EtherChannel bundles multiple physical links into a single logical link, increasing bandwidth and providing redundancy between switches. Because Spanning Tree Protocol (STP) sees the bundle as one logical link rather than several parallel ones, it doesn’t block any of the member links — so you get the full aggregate bandwidth without STP shutting links down to prevent loops.
Two protocols negotiate EtherChannel formation: Cisco’s proprietary Port Aggregation Protocol (PAgP) and the IEEE-standard Link Aggregation Control Protocol (LACP). You can also build an EtherChannel with no negotiation protocol at all — this is called a static (or unconditional) EtherChannel, configured with the on mode.
Each EtherChannel is a channel group. You add a physical port to a group with the channel-group group-number mode {on | auto | desirable | active | passive} command in interface configuration mode. The protocols track each member link, and when a link fails or is restored, they handle removing or adding it to the bundle without STP needing to reconverge.
How Many Links Can Be Bundled?
A single EtherChannel supports up to 8 active links. This limit comes from the load-balancing hash, which produces a value from 0 to 7 (eight possible outcomes) to distribute traffic across the member links.
There’s an important nuance with LACP: you can configure up to 16 ports in an LACP bundle, but only 8 can be active at any one time — the remaining ports are placed in hot-standby mode. If an active link fails, a hot-standby link automatically takes its place. PAgP, by contrast, supports up to 8 active links with no hot-standby capability. So the practical forwarding maximum for both protocols is 8 active links; LACP simply adds up to 8 standby links for faster failover on platforms that support it.
Port Aggregation Protocol (PAgP)
PAgP is a Cisco-proprietary protocol that runs only on Cisco switches (or switches licensed by other vendors to support it). It automatically creates an EtherChannel by exchanging PAgP packets between EtherChannel-capable ports to negotiate the channel.
PAgP packets carry the neighbor switch’s information. The receiving switch learns the neighbor’s identity and its capability to support PAgP, then dynamically groups similarly configured ports into a single logical link. When PAgP is enabled, packets are sent every 30 seconds. PAgP uses the Layer 2 multicast address 01-00-0C-CC-CC-CC.
For an EtherChannel to form, all ports must have the same speed, duplex setting, and VLAN configuration. Any change to a member port’s configuration can affect the other ports in the channel. PAgP performs a configuration check on participating interfaces and confirms the neighbor is also using PAgP — interfaces without matching configurations won’t participate, which prevents accidental switching loops.
PAgP Modes
PAgP has three modes: On, Desirable, and Auto.
- On — The interface does not exchange PAgP packets; it forces the port into an EtherChannel without negotiation. A port in
onmode forms an EtherChannel only when the port on the other end is also set toon. - Desirable — The interface actively initiates negotiation, sending PAgP packets every 30 seconds to ask the other side to form a channel.
- Auto — The interface is passive: it responds to PAgP packets it receives but does not initiate negotiation.
Mode compatibility matters. If one side is set to auto, it waits for the other side to start negotiating — so if both sides are auto, no PAgP packets are ever exchanged and the EtherChannel never forms. Likewise, if one side is on, the other side must also be on; an on port doesn’t negotiate, so pairing it with a desirable or auto port will fail.
Note: PAgP modes are On, Desirable, Auto.
PAgP Configuration Example
Switch1
Switch1>enable
Switch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch1(config)#interface range ethernet0/0-3
Switch1(config-if-range)#channel-group 1 mode auto
Switch1(config-if-range)#exit
Switch1(config)#interface port-channel 1
Switch1(config-if)#switchport mode trunk
Switch1(config-if)#exit
Switch1(config)#end
Switch1#Switch2
Switch2>enable
Switch2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch2(config)#interface range ethernet0/0-3
Switch2(config-if-range)#channel-group 1 mode desirable
Switch2(config-if-range)#exit
Switch2(config)#interface port-channel 1
Switch2(config-if)#switchport mode trunk
Switch2(config-if)#exit
Switch2(config)#end
Switch2#Here Switch1 is auto (passive) and Switch2 is desirable (active), which is a valid combination — the desirable side initiates and the auto side responds.
Advantages and Disadvantages of PAgP
PAgP offers automatic negotiation, simplifying setup in all-Cisco networks, and provides redundancy and load balancing across the bundled links. Its main drawback is that it’s Cisco-proprietary, so it can’t be used in multi-vendor environments — which is why LACP is generally preferred for new deployments and mixed-vendor networks.
Link Aggregation Control Protocol (LACP)
LACP is an open protocol, originally published by the IEEE as 802.3ad and later defined under IEEE 802.1AX for local and metropolitan area networks. Like PAgP, it lets several physical ports be bundled into a single logical channel by negotiating automatically through LACP packets — detecting each side’s configuration and checking compatibility.
The key difference is that LACP is a vendor-neutral standard, so it can establish EtherChannels between devices from different vendors (Cisco, Juniper, HP, and others), whereas PAgP is Cisco-only. Both protocols can be used on Cisco devices. LACP uses the multicast address 01-80-C2-00-00-02.
LACP Modes
LACP has three modes: On, Active, and Passive.
