EtherChannel: LACP ו־PAgP

Uniting several cables into one logical link — more bandwidth and redundancy without loops.

Analogy: Instead of a single-lane road between two buildings — open four lanes but mark them as one road. STP sees a single logical link and therefore doesn't block anything, and the bandwidth accumulates.

Four physical links gi0/1 to gi0/4 between SW1 and SW2 are bundled into logical Port-Channel 1. STP sees a single link, so nothing is blocked and bandwidth adds up. LACP is the open 802.3ad standard with active and passive modes and works with third-party gear; PAgP is Cisco proprietary with desirable and auto modes.

The two protocols

LACP (802.3ad) — Open standard, also works with non-Cisco equipment. Modes: active / passive.
PAgP — Cisco proprietary protocol only. Modes: desirable / auto.
on — Manual forcing without negotiation; dangerous, an error on one side creates a loop.
Working combinations: active+active, active+passive, desirable+desirable, desirable+auto.
All ports in the group must be identical: speed, Duplex, Access/Trunk mode, and VLAN list.

! LACP between two switches
Switch(config)# interface range gi0/1 - 4
Switch(config-if-range)# channel-group 1 mode active
Switch(config-if-range)# exit

! Settings on the logical interface — not on the physical ones!
Switch(config)# interface port-channel 1
Switch(config-if)# switchport mode trunk
Switch(config-if)# switchport trunk allowed vlan 10,20

! Checks
Switch# show etherchannel summary
Switch# show etherchannel port-channel
Switch# show interfaces port-channel 1

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Architecture and Theory — Under the Hood

EtherChannel bundles up to 8 physical links into one logical link (Port-Channel). STP sees one link — therefore there is no blocking, and bandwidth is aggregated.

  SW1 gi0/1 ==\                 /== gi0/1 SW2
  SW1 gi0/2 ===>  Port-Channel 1  <=== gi0/2 SW2
        (LACP active <-> active)

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passive+passive or auto+auto will never come up — neither side initiates negotiation. This is the classic fault in exams and in the field.
Load Balancing does not distribute by packet but by Hash (src-mac / dst-ip / src-dst-ip) — therefore, a single TCP flow will never pass over more than one link, and a large backup between two servers will not utilize the full bandwidth.
mode on = no negotiation from either side — risky, creates loops if one side is not configured.
Load-Balancing according to hash (src-mac, dst-ip, src-dst-ip) — a single flow will not traverse all links.

Practical Configuration (CLI)

SW1(config)# interface range gi0/1-2
SW1(config-if-range)# shutdown
SW1(config-if-range)# switchport mode trunk
SW1(config-if-range)# switchport trunk allowed vlan 10,20
SW1(config-if-range)# channel-group 1 mode active     ! LACP
SW1(config-if-range)# no shutdown

SW1(config)# interface port-channel 1
SW1(config-if)# switchport mode trunk
SW1(config-if)# switchport trunk allowed vlan 10,20

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Real-world scenarios in an organization

Uplink connection between an access switch and a Core switch — 2x1G instead of 1G, plus redundancy.
Connecting an ESXi/Windows server with NIC Teaming to the switch.
Prevents bottleneck without upgrading to 10G equipment.

Troubleshooting

SW# show etherchannel summary     ! (SU) = OK, (SD)/(I) = problem
SW# show etherchannel port-channel
SW# show interfaces port-channel 1
SW# show lacp neighbor
SW# show run interface gi0/1

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Flag (I) = independent — the port did not join the channel, usually due to misconfiguration.
passive vs. passive = the channel will never form; one side must be active.
Mixing LACP and PAgP between the two sides does not work.
Traffic is not evenly distributed → this is normal hash behavior; you can change port-channel load-balance.

Glossary and Quick Command Line

LACP: active/passive | PAgP: desirable/auto | on: no negotiation
channel-group <n> mode active
show etherchannel summary — the main command; look for (SU)
Maximum 8 active links per channel

EtherChannel: LACP and PAgP

What you will learn here

Four physical links = one logical Port-Channel

The two protocols

Check yourself