ethernet bonding is the Linux term, NIC teaming the Windows term, and link aggregation the standards term. When both ends use LACP from IEEE 802.3ad, equipment from different vendors interoperates.
Keep readingRelated concepts
Read next · Switching Spanning Tree Protocol Why STP does not block an EtherChannel: it sees one logical link. Open this next11 min- Switching · 14 min What Is a VLAN? The VLAN settings every member link in a bundle must share.
- Switching · 9 min The MAC Address Table, and How to Find Which Port a Device Is On How a switch, the usual home of an EtherChannel, forwards frames.