- On — As with PAgP,
onmode forces the port into a channel without exchanging LACP packets (a static EtherChannel). - Active — Places the port in an active negotiating state; it initiates negotiation by sending LACP packets.
- Passive — Places the port in a passive state; it responds to LACP packets it receives but does not initiate negotiation.
As with PAgP, the modes must be compatible for the channel to form. Two passive ports will never form a channel, because neither initiates. The on mode again creates a static EtherChannel with no LACP negotiation, so both ends must be on.
Note: LACP modes are On, Active, Passive.
LACP Configuration Example
Switch1
Switch1>enable
Switch1#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch1(config)#interface range ethernet0/0-1
Switch1(config-if-range)#channel-group 1 mode active
Switch1(config-if-range)#exit
Switch1(config)#interface port-channel 1
Switch1(config-if)#switchport mode trunk
Switch1(config-if)#exit
Switch1(config)#end
Switch1#Switch2
Switch2>enable
Switch2#configure terminal
Enter configuration commands, one per line. End with CNTL/Z.
Switch2(config)#interface range ethernet0/0-1
Switch2(config-if-range)#channel-group 1 mode passive
Switch2(config-if-range)#exit
Switch2(config)#interface port-channel 1
Switch2(config-if)#switchport mode trunk
Switch2(config-if)#exit
Switch2(config)#end
Switch2#The LACP configuration is nearly identical to PAgP — only the mode keywords differ. active initiates LACP negotiation, while passive only responds to incoming LACP packets without initiating. Here Switch1 (active) and Switch2 (passive) form a valid pair.
Comparison of PAgP and LACP
| Feature | PAgP | LACP |
|---|---|---|
| Standard | Cisco proprietary | IEEE standard (802.3ad / 802.1AX) |
| Modes | On, Desirable, Auto | On, Active, Passive |
| Multicast address | 01-00-0C-CC-CC-CC | 01-80-C2-00-00-02 |
| Interoperability | Cisco devices only | Multi-vendor |
| Active links | Up to 8 | Up to 8 (plus up to 8 hot-standby) |
| Best suited for | All-Cisco environments | Multi-vendor / standards-based |
For most modern and mixed-vendor networks, LACP is the recommended choice because of its interoperability. PAgP remains a valid option in all-Cisco environments.
Load Balancing and Troubleshooting
Load balancing. EtherChannel distributes traffic across member links using a hash of address information. You can set the method with the port-channel load-balance command — for example, port-channel load-balance src-dst-ip to hash on source and destination IP. Note that a single traffic flow (one source-destination pair) is generally carried on a single physical link due to the hash, so aggregate bandwidth benefits multiple flows rather than speeding up any single flow. Check which methods your platform supports before setting one.
Troubleshooting. The most common problems are mode mismatches and mismatched port settings. Useful commands:
show etherchannel summary— shows each channel group, its protocol, and the status of member ports (a quick way to spot a channel that didn’t form).show interfaces— verify that member links have matching speed and duplex.show lacp neighbor— view LACP peer information.
Remember that PAgP and LACP are mutually exclusive: both ends of a link must use the same negotiation protocol (or both use on). Mismatched protocols will prevent the channel from forming, and the ports will revert to operating as individual links.
Conclusion
EtherChannel improves network redundancy and bandwidth by bundling physical links into one logical link that STP treats as a single path. PAgP (Cisco-proprietary) and LACP (the IEEE standard) both automate the negotiation, with compatible-mode pairing required on each end — desirable/auto for PAgP, active/passive for LACP, or on/on for a static bundle. For CCNA study and real-world deployments alike, knowing the modes, their compatibility rules, and how to verify a channel with show etherchannel summary is the foundation for working with link aggregation on Cisco switches.
FAQs
What is the main difference between PAgP and LACP?
PAgP is Cisco’s proprietary EtherChannel negotiation protocol and works only between Cisco (or Cisco-licensed) devices. LACP is an open IEEE standard (802.3ad / 802.1AX) that works across vendors. Both negotiate compatible modes to bundle links, but LACP is preferred where multi-vendor interoperability is needed.
How do I configure EtherChannel using LACP on Cisco switches?
Enter interface range configuration mode, apply channel-group 1 mode active on one side and mode passive (or active) on the other, then configure the resulting port-channel interface (for example, as a trunk). Verify the result with show etherchannel summary.
Why use EtherChannel in modern networks?
It aggregates multiple links for higher total bandwidth and provides fault tolerance — if one link fails, traffic shifts to the remaining links. Because STP sees the bundle as one logical link, none of the members are blocked, so you use all the available capacity without loops.
What are common EtherChannel troubleshooting steps?
Check mode compatibility with show etherchannel summary, confirm all member ports share the same speed, duplex, and VLAN settings, and make sure both ends use the same negotiation protocol. Mismatched modes or settings are the usual reason a channel fails to form.
How many links can an EtherChannel have?
Up to 8 active links per bundle for both PAgP and LACP. LACP additionally allows up to 8 hot-standby links (16 configured total), which automatically become active if an active link fails — a feature PAgP doesn’t offer.